Summary Learn Java

Q)Explain about a final class.
=>Once a class is made final, inheritance is prevented.I.e. sub classes can’t be created from a final class.

For eg.
final class A{
}
class B extends A{
}//ERROR.

=>Generally, the most specialized class in the hierarchy is made final.

=>To make a class immutable(non modifiable) also, that class can be made final.
For eg. String class.

abstract modifier
-------------------
=>"abstract" is a keyword in Java.
=>"abstract" keyword can be applied to classes and methods but not to variables.

Q)Explain about an abstract method.

=>If "abstract" modifier is applied to a method, it becomes an abstract method.

=>An abstract method can’t have body.I.e. an abstract method has declaration but
no definition.

For eg. abstract void move();

=>an abstract method in Java is similar to function prototype in C.

=>If a method is declared abstract, its class also must be declared abstract.I.e. a non abstract class can’t
contain abstract methods.

class Vehicle{
abstract void move();
}//ERROR.

=>In the above example, Vehicle class is not declared abstract.Therefore, compilation error comes.


Q)What is wrong with the following code?
abstract class Vehicle{
abstract void move(){
}
}

Ans:- An abstract method can’t have body.


Q)What is the purpose of abstract methods?
=>To enforce method overriding in the sub classes, abstract methods are declared in a super class.
=>Once a method is declared abstract in a class, all its sub classes must implement/override that method.


Q)What is wrong with the following code?