Java Abstract Class.

Java Abstract class

• An abstract class is a class where some or all methods do not have any code written for them, and such classes must be notified with the abstract modifier in front.
• An abstract class cannot be instantiated as an object.
• The methods with no implementation are called abstract methods and must be notified with the abstractt modifier in front.
• A sub-class which extends the abstract class must provide the implementations of all those methods. To not do so means that the sub-class also is abstract.
• A class that is not an abstract class is called a concrete class, and an abstract class can extends a such concrete class.
  Abstract class example:

  // This is an abstract class that cannot be instantiated
  abstract class Shape {
  
    abstract public double GetArea();
  
    abstract public double GetPerim();
  
    public void Draw() {
      System.out.println("Abstract drawing mechanism!");
    }
  }
  // Circle is a concrete class that implements the 
  // abstract methods in the abstract class shape
  
  class Circle extends Shape {
  
    private final int itsRadius;
  
    public Circle(int radius) {
      itsRadius = radius;
    }
  
    @Override
    public double GetArea() {
      return 3.14 * itsRadius * itsRadius;
    }
  
    @Override
    public double GetPerim() {
      return 2 * 3.14 * itsRadius;
    }
  
    @Override
    public void Draw() {
      System.out.println("Circle drawing routine here!");
      super.Draw();
    }
  }
  // Rectangle is a concrete class that implements the 
  // abstract methods in the abstract class shape
  
  class Rectangle extends Shape {
  
    private final int itsWidth;
    private final int itsLength;
  
    public Rectangle(int len, int width) {
      itsLength = len;
      itsWidth = width;
    }
  
    @Override
    public double GetArea() {
      return itsLength * itsWidth;
    }
  
    @Override
    public double GetPerim() {
      return 2 * itsLength + 2 * itsWidth;
    }
  
    public int GetLength() {
      return itsLength;
    }
  
    public int GetWidth() {
      return itsWidth;
    }
  
    @Override
    public void Draw() {
      for (int i = 0; i < itsLength; i++) {
        for (int j = 0; j < itsWidth; j++) {
          System.out.print("x ");
        }
        System.out.print("\n");
      }
      super.Draw();
    }
  }
  
  public class AbstractDemo {
  
    public static void main(String args[]) {
      int choice;
      boolean fQuit = false;
      Shape sp = null;
      while (fQuit == false) {
        System.out.print("(1)Circle (2)Rectangle (0)Quit: ");
        Scanner scan = new Scanner(System.in);
        choice = scan.nextInt();
        switch (choice) {
          case 1:
            sp = new Circle(4);
            break;
          case 2:
            sp = new Rectangle(4, 6);
            break;
          default:
            fQuit = true;
        }
        if (fQuit == false) {
          System.out.println("Area:"+ sp.GetArea()+" and perimeter:"+sp.GetPerim());
          sp.Draw();                
          System.out.println();
        }
      }
    }
  }

  The result of this is:

  (1)Circle (2)Rectangle (0)Quit: 1
  Area:50.24 and perimeter:25.12
  Circle drawing routine here!
  Abstract drawing mechanism!
  
  (1)Circle (2)Rectangle (0)Quit: 2
  Area:24.0 and perimeter:20.0
  x x x x x x 
  x x x x x x 
  x x x x x x 
  x x x x x x 
  Abstract drawing mechanism!
  
  (1)Circle (2)Rectangle (0)Quit: 0

  You can download this example here (needed tools can be found in the right menu on this page).