Question

I have a class:

public abstract class LogicGate extends JPanel implements PropertyChangeListener {

    private Image image;
    private URL url;
    private OutputTerminal output;
    private Terminal input0;
    private Terminal input1;

    public LogicGate(String fileName) {
        this.url = getClass().getResource(fileName);
        this.image = new javax.swing.ImageIcon(url).getImage();
        this.setSize(image.getWidth(null), image.getHeight(null));
        this.output = new OutputTerminal();
    }
}

and a subclass:

public class ANDGate extends LogicGate {

    private OutputTerminal output;
    private Terminal input0;
    private Terminal input1;

    public ANDGate() {
        super("images/AND.gif");
        System.out.println(this.output);
    }
}

Yet when I invoke a new ANDGate object, output is null, when it should have been assigned (as per the super constructor).

Now clearly I have a made an assumption in understanding subclassing constructors; what am I doing wrong?

Answer 1

This situation is called field hiding – the subclass field output is “hiding” the field of the same name in the super class.

You have defined

private OutputTerminal output;

in both your super class and your subclass. References to output in the subclass will be to its field, but you’re setting output in the super class – the subclass field will remain null.

To fix:

• delete the declaration of output in the subclass
• change the declaration of output in the super class to protected (so the subclass can access it)