Thread: Why and where abstract methods & classes and static methods are used?

By declaring an abstract method in an abstract class and then inheriting the abstract method, you are able to create many different implementations of one method and invoke that method in your code without having to know what subclass you are invoking it on. Alternatively, you can use a non-abstract method to call an abstract method, which when you implement it will have a proper algorithm.

For example, look at the code below (will not compile as is since classes were not created properly, but the rest of it would work):

public abstract class A {
	public int getValue() {
		return calculate(5);
	}
	public abstract int calculate(int n);
}
 
public class B extends A{
	@Override
	public int calculate(int n) {
		return n+5;
	}
}
 
public class C extends A{
	@Override
	public int calculate(int n) {
		return n+10;
	}
}
 
public class Runner {
	public static void main(String args[]) {
		A obj1 = new B();
		A obj2 = new C();
 
		//Call obj1's implementation
		System.out.println(obj1.calculate(7)+" equals 12");
		System.out.println(obj1.getValue()+" equals 10");
		//Call obj2's implementation
		System.out.println(obj2.calculate(7)+" equals 17");
		System.out.println(obj2.getValue()+" equals 15");
 
		//Add both objects to an array
		A[] objects = new A[2];
		objects[0] = obj1;
		objects[1] = obj2;
 
		//Print the getValue() method of each object in the array
		for(int i=0;i<objects.length;i++) {
			System.out.println(objects[i].getValue());
		}
	}
}

That code will produce the output:

12 equals 12
10 equals 10
17 equals 17
15 equals 15
10
15

I'll let you read over that. If you need anything clarified, feel free to ask.

I'd start by defining each.

Please read the forum rules. I have merged your duplicate thread. You've got two people replying to your same question in different locations - not very considerate of the volunteers time.

And post #4 applies not just to double posts in a single forum, but to cross posts as well.

This thread has been cross posted here:

http://www.java-forums.org/advanced-java/61365-why-where-abstract-methods-classes-static-methods-used.html

Although cross posting is allowed, for everyone's benefit, please read:

Java Programming Forums Cross Posting Rules

The Problems With Cross Posting

an abstract method have no body while the body of the abstract method is in some other class.
abstract class provide default functionality for the subclass.

I provide an example of why and where for abstraction here.