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).
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