Abstract Class Demo in Java.

AbstractClass.java

/*
 * Abstract Class Demo.
 *
 * - A class which has at least one abstract method is called as abstract class.
 * - Abstract class is a class which has no complete implementation.
 * - Abstract class cannot be instantiated.
 * - An abstract class can have abstract and concrete methods.
 *
 * Abstract method is a method which has no body or implementation.
 * Concrete method is a method which has the body (e.g. closing curly brackets).
 *
 * Note:
 *   If a class extends abstract class,
 *   it must provide an implementation for all abstract methods of its superclass,
 *   otherwise class it-self becomes subclass.
 *
 * How to run:
 *   javac AbstractClass.java
 *   java AbstractDemo
 */

abstract class A {
  abstract void getA(); // Abstract method which has no body.

  void display() { // Concrete method which has the body.
    System.out.println("From display()");
  }
}

class B extends A { // Here the class B is a concrete class.
  void getA() { // A method with an implementation.
    System.out.println("From getA()");
  }
  // display() is inherited from class A.
}

class AbstractDemo {
  public static void main(String[] args) {
    B b1 = new B();
    b1.getA();
    b1.display();

    // A a1 = new A(); // error: A is abstract; cannot be instantiated
  }
}