Question about oop & polymorphism

Question

Hi, I'm trying to understand the best way to modify variables in a superclass when using multiple subclasses. For a example I created a class named Rectangle with a height and width:

abstract public class Rectangle {

    private int height = 10;
    private int width = 20;

    public int getHeight(){
        return height;
    }

    public void setHeight(int x){
        height = x;
    }

    public int getWidth(){
        return width;
    }

    abstract void printResult();
}

and two classes: Area which calcs area and Circuit wich calcs the Circuit

public class Area extends Rectangle {

    public int calcArea(){
        int height = getHeight();
        int width = getWidth();
        return height * width;
    }

    public void printResult(){
        System.out.println("Area: " + calcArea());
    }
}

public class Circuit extends Rectangle {

    public int calcCircuit(){
        int height = getHeight();
        int width = getWidth();
        return height * 2 + width * 2;
    }

    public void printResult(){
        System.out.println("Circuit: " + calcCircuit());
    }
}

and then I use a class for the output

public class RectangleOutput {

    public static void main(String[] args) {
        Rectangle area = new Area();
        Rectangle circuit = new Circuit();
        area.printResult();
        circuit.printResult();
        area.setHeight(30); //modifying height
        area.printResult();
        circuit.printResult();
        /* Output:
         * Area: 200
         * Circuit: 60
         * Area: 600
         * Circuit: 60
         */

    }
}

My problem now is that I don't know the best way to modify the variables in the superclass. When changing the height with the area-object it won't work on the circuit-object. Could someone please help me or give some tips on the best oop practise? Thanks!

Answer 1 · 2017-02-22T01:40:29Z

February 22, 2017 1:40am

i am new to java but i think you need some adjustments here. area and circuit should be methods called on rectangle objects, not objects themselves. a subclass of rectangle inheriting from the rectangle class would be something like square - a rectangle with 4 equal sides. a square subclass would then inherit the rectangle methods area and circuit. you seem to be missing a setWidth method, and you are not using the "this" keyword in your setters. in your main, you would then create a new rectangle object, be able to set and get width and height, and thus calculate and return area and circuit, and any of those could be printed to the console.

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Björn Norén

Björn Norén

Question about oop & polymorphism

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james south

james south