java - 面板中的背景颜色不会改变

Question

下面的代码生成一个带有按钮的窗口，但是当我运行 i 并实际按下按钮时会弹出一条错误消息。根据 Spring 工具提示:

Cannot make a static reference to the non-static method setBackground(Color) from the type JComponent

据我所知，这个程序是从我的 Java 教科书逐行输入的。这是一本较旧的书，因此可能存在不兼容性，但似乎不太可能。

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

public class ButtonTest
{
    public static void main(String[] args)
    {
        final ButtonFrame frame = new ButtonFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.show();
}
}

class ButtonFrame extends JFrame
{
    public ButtonFrame()
    {
    setTitle("Button Test");
    setSize(Default_width, Default_height);

        //panel
        ButtonPanel panel = new ButtonPanel();
        Container contentPane=getContentPane();
        contentPane.add(panel);
    }

    public static final int Default_width = 300;
public static final int Default_height = 200;
}

class ButtonPanel extends JPanel
{
public ButtonPanel()
{
    JButton yellowButton = new JButton("Yellow");
    JButton blueButton = new JButton("Blue");
    JButton redButton = new JButton("Red");

    add(yellowButton);
    add(blueButton);
    add(redButton);

    ColorAction yellowAction= new ColorAction(Color.YELLOW);
    ColorAction redAction = new ColorAction(Color.RED);
    ColorAction blueAction = new ColorAction(Color.BLUE);

    yellowButton.addActionListener(yellowAction);
    blueButton.addActionListener(blueAction);
    redButton.addActionListener(redAction);
    }
}

    class ColorAction implements ActionListener
    {
        public ColorAction(Color c)
    {
        backgroundColor=c;
    }

    public void actionPerformed(ActionEvent event)
    {
    ButtonPanel.setBackground(backgroundColor);
    }

        private Color backgroundColor;
}

Answer 1

一种方法是 nest ColorAction 作为 ButtonPanel 中的内部类，它可以隐式访问封闭面板。

附录:正如@Andrew Thompson 和@nachokk 在评论中指出的那样，隐式可访问性可以通过使用封闭类名限定 this 来明确显示。参见 JLS §15.8.4. Qualified this了解详情。在此示例中，这两个调用是等效的:

 setBackground(backgroundColor);
 ButtonPanel.this.setBackground(backgroundColor);

作为更通用的替代方案，考虑将目标面板和颜色封装在 Action 中，如 here 所述.

class ButtonPanel extends JPanel {

    public ButtonPanel() {
        JButton yellowButton = new JButton("Yellow");
        JButton blueButton = new JButton("Blue");
        JButton redButton = new JButton("Red");

        add(yellowButton);
        add(blueButton);
        add(redButton);

        ColorAction yellowAction = new ColorAction(Color.YELLOW);
        ColorAction redAction = new ColorAction(Color.RED);
        ColorAction blueAction = new ColorAction(Color.BLUE);

        yellowButton.addActionListener(yellowAction);
        blueButton.addActionListener(blueAction);
        redButton.addActionListener(redAction);
    }

    private class ColorAction implements ActionListener {

        public ColorAction(Color c) {
            backgroundColor = c;
        }

         @Override
         public void actionPerformed(ActionEvent event) {
            setBackground(backgroundColor);
        }
        private Color backgroundColor;
    }
}