为什么即使单击按钮，JPanel 的颜色也不会改变？

Question

我在做 Head First Java 时偶然发现了这个问题，但我不知道如何解决它。

我想在单击按钮时更改 JPanel 小部件的颜色我正在使用 Mac OS。

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

public class SimpleGui implements ActionListener {
    JFrame frame;
    JButton button;

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

    public void go() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        button = new JButton("changes colour");
        button.addActionListener(this);

        MyPanel drawPanel = new MyPanel();

        frame.getContentPane().add(BorderLayout.SOUTH,button);
        frame.getContentPane().add(BorderLayout.CENTER,drawPanel);

        frame.setSize(300, 300);
        frame.setVisible(true);
   } //close go


    public void actionPerformed(ActionEvent event) {
        frame.repaint();
        button.setText("color changed");
    }
} // close actionPerformed

// the widget whose color i want to change 
class MyPanel extends JPanel {

    public void paintComponent(Graphics g) {
        g.setColor(Color.green); // i choose green as a color 
        g.fillRect(20, 50, 100, 100);
    } //close paintComponent
} //close MyPanel

Answer 1

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

public class SimpleGui implements ActionListener {
JFrame frame;
JButton button;

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

public void go() {
    frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    button = new JButton("changes colour");
    button.addActionListener(this);

    MyPanel drawPanel = new MyPanel();

    frame.getContentPane().add(BorderLayout.SOUTH,button);
    frame.getContentPane().add(BorderLayout.CENTER,drawPanel);

    frame.setSize(300, 300);
    frame.setVisible(true);
   } //close go


@Override
public void actionPerformed(ActionEvent event) {
    frame.repaint();
    button.setText("color changed");
}
} // close actionPerformed

// the widget whose color i want to change 
class MyPanel extends JPanel {
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    this.setBackground(Color.white);
    g.setColor(Color.green); // i choose green as a color 
    g.fillRect(20, 50, 100, 100);
} //close paintComponent
} //close MyPanel

paintCompenent 应该是 paintComponent

Answer 2

but the JFrame widow have green rectangle even before i click on the button.

那是因为您已经在 paintComponent 方法中对绿色进行了硬编码。所以它永远是绿色的。

您的 class 应该有一个属性来设置矩形的颜色。类似于：

class MyPanel extends JPanel 
{
    private Color rectangleColor = getBackground();

    @Override
    public void paintComponent(Graphics g) 
    {
        super.paintComponent(g);

        g.setColor( rectangleColor); // i choose green as a color 
        g.fillRect(20, 50, 100, 100);
    }

    public void setRectangleColor(Color rectangleColor)
    {
        this.rectangleColor = rectangleColor;
        repaint();
    }
}

然后在你使用的按钮的ActionListener中：

//frame.repaint();
drawPanel.setRectangleColor( Color.GREEN );
button.setText("color changed");

“drawPanel”变量也需要是您 class 中的一个实例变量。

现在有了这个设计，您可以拥有多个按钮。每个按钮都可以将矩形更改为不同的颜色。

为什么即使单击按钮，JPanel 的颜色也不会改变？

Why the color of JPanel not changing even when the button is clicked?

java

swing

jpanel