java Interview Question

Commonly Asked Java Programming Interview Questions 

What is “this” keyword in java?
Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.
Usage of this keyword

• Used to refer current class instance variable.
• To invoke current class constructor.
• It can be passed as an argument in the method call.
• It can be passed as argument in the constructor call.
• Used to return the current class instance.
• Used to invoke current class method (implicitly)

What is an abstract class? How abstract classes are similar or different in Java from C++?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.

        Like C++, in Java, an instance of an abstract class cannot be created, we can have references of abstract class type though.

• Like C++, an abstract class can contain constructors in Java. And a constructor of abstract class is called when an instance of a inherited class is created
• In Java, we can have an abstract class without any abstract method. This allows us to create classes that cannot be instantiated, but can only be inherited.