Area of an object- Abstract Class - Java

Question

I am learning Java using the book Java: The Complete Reference. Currently I am working on the topic of abstract classes.

Please Note: There are similar questions on stackoverflow. I searched them but I couldn't understand the concept.

If I run the below program, it produces the correct output, but I didn't understand the concept.

What is the need of reference variable of an Abstract class here. I can get the output without the reference variable of an abstract class.

First I ran the below program and got the desired output.

abstract class Figure {
  double dim1;
  double dim2; 

  Figure(double a, double b) {
    dim1 = a;
    dim2 = b;
  }

  // area is now an an abstract method 
  abstract double area();
}

class Rectangle extends Figure {
  Rectangle(double a, double b) {
    super(a, b);
  }

  // override area for rectangle
  double area() {
    System.out.println("Inside Area for Rectangle.");
    return dim1 * dim2;
  }
}

class Triangle extends Figure {
  Triangle(double a, double b) {
    super(a, b);
  }

  // override area for right triangle
  double area() {
    System.out.println("Inside Area for Triangle.");
    return dim1 * dim2 / 2;
  }
}

class AbstractAreas {
  public static void main(String args[]) {

    Rectangle r = new Rectangle(9, 5);
    Triangle t = new Triangle(10, 8);

    Figure figref; 

    figref = r;
    System.out.println("Area is " + figref.area());

    figref = t;
    System.out.println("Area is " + figref.area());
  }
}

And I tried the below code without creating/using abstract class reference.

class AbstractAreas {
  public static void main(String args[]) {

    Rectangle r = new Rectangle(9, 5);
    Triangle t = new Triangle(10, 8);

    // Figure figref; 

    // figref = r;
    System.out.println("Area is " + r.area());

    // figref = t;
    System.out.println("Area is " + t.area());
  }
}

It also gave the same output as the first program.

Can anyone please explain what is the need of calling "area method" using abstract class reference.

Answer 1

It's meant simply as a demonstration that even though you declared the variable as the abstract type, you can assign an instance of a concrete subclass to it and get the overriden behavior from the subclass.

Practical use example would be if you needed a collection of them:

List<Figure> figureList = new ArrayList<Figure>();
figureList.add(new Rectangle(9, 5));
figureList.add(new Triangle(10, 8));

for (Figure f : figureList) {
    System.out.println(f.area());
}

Or if you want to pass any subclass of Figure to a method that used the area():

public void printArea(Figure f) {
    System.out.println("Area is: " + f.area());
}
...
myObject.printArea(new Rectangle(9, 5));
myObject.printArea(new Triangle(10, 8));