Java Generics ?

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JProfiler Links ?

One Link

Difference between volatile and transient keyword in Java ?

1) transient keyword is used along with instance variables to exclude them from serialization process.

if a field is transient its value will not be persisted.

volatile keyword can also be used in variables to indicate compiler and JVM that always read its value from main memory and follow happens-before relationship on visibility of volatile variable among multiple thread.

2) transient keyword can not be used along with static keyword but volatile can be used along with static.

3) transient variables are initialized with default value during de-serialization and there assignment or restoration of value has to be handled by application code.

What is the difference between JAR and WAR files?

JAR files (Java ARchive) allows aggregating many files into one, it is usually used to hold Java classes in a library. WAR files (Web Application aRchive) stores XML, java classes, and JavaServer pages for Web Application purposes.

Object Lifecycle in Java ?

One Link

What is Meta data?

Meta data is some kind of description that you can supply along with your data.

Why use annotation? OR benefits of annotation in java ?

Meta data is some kind of description that you can supply along with your data. In java its possible to do with annotations, XML, there are other methods.

In order to take advantage of meta data you should have something that will process this data and decide what happens once the meta data is recognized.

Annotations have a lot of advantages over XML, to name a few :

Static type checking - the compiler will check for you where the annotation (once defined properly) is applicable and how

Clean code - its much easier to see (visually) the meta data defined in annotations

However it comes at a price:

XML doesn't require recompilation when you want to change something. With annotation you'll have to recompile.

What is annotation?

Annotations provide data about a class that is not part of the programming logic itself. They have no direct effect on the code they annotate.

Annotations == Add notes to (a text or diagram) giving explanation or comment

Sorting Techniques in java (Bubble Sort,Quick Sort)?

Reference:-                   One Link                   Two Link Quick Sort:-                    Three Link

What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

• final Classes- A final class cannot have subclasses.

• final Variables- A final variable cannot be changed once it is initialized.

• final Methods- A final method cannot be overridden by subclasses.

What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

• Public- public classes, methods, and fields can be accessed from everywhere.

• Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.

• Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.

• Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

• Situation	public	protected	default	private
  Accessible to class   from same package?	yes	yes	yes	no
  Accessible to class   from different package?	yes	no, unless it is a subclass	no	no

What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.	Instance methods are not declared as static.

creating an  instance of the class.

How are this() and super() used with constructors?

Constructors use this to refer to another constructor in the same class with a different parameter list. Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

What are the differences between Contructors and Methods?

Constructors Methods Purpose Create an instance of a class Group Java statements Modifiers Cannot be abstract, final, native, static, or synchronized Can be abstract, final, native, static, or synchronized Return Type No return type, not even void void or a valid return type Name Same name as the class (first letter is capitalized by convention) -- usually a noun Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action this Refers to another constructor in the same class. If used, it must be the first line of the constructor Refers to an instance of the owning class. Cannot be used by static methods. super Calls the constructor of the parent class. If used, must be the first line of the constructor Calls an overridden method in the parent class Inheritance Constructors are not inherited Methods are inherited

Can constructor be inherited?

No, constructor cannot be inherited, though a derived class can call the base class constructor.

How does the Java default constructor be provided?

If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

What is Constructor?

A constructor is a special method whose task is to initialize the object of its class. It is special because its name is the same as the class name. They do not have return types, not even void and therefore they cannot return values. They cannot be inherited, though a derived class can call the base class constructor. Constructor is invoked whenever an object of its associated class is created.

Can there be an abstract class with no abstract methods in it?

Yes, there can be an abstract class without abstract methods.

When you declare a method as abstract, can other nonabstract methods access it?

Yes, other nonabstract methods can access a method that you declare as abstract.

When should I use abstract classes and when should I use interfaces?

Use Interfaces when.

• You see that something in your design will change frequently.

• If various implementations only share method signatures then it is better to use Interfaces.

• you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when.

• If various implementations are of the same kind and use common behavior or status then abstract class is better to use.

• When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.

• Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

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What are the differences between Interface and Abstract class?

Abstract Class	Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.	An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.	A Class may implement several interfaces.
An abstract class can have non-abstract methods.	All methods of an Interface are abstract.
An abstract class can have instance variables.	An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.	An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .	An Interface cannot contain constructors .
Abstract classes are fast.	Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

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Can we instantiate an abstract class?

An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

What is an abstract class?

Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Note:

• If even a single method is abstract, the whole class must be declared abstract.
• Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
• You can’t mark a class as both abstract and final.

What is a marker interface?

Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface andCloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

Do interfaces have member variables?

Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

Can we create an object for an interface?

Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

Can we instantiate an interface?

You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

What is an Interface?

An interface is a description of a set of methods that conforming implementing classes must have.Note:

• You can’t mark an interface as final.
• Interface variables must be static.
• An Interface cannot extend anything but another interfaces.

How do you prevent a method from being overridden?

To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                       public final void exampleMethod() {

                         //  Method statements

                         }

What is super?

super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword. Note:

• You can only go back one level.

• In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

How to invoke a superclass version of an Overridden method?

To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method.

// From subclass

super.overriddenMethod();

Is it possible to override the main method?

NO, because main is a static method. A static method can't be overridden in Java.

Can overloaded methods be override too?

Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

What are the differences between method overloading and method overriding?

	Overloaded Method	Overridden Method
Arguments	Must change	Must not change
Return type	Can change	Can’t change except for covariant returns
Exceptions	Can change	Can reduce or eliminate. Must not throw new or broader checked exceptions
Access	Can change	Must not make more restrictive (can be less restrictive)
Invocation	Reference type determines which overloaded version is selected. Happens at compile time.	Object type determines which method is selected. Happens at runtime.