更改 JLabel 中图标的颜色

Question

I need to make a program that shows three buttons in a jframe along with an initially red circle. The three buttons must say "RED" "GREEN", and "BLUE", and when you click the buttons, the red circle should change to whatever color you clicked on.

At first I tried to actually alter the color of the icon, but I thought that it would be easier just to make three circles each a different color and add an action listener to each button which will the correct color circle to the frame while replacing the previous one whenever the user clicks a color. I am having trouble figuring how to do that. Should I make three separate classes for each circle? Or is there an easier way?

Another thing is, I must use JLabel so I can call the repaint() method at the end of each color change that's part of the project. I also need to add a static method in the main method which returns an action listener which I haven't figured out how to do yet.

Here's what I have so far:

/**
 * Write a description of class CircleIcon here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;

public class CircleIcon implements Icon
{
    // Instance variables - replace the example below with your own

    /**
     * Constructor for objects of class CircleIcon
     */
    private int size;

    public CircleIcon(int aSize)
    {
        // Initialise instance variables
        size = aSize;
    }

    public int getIconWidth() {
        return size;
    }

    public int getIconHeight() {
        return size;
    }

    public void paintIcon(Component c, Graphics g, int x, int y) {
        Graphics2D g2 = (Graphics2D) g;
        Ellipse2D.Double circle = new Ellipse2D.Double(x, y, size, size);
        g2.setColor(Color.RED);
        g2.fill(circle);
    }
}

CircleIconTester class:

/**
 * Write a description of class CircleIconTester here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.geom.*;
import java.awt.color.*;

public class CircleIconTester
{
    public static void main(String[] args) {
        JFrame frame = new JFrame();

        CircleIcon circle = new CircleIcon(50);
        JLabel label = new JLabel(circle);
        frame.add(label);

        JButton red = new JButton("RED");
        JButton blue = new JButton("BLUE");
        JButton green = new JButton("GREEN");
        frame.add(red);
        frame.add(blue);
        frame.add(green);

        ActionListener redAL = new ActionListener() {
            public void actionPerformed(ActionEvent event) {
            }
        };

        red.addActionListener(redAL);

        frame.setLayout(new FlowLayout());

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

 }

Answer 1

Changing the (foreground) color of a JLabel is a simple call of the setForeground(...) method on the JLabel object. But your icon implementation must get the properties of the component it is placed in. Luckily the paintIcon() method returns the parent component the icon is placed in. See the documentation of paintIcon() :

 void paintIcon(Component c, Graphics g, int x, int y)

Draw the icon at the specified location. Icon implementations may use the Component argument to get properties useful for painting, eg the foreground or background color.

The documentation even mentions that you can use it for getting the color. Inside your paintIcon() method you can use the getForeground() method to get the foreground color of the JLabel .

public void paintIcon(Component c, Graphics g, int x, int y){
    Graphics2D g2 = (Graphics2D) g;
    Ellipse2D.Double circle = new Ellipse2D.Double(x, y, size, size);
    g2.setColor(c.getForeground()); // <-- get foreground color from parent.
    g2.fill(circle);
}

Now you have to set the correct foreground color in your action listener. As you want to build an action listener with a static method you can do that. Create a new static method BuildActionListener which gets two arguments. One for the JLabel object to change and one for the foreground color to use. It returns an ActionListener object which changes the foreground color:

/**
 * Build an action listener to change the color of the label.
 *
 * @param label The label to change.
 * @param color The color to use.
 * @returns The action listener which changes the color.
 */
public static ActionListener BuildActionListener(JLabel label, Color color) {
    return new ActionListener(){
        public void actionPerformed(ActionEvent event){
            label.setForeground(color);
        }
    };
}

Use this helper method to assign custom action listeners for each button:

red.addActionListener(BuildActionListener(label, Color.RED));
blue.addActionListener(BuildActionListener(label, Color.BLUE));
green.addActionListener(BuildActionListener(label, Color.GREEN));

And to start with an red circle (and not with a black circle), set the foreground color of the label somewhere at the beginning:

JLabel label = new JLabel(circle);
label.setForeground(Color.RED);

Answer 2

Simply the code is:

label.setForeground(Color.red);

Answer 3

Try the following. It works:

red.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent ae) {
        label.setForeground(Color.red);
    }
});