Java-MouseListener在另一个类中？

Question

I realize this is a repeat question, but my circumstances are a little bit different. I need to have a MouseListener in another class that can altar the background color of the object that calls it. Please help me.

public class LeftListPanel extends JPanel {

public LeftListPanel() {
    setBackground(Settings.BACKGROUND_COLOR);
    setLayout(null);

    addPersonalStatsTab();
}

private void addPersonalStatsTab() {
    JPanel personalStatsPanel = new JPanel();
    personalStatsPanel.addMouseListener(new CustomMouseListener());

    JLabel personalStatsText = new JLabel("Text");
    personalStatsPanel.add(personalStatsText);

    add(personalStatsPanel);
}

Then I have an inner-nested class for the MouseListener because this is the only place this MouseListener will be called.

class CustomMouseListener implements MouseListener {

    @Override
    public void mouseReleased(MouseEvent e) {

    }

    @Override
    public void mouseEntered(MouseEvent e) {
        setBackground(Settings.BACKGROUND_COLOR.brighter());
    }

    @Override
    public void mouseExited(MouseEvent e) {
        setBackground(Settings.BACKGROUND_COLOR);
    }

}

The setBackground(COLOR) lines are those who don't work... this.setBack and super.setBack ARE NOT working in this case.. I'M DESPERATE FOR HELP!

Answer 1

The reason you don't see the background changes is that when you call setBackground , you are de-referring (implicitly) the this object, ie the instance of LeftListPanel . So, you are actually changing its background, but you don't see it because inside the LeftListPanel instance there is another JPanel (instantiated at the addPersonalStatsTab method) which occupies the whole visible space (or even it is not visible at all, because of that weird null layout; I don't know exactly).

• Fist of all, I recommend to you not to set null as a layout. Chose a proper layout, or let it be defaulted - do not call setLayout(null) .

• Then, set personalStatsPanel as a private member of LeftListPanel . And when calling to setBackground , use it as the scope reference:

    LeftListPanel.this.personalStatsPanel.setBackground(...); 

Answer 2

This works, I instead just created a private method where I pass in the panel I want to apply it too.

private void CustomMouseListener(JPanel panel) {
    panel.addMouseListener(new MouseAdapter() {

        @Override
        public void mouseReleased(MouseEvent e) {

        }

        @Override
        public void mouseEntered(MouseEvent e) {
            panel.setBackground(Settings.BACKGROUND_COLOR.brighter());
        }

        @Override
        public void mouseExited(MouseEvent e) {
            panel.setBackground(Settings.BACKGROUND_COLOR);
        }

    });
}

Thank you all for your time and suggestions :)

Answer 3

You could...

Pass a reference of the component you want changed to the CustomMouseListener

class CustomMouseListener implements MouseListener {

    private JPanel panel;

    public CustomMouseListener(JPanel panel) {
        this.panel = panel;
    }

    @Override
    public void mouseReleased(MouseEvent e) {

    }

    @Override
    public void mouseEntered(MouseEvent e) {
        panel.setBackground(Settings.BACKGROUND_COLOR.brighter());
    }

    @Override
    public void mouseExited(MouseEvent e) {
        panel.setBackground(Settings.BACKGROUND_COLOR);
    }

}

This is okay if you want to use the listener on a limited number of components, but if you want to use the same listener on a number of components...

You could...

Use the source property of the MouseEvent to get which component triggered the event

@Override
public void mouseEntered(MouseEvent e) {
    if (!(e.getSource() instanceof JPanel)) {
        return;
    }
    JPanel panel = (JPanel)e.getSource();
    panel.setBackground(Settings.BACKGROUND_COLOR.brighter());
}

or, a better solution would be to do something more like...

@Override
public void mouseEntered(MouseEvent e) {
    e.getComponent().setBackground(Settings.BACKGROUND_COLOR.brighter());
}

since the information is already provided to you (just not, this returns an instance of Component , so if you need to access the Swing specific properties, you'd still need to cast it).

Why is this approach better?

CustomMouseListener listener = new CustomMouseListener();
panel1.addMouseListener(listener);
panel2.addMouseListener(listener);
panel3.addMouseListener(listener);
panel4.addMouseListener(listener);
panel5.addMouseListener(listener);
panel6.addMouseListener(listener);
panel7.addMouseListener(listener);

because it's agnostic, meaning you can create a single instance of the listener and re-use on multiple components