Event handling in Java not working as expected

Question

I cannot get the code linked below to do exactly what I want it to do.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Gui {

    static JFrame frame;
    static JLabel label;

    public static void main (String[] args) {
        Gui gui = new Gui();
        gui.go();
    }

    public void go () {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400,300);
        frame.setVisible(true);

        MyDrawPanel panel = new MyDrawPanel();
        frame.getContentPane().add(BorderLayout.CENTER, panel);

        label = new JLabel("I'm a label");
        frame.getContentPane().add(BorderLayout.WEST, label);

        JButton colorButton = new JButton("Change Colors");
        ColorButtonListener colorButtonListener = new ColorButtonListener();
        colorButton.addActionListener(colorButtonListener); 
        frame.getContentPane().add(BorderLayout.SOUTH, colorButton);

        JButton labelButton = new JButton("Change Label");
        LabelButtonListener labelButtonListener = new LabelButtonListener();
        labelButton.addActionListener(labelButtonListener); 
        frame.getContentPane().add(BorderLayout.EAST, labelButton);
    }
}

class ColorButtonListener implements ActionListener {
    JFrame frame = Gui.frame;
    public void actionPerformed (ActionEvent event) {
        frame.repaint();
    }
}

class LabelButtonListener implements ActionListener {
    JLabel label = Gui.label;
    public void actionPerformed (ActionEvent event) {
        if (label.getText() == "That hurt") {
            label.setText("I'm a label");
        } else {
            label.setText("That hurt");
        }
    }
}

class MyDrawPanel extends JPanel {
    public void paintComponent (Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        int red = (int) (Math.random() * 256);
        int green = (int) (Math.random() * 256);
        int blue = (int) (Math.random() * 256);
        Color startColor = new Color(red, green, blue);

        red = (int) (Math.random() * 256);
        green = (int) (Math.random() * 256);
        blue = (int) (Math.random() * 256);
        Color endColor = new Color(red, green, blue);

        GradientPaint gradient = new GradientPaint(70, 70, startColor, 150, 150, endColor);
        g2d.setPaint(gradient);
        g2d.fillOval(0, 0, this.getWidth(), this.getHeight());

    }
}

There is a panel class used to draw a circle and then the panel is positioned in the center region of the frame.

A label is positioned in the west region of the frame, two buttons colorButton(positioned south) and labelButton(positioned east) should control the circle and the label respectively. 2 classes ColorButtonListener and LabelButtonListener implement the ActionListener interface and are registered with the colorButton and labelButton respectively. The color button when clicked should paint a circle with random colors and the label button when clicked should toggle between the text "I'm a label" and "That hurt".

Now, the issue I am having is with the label button. When clicking it, it not only changes the text(as expected), but also redraws the circle. This button should not be redrawing the circle. The color button works as expected.

Answer 1

This is because setText() methods internally calls repaint() if new text is not same as old text. So results in color change whenever you click that button too.

Answer 2

The problem is you don't control when a repaint might occur. Instead of changing the color every time paintComponent is called, which you can't control, change the color only when you want to and reference from within the paintComponent method, for example

class MyDrawPanel extends JPanel {
    private Color startColor;
    private Color endColor;

    // And setters or some other way
    // to randomise the colors

    public void paintComponent (Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        GradientPaint gradient = new GradientPaint(70, 70, startColor, 150, 150, endColor);
        g2d.setPaint(gradient);
        g2d.fillOval(0, 0, this.getWidth(), this.getHeight());

    }
}

Check out Painting in AWT and Swing for more details