如何使用 actionListener 更改 JButton 的颜色

Question

I have the following JFrame. enter image description here

It contains a JButton[100] of square and circle JButtons. I want to change their color when I click on them. So I use actionListener for that. But when I write the actionListener to change the color it gives me an exception

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: Index 100 out of bounds for length 100

and it says that the exception is at the b[i].setBackground(Color.blue); inside the actionListener. I've searched a lot and nothing is changing the color but just gives the exception. Here is my code

Subject.java

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.*;

public class Subject extends JFrame{
    public Subject() {
        super("Subject");
        p = new JPanel(new GridLayout(10,10));
        b = new JButton[100];
        Random r = new Random();
        for(i=0;i<100;i++) {
            y = r.nextInt(2) +1;
            if(y==1) {
                b[i] = new JButton();
                b[i].setBackground(Color.black);
                b[i].setPreferredSize(new Dimension(35,35));
                p.add(b[i]);
            }else {
                b[i] = new Circle();
                b[i].setBackground(Color.black);
                p.add(b[i]);
            }
            b[i].addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    b[i].setBackground(Color.blue);
                }
            }); 
        }
        add(p);
        pack();
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLocation(100,200);
    }
    
    private JPanel p;
    private JButton[] b;
    private int i, y;
    
}

Circle.java

import java.awt.*;
import java.awt.geom.Ellipse2D;
import javax.swing.*;

public class Circle extends JButton{
    public Circle() {
        setBackground(Color.red);
        setFocusable(false);
     
        /*
         These statements enlarge the button so that it 
         becomes a circle rather than an oval.
        */
        Dimension size = getPreferredSize();
        size.width = size.height = Math.min(35,35);
        setPreferredSize(size);
     
        /*
         This call causes the JButton not to paint the background.
         This allows us to paint a round background.
        */
        setContentAreaFilled(false);
      }
     
      protected void paintComponent(Graphics g) {
        if (getModel().isArmed()) {
          g.setColor(Color.yellow);
        } else {
          g.setColor(getBackground());
        }
        g.fillOval(0, 0, getSize().width - 1, getSize().height - 1);
        JLabel l = new JLabel("Click Me");
        
        super.paintComponent(g);
      }
     
      protected void paintBorder(Graphics g) {
        g.setColor(Color.darkGray);
        g.drawOval(0, 0, getSize().width - 1, getSize().height - 1);
      }
     
      // Hit detection.
      Shape shape;
     
      public boolean contains(int x, int y) {
        // If the button has changed size,  make a new shape object.
        if (shape == null || !shape.getBounds().equals(getBounds())) {
          shape = new Ellipse2D.Float(0, 0, getWidth(), getHeight());
        }
        return shape.contains(x, y);
      }
}

I've also tried the following but nothing has changed.

b[i].addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    if (e.getSource()==b[i])
                    {
                        b[i].setBackground(Color.blue);
                    }
                }
            }); 

Does anyone have any advice on what to do to fix it? Thank you!

Answer 1

Simply change the following line in your actionPerformed() method...

b[i].setBackground(Color.blue);

to this...

((JButton) e.getSource()).setBackground(Color.blue);

You are creating a separate, anonymous class (that implements ActionListener interface) for each JButton . Hence the ActionEvent source can only be the JButton that you created the anonymous class for.

Note that after you compile your java code, you will have a separate class file for each ActionListener . The class file names will begin with Subject$ and end with .class , for example:

Subject$1.class
Subject$2.class
Subject$3.class

There will be at least 100 of these classes since you have created one for each JButton .