What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
Can an abstract class be final?
An abstract class may not be declared as final.
What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.
What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.
What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
Can try statements be nested?
Try statements may be tested.
What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.
What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.
What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.
How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.
Can an unreachable object become reachable again?
An unreachable object may become reachable again। This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
Can an abstract class be final?
An abstract class may not be declared as final.
What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.
What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.
What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
Can try statements be nested?
Try statements may be tested.
What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.
What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.
What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.
How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.
Can an unreachable object become reachable again?
An unreachable object may become reachable again। This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
ava Interview Questions and Answers
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When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in which it is used.
What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()
Which Component subclass is used for drawing and painting?
Canvas
What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.
What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.
What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a Boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.
Can there be an abstract class with no abstract methods in it?
yes.
Can an Interface be final?
yes.
Can an Interface have an inner class?
Yes public interface abc { static int i=0; void dd(); class a1 { a1() { int j; System.out.println("in interfia"); }; public static void main(String a1[]) { System.out.println("in interfia"); } } }
Can we define private and protected modifiers for variables in interfaces?
Yes.
What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
What is a local, member and a class variable?
Variables declared within a method are "local" variables.
Variables declared within the class i.e not within any methods are "member" variables (global variables).
Variables declared within the class i.e not within any methods and are defined as "static" are class variables
I made my class Cloneable but I still get 'Can't access protected method clone. Why?
Yeah, some of the Java books, in particular "The Java Programming Language", imply that all you have to do in order to have your class support clone() is implement the Cloneable interface. Not so. Perhaps that was the intent at some point, but that's not the way it works currently. As it stands, you have to implement your own public clone() method, even if it doesn't do anything special and just calls super.clone().
What are the different identifier states of a Thread?
The different identifiers of a Thread are:
R - Running or runnable thread
S - Suspended thread
CW - Thread waiting on a condition variable
MW - Thread waiting on a monitor lock
MS - Thread suspended waiting on a monitor लाक
What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn't even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte().
What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work. out is a static field in the java.lang.System class.
Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
Is null a keyword?
The null value is not a keyword.
Which characters may be used as the second character of an identifier,but not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
What is the difference between notify() and notifyAll()?
notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a "writer" lock on a file might permit all "readers" to resume).
Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()?
The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That's just the way it works, you'll get used to it. It's really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn't need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can't use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at all, because Math is a "final" class which means it can't be extended.
Why are there no global variables in Java?
Global variables are considered bad form for a variety of reasons: · Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables).
· State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state.
· When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once.
For these reasons, Java decided to ban global variables.
What does it mean that a class or member is final?
A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass.
Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared. For example,
public final double c = 2.998;
It's also possible to make a static field final to get the effect of C++'s const statement or some uses of C's #define, e।g. public static final double c = 2.998;
What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the current class. It exists only to be overridden in subclasses. It has no body. For example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract.
What is the main difference between Java platform and other platforms?
The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms.
The Java platform has three elements:
Java programming language
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)
What is the Java Virtual Machine?
The Java Virtual Machine is a software that can be ported onto various hardware-based platforms.
What is the Java API?
The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.
What is the package?
The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
What is native code?
The native code is code that after you compile it, the compiled code runs on a specific hardware platform.
Explain the user defined Exceptions?
User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}
Is Java code slower than native code?
Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
Can main() method be overloaded?
Yes. the main() method is a special method for a program entry. You can overload main() method in any ways. But if you change the signature of the main method, the entry point for the program will be gone.
What is the serialization?
The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.
Explain the new Features of JDBC 2.0 Core API?
The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:
Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
Java applications can now use the ResultSet.updateXXX methods.
New data types - interfaces mapping the SQL3 data types
Custom mapping of user-defined types (UTDs)
Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.
How you can force the garbage collection?
Garbage collection automatic process and can't be forced.
Explain garbage collection?
Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.
Describe the principles of OOPS.
There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
Explain the Encapsulation principle.
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Explain the Inheritance principle.
Inheritance is the process by which one object acquires the properties of another object.
Explain the Polymorphism principle.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
Explain the different forms of Polymorphism.
From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:
Method overloading
Method overriding through inheritance
Method overriding through the Java interface
What are Access Specifiers available in Java?
ccess specifiers are keywords that determines the type of access to the member of a class. These are:
Public
Protected
Private
Defaults
Describe the wrapper classes in Java.
Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
Primitive Wrapper
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
double java.lang.Double
float java.lang.Float
int java.lang.Integer
long java.lang.Long
short java.lang.Short
void java.lang.Void
Question: Read the following program:
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}
What is the result?
A. The output is “Equal”
B. The output in “Not Equal”
C. An error at " if (x = y)" causes compilation to fall.
D. The program executes but no output is show on console.
Answer: C
Use the Externalizable interface when you need complete control over your Bean's serialization (for example, when writing and reading a specific file format).
No। Earlier order is maintained.
The superclass constructor runs before the subclass constructor. The subclass's version of the overridable method will be invoked before the subclass's constructor has been invoked. If the subclass's overridable method depends on the proper initialization of the subclass (through the subclass constructor), the method will most likely fail. Is that true?
Yes. It is true
Why are the interfaces more flexible than abstract classes?
--An interface-defined type can be implemented by any class in a class hierarchy and can be extended by another interface. In contrast, an abstract-class-defined type can be implemented only by classes that subclass the abstract class.
--An interface-defined type can be used well in polymorphism. The so-called interface type vs. implementation types.
--Abstract classes evolve more easily than interfaces. If you add a new concrete method to an abstract class, the hierarchy system is still working. If you add a method to an interface, the classes that rely on the interface will break when recompiled.
--Generally, use interfaces for flexibility; use abstract classes for ease of evolution (like expanding class functionality).
What are new language features in J2SE 5.0?
Generally:
1. generics
2. static imports
3. annotations
4. typesafe enums
5. enhanced for loop
6. autoboxing/unboxing
7. varargs
8. covariant return types
What is covariant return type?
A covariant return type lets you override a superclass method with a return type that subtypes the superclass method's return type. So we can use covariant return types to minimize upcasting and downcasting.
class Parent {
Parent foo () {
System.out.println ("Parent foo() called");
return this;
}
}
class Child extends Parent {
Child foo () {
System.out.println ("Child foo() called");
return this;
}
}
class Covariant {
public static void main(String[] args) {
Child c = new Child();
Child c2 = c.foo(); // c2 is Child
Parent c3 = c.foo(); // c3 points to Child
}
}
What is the result of the following statement?
int i = 1, float f = 2.0f;
i += f; //ok, the cast done automatically by the compiler
i = i + f; //error
The compound assignment operators automatically include cast operations in their behaviors.
What is externalization? Where is it useful?
Use the Externalizable interface when you need complete control over your Bean's serialization (for example, when writing and reading a specific file format).
What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc);
}
}
A Error array not initialized
B 5
C null
D Print some junk characters
Answer : D
It will print some junk characters to the output. Here it will not give any compile time or runtime error because we have declared and initialized the array properly. Event if we are not assigning a value to the array, it will always initialized to its defaults.
What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc[0]);
}
}
A Error array not initialized
B 5
C 0
D Print some junk characters
Answer : C।
What is a marker interface ?
An interface that contains no methods. E.g.: Serializable, Cloneable, SingleThreadModel etc. It is used to just mark java classes that support certain capability.
What are tag interfaces?
Tag interface is an alternate name for marker interface.
What are the restrictions placed on static method ?
We cannot override static methods. We cannot access any object variables inside static method. Also the this reference also not available in static methods.
What is JVM?
JVM stands for Java Virtual Machine. It is the run time for java programs. All are java programs are running inside this JVM only. It converts java byte code to OS specific commands. In addition to governing the execution of an application's byte codes, the virtual machine handles related tasks such as managing the system's memory, providing security against malicious code, and managing multiple threads of program execution.
What is JIT?
JIT stands for Just In Time compiler. It compiles java byte code to native code.
What are ClassLoaders?
A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a "class file" of that name from a file system.
Every Class object contains a reference to the ClassLoader that defined it.
Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader.
Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes.
What is Service Locator pattern?
The Service Locator pattern locates J2EE (Java 2 Platform, Enterprise Edition) services for clients and thus abstracts the complexity of network operation and J2EE service lookup as EJB (Enterprise JavaBean) Interview Questions - Home and JMS (Java Message Service) component factories. The Service Locator hides the lookup process's implementation details and complexity from clients. To improve application performance, Service Locator caches service objects to eliminate unnecessary JNDI (Java Naming and Directory Interface) activity that occurs in a lookup operation.
What is Session Facade pattern?
Session facade is one design pattern that is often used while developing enterprise applications. It is implemented as a higher level component (i.e.: Session EJB), and it contains all the iteractions between low level components (i.e.: Entity EJB). It then provides a single interface for the functionality of an application or part of it, and it decouples lower level components simplifying the design. Think of a bank situation, where you have someone that would like to transfer money from one account to another. In this type of scenario, the client has to check that the user is authorized, get the status of the two accounts, check that there are enough money on the first one, and then call the transfer. The entire transfer has to be done in a single transaction otherwise is something goes south, the situation has to be restored.
As you can see, multiple server-side objects need to be accessed and possibly modified. Multiple fine-grained invocations of Entity (or even Session) Beans add the overhead of network calls, even multiple transaction. In other words, the risk is to have a solution that has a high network overhead, high coupling, poor reusability and mantainability.
The best solution is then to wrap all the calls inside a Session Bean, so the clients will have a single point to access (that is the session bean) that will take care of handling all the rest.
What is Data Access Object pattern?
The Data Access Object (or DAO) pattern: separates a data resource's client interface from its data access mechanisms adapts a specific data resource's access API to a generic client interface
The DAO pattern allows data access mechanisms to change independently of the code that uses the data.
The DAO implements the access mechanism required to work with the data source. The data source could be a persistent store like an RDBMS, an external service like a B2B exchange, a repository like an LDAP database, or a business service accessed via CORBA Internet Inter-ORB Protocol (IIOP) or low-level sockets. The business component that relies on the DAO uses the simpler interface exposed by the DAO for its clients. The DAO completely hides the data source implementation details from its clients. Because the interface exposed by the DAO to clients does not change when the underlying data source implementation changes, this pattern allows the DAO to adapt to different storage schemes without affecting its clients or business components. Essentially, the DAO acts as an adapter between the component and the data source.
Can we make an EJB singleton?
This is a debatable question, and for every answer we propose there can be contradictions. I propose 2 solutions of the same. Remember that EJB's are distributed components and can be deployed on different JVM's in a Distributed environment
i) Follow the steps as given below
Make sure that your serviceLocator is deployed on only one JVM.
In the serviceLocator create a HashTable/HashMap(You are the right judge to choose between these two)
When ever a request comes for an EJB to a serviceLocator, it first checks in the HashTable if an entry already exists in the table with key being the JNDI name of EJB. If key is present and value is not null, return the existing reference, else lookup the EJB in JNDI as we do normally and add an entry into the Hashtable before returning it to the client. This makes sure that you maintain a singleton of EJB.
ii) In distributed environment our components/Java Objects would be running on different JVM's। So the normal singleton code we write for maintaining single instance works fine for single JVM, but when the class could be loaded in multiple JVM's and Instantiated in multiple JVM's normal singleton code does not work. This is because the ClassLoaders being used in the different JVM's are different from each other and there is no defined mechanism to check and compare what is loaded in another JVM. A solution could be(Not tested yet. Need your feedback on this) to write our own ClassLoader and pass this classLoader as argument, whenever we are creating a new Instance and make sure that only one instance is created for the proposed class. This can be done easily.
How can we make a class Singleton ?
A) If the class is Serializable
class Singleton implements Serializable
{
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
If the singleton implements Serializable, then this
* method must be supplied.
*/
protected Object readResolve() {
return instance;
}
/**
This method avoids the object fro being cloned
*/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}
B) If the class is NOT Serializable
class Singleton
{
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
This method avoids the object from being cloned
**/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}
How is static Synchronization different form non-static synchronization?
When Synchronization is applied on a static Member or a static block, the lock is performed on the Class and not on the Object, while in the case of a Non-static block/member, lock is applied on the Object and not on class. [Trail 2: There is a class called Class in Java whose object is associated with the object(s) of your class. All the static members declared in your class will have reference in this class(Class). As long as your class exists in memory this object of Class is also present. Thats how even if you create multiple objects of your class only one Class object is present and all your objects are linked to this Class object. Even though one of your object is GCed after some time, this object of Class is not GCed untill all the objects associated with it are GCed.
This means that when ever you call a "static synchronized" block, JVM locks access to this Class object and not any of your objects. Your client can till access the non-static members of your objects.
What are class members and Instance members?
Any global members(Variables, methods etc.) which are static are called as Class level members and those which are non-static are called as Instance level members.
Name few Garbage collection algorithms?
Here they go:
Mark and Sweep
Reference counting
Tracing collectors
Copying collectors
Heap compaction
Mark-compact collectors
java follows a philosophy of automatic garbage collection, you can suggest or encourage the JVM to perform garbage collection but you can not force it. Once a variable is no longer referenced by anything it is available for garbage collection. You can suggest garbage collection with System.gc(), but this does not guarantee when it will happen. Local variables in methods go out of scope when the method exits. At this point the methods are eligible for garbage collection. Each time the method comes into scope the local variables are re-create
Does Java pass by Value or reference?
Its uses Reference while manipulating objects but pass by value when sending method arguments. Those who feel why I added this simple question in this section while claiming to be maintaining only strong and interesting questions, go ahead and answer following questions.
a)What is the out put of:
import java.util.*;
class TestCallByRefWithObject
{
ArrayList list = new ArrayList(5);
public void remove(int index){
list.remove(index);
}
public void add(Object obj){
list.add(obj);
}
public void display(){
System.out.println(list);
}
public static void main(String[] args)
{
TestCallByRefWithObject test = new TestCallByRefWithObject();
test.add("1");
test.add("2");
test.add("3");
test.add("4");
test.add("5");
test.remove(4);
test.display();
}
}
b) And now what is the output of:
import java.util.*;
class TestCallByRefWithInt
{
int i = 5;
public void decrement(int i){
i--;
}
public void increment(int i){
i++;
}
public void display(){
System.out.println("\nValue of i is : " +i);
}
public static void main(String[] args)
{
TestCallByRefWithInt test = new TestCallByRefWithInt();
test.increment(test.i);
test.display();
}
}
Why Thread is faster compare to process?
A thread is never faster than a process. If you run a thread(say there's a process which has spawned only one thread) in one JVM and a process in another and that both of them require same resources then both of them would take same time to execute. But, when a program/Application is thread based(remember here there will be multiple threads running for a single process) then definetly a thread based appliation/program is faster than a process based application. This is because, when ever a process requires or waits for a resource CPU takes it out of the critical section and allocates the mutex to another process.
Before deallocating the ealier one, it stores the context(till what state did it execute that process) in registers. Now if this deallocated process has to come back and execute as it has got the resource for which it was waiting, then it can't go into critical section directly. CPU asks that process to follow scheduling algorithm. So this process has to wait again for its turn. While in the case of thread based application, the application is still with CPU only that thread which requires some resource goes out, but its co threads(of same process/apllication) are still in the critical section. Hence it directly comes back to the CPU and does not wait outside. Hence an application which is thread based is faster than an application which is process based.
Be sure that its not the competion between thread and process, its between an application which is thread based or process based.
When and How is an object considered as Garbage by a GC?
An object is considered garbage when it can no longer be reached from any pointer in the running program. The most straightforward garbage collection algorithms simply iterate over every reachable object. Any objects left over are then considered garbage.
What are generations in Garbage Collection terminology? What is its relevance?
Garbage Collectors make assumptions about how our application runs. Most common assumption is that an object is most likely to die shortly after it was created: called infant mortality. This assumes that an object that has been around for a while, will likely stay around for a while. GC organizes objects into generations (young, tenured, and perm). This tells that if an object lives for more than certain period of time it is moved from one generation to another generations( say from young -> tenured -> permanent). Hence GC will be run more frequently at the young generations and rarely at permanent generations. This reduces the overhead on GC and gives faster response time.
What is a Throughput Collector?
The throughput collector is a generational collector similar to the default collector but with multiple threads used to do the minor collection. The major collections are essentially the same as with the default collector. By default on a host with N CPUs, the throughput collector uses N garbage collector threads in the collection. The number of garbage collector threads can be controlled with a command line option.
When to Use the Throughput Collector?
Use the throughput collector when you want to improve the performance of your application with larger numbers of processors. In the default collector garbage collection is done by one thread, and therefore garbage collection adds to the serial execution time of the application. The throughput collector uses multiple threads to execute a minor collection and so reduces the serial execution time of the application. A typical situation is one in which the application has a large number of threads allocating objects. In such an application it is often the case that a large young generation is needed
What is Aggressive Heap?
The -XX:+AggressiveHeap option inspects the machine resources (size of memory and number of processors) and attempts to set various parameters to be optimal for long-running, memory allocation-intensive jobs. It was originally intended for machines with large amounts of memory and a large number of CPUs, but in the J2SE platform, version 1.4.1 and later it has shown itself to be useful even on four processor machines. With this option the throughput collector (-XX:+UseParallelGC) is used along with adaptive sizing (-XX:+UseAdaptiveSizePolicy). The physical memory on the machines must be at least 256MB before Aggressive Heap can be used.
What is a Concurrent Low Pause Collector?
The concurrent low pause collector is a generational collector similar to the default collector. The tenured generation is collected concurrently with this collector. This collector attempts to reduce the pause times needed to collect the tenured generation. It uses a separate garbage collector thread to do parts of the major collection concurrently with the applications threads. The concurrent collector is enabled with the command line option -XX:+UseConcMarkSweepGC. For each major collection the concurrent collector will pause all the application threads for a brief period at the beginning of the collection and toward the middle of the collection. The second pause tends to be the longer of the two pauses and multiple threads are used to do the collection work during that pause. The remainder of the collection is done with a garbage collector thread that runs concurrently with the application. The minor collections are done in a manner similar to the default collector, and multiple threads can optionally be used to do the minor collection.
When to Use the Concurrent Low Pause Collector?
Use the concurrent low pause collector if your application would benefit from shorter garbage collector pauses and can afford to share processor resources with the garbage collector when the application is running. Typically applications which have a relatively large set of long-lived data (a large tenured generation), and run on machines with two or more processors tend to benefit from the use of this collector. However, this collector should be considered for any application with a low pause time requirement. Optimal results have been observed for interactive applications with tenured generations of a modest size on a single processor.
What is Incremental Low Pause Collector?
The incremental low pause collector is a generational collector similar to the default collector. The minor collections are done with the same young generation collector as the default collector. Do not use either -XX:+UseParallelGC or -XX:+UseParNewGC with this collector. The major collections are done incrementally on the tenured generation. This collector (also known as the train collector) collects portions of the tenured generation at each minor collection. The goal of the incremental collector is to avoid very long major collection pauses by doing portions of the major collection work at each minor collection. The incremental collector will sometimes find that a non-incremental major collection (as is done in the default collector) is required in order to avoid running out of memory.
When to Use the Incremental Low Pause Collector?
Use the incremental low pause collector when your application can afford to trade longer and more frequent young generation garbage collection pauses for shorter tenured generation pauses. A typical situation is one in which a larger tenured generation is required (lots of long-lived objects), a smaller young generation will suffice (most objects are short-lived and don't survive the young generation collection), and only a single processor is available.
How do you enable the concurrent garbage collector on Sun's JVM?
-Xconcgc options allows us to use concurrent garbage collector (1.2.2_07+)we can also use -XX:+UseConcMarkSweepGC which is available beginning with J2SE 1.4.1.
What is a platform?
A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000 and XP, Linux, Solaris, and MacOS.
What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.
How to make a class or a bean serializable?
By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.
What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
Name Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
What is the List interface?
The List interface provides support for ordered collections of objects.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
What is tunnelling?
Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data.
What is meant by "Abstract Interface"?
First, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.
Can Java code be compiled to machine dependent executable file?
Yes. There are many tools out there. If you did so, the generated exe file would be run in the specific platform, not cross-platform.
Do not use the String contatenation operator in lengthy loops or other places where performance could suffer. Is that true?
Yes.
What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
What is the List interface?
The List interface provides support for ordered collections of objects.
Is sizeof a keyword?
The sizeof operator is not a keyword in Java.
Which class is the superclass for every class.
Object.
Which Container method is used to cause a container to be laid out and redisplayed?
validate()
What's the difference between a queue and a stack?
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
What comes to mind when you hear about a young generation in Java?
Garbage collection.
You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.
What comes to mind when someone mentions a shallow copy in Java?
Object cloning.
If you're overriding the method equals() of an object, which other method you might also consider?
hashCode()
You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?
ArrayList
How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().
How can you minimize the need of garbage collection and make the memory use more effective?
Use object pooling and weak object references.
There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?
If these classes are threads I'd consider notify() or notifyAll(). For regular classes you can use the Observer interface.
What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
You do not need to specify any access level, and Java will use a default package access level.
What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
Difference between Swing and AWT?
AWT are heavy-weight components. Swings are light-weight components. Hence swing works faster than AWT.
What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected). It means that it is visible to all within a particular package.
What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
What is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class. Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a super class can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a no static method. In other words, you can't change a static method into an instance method in a subclass.
What is final?
A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).
What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.
What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
What if I write static public void instead of public static void?
Program compiles and runs properly.
What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
It is empty. But not null.
How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.
What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes
Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
What is the default value of an object reference declared as an instance variable?
Null unless we define it explicitly.
Can a top level class be private or protected?
No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.
What type of parameter passing does Java support?
In Java the arguments are always passed by value .
Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
How can I customize the serialization process? i.e. how can one have a control over the serialization process?
Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilized.
What is Externalizable interface?
Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
When you serialize an object, what happens to the object references included in the object?
The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
Does Java provide any construct to find out the size of an object?
No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc
Give a simplest way to find out the time a method takes for execution without using any profiling tool?
Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amount of processing.
Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these reasons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.
What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).
How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.
If I want an object of my class to be thrown as an exception object, what should I do?
The class should extend from Exception class. Or you can extend your class from some more precise exception type also.
If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
One can not do anything in this scenario. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.
What are the different ways to handle exceptions?
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.
What is the basic difference between the 2 approaches to exception handling.
1. try catch block and
2. specifying the candidate exceptions in the throws clause?
When should you use which approach?
In the first approach as a programmer of the method, you yourself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.
Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
If I write System.exit (0); at the end of the try block, will the finally block still execute?
No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.
How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection .
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
How are this() and super() used with constructors?
This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
What is daemon thread and which method is used to create the daemon thread?
Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
Can applets communicate with each other?
At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.
It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.
What are the steps in the JDBC connection?
While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.
Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
Is Empty .java file a valid source file?
Yes, an empty .java file is a perfectly valid source file.
Can a .java file contain more than one java classes?
Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
Is String a primitive data type in Java?
No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.
Is main a keyword in Java?
No, main is not a keyword in Java.
Is next a keyword in Java?
No, next is not a keyword.
Is delete a keyword in Java?
No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.
Is exit a keyword in Java?
No. To exit a program explicitly you use exit method in System object.
What happens if you don't initialize an instance variable of any of the primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a Boolean will be initialized to false.
What will be the initial value of an object reference which is defined as an instance variable?
The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerException everywhere you try to use such default initialized references.
What are the different scopes for Java variables?
The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessible only during the course of method execution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain about the local variable not being initialized..
How many objects are created in the following piece of code? MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
Can a public class MyClass be defined in a source file named YourClass.java?
No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
Can main method be declared final?
Yes, the main method can be declared final, in addition to being public static.
What will be the output of the following statement?
System.out.println ("1" + 3);
It will print 13.
What will be the default values of all the elements of an array defined as an instance variable?
If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of Boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.
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