背景色不会在面板上改变

Question

下面的代码生成一个带有按钮的窗口，但是当我运行i并按下该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教科书中逐行输入的。这是一本旧书，所以可能有不兼容之处，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

一种方法是将http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html ColorAction作为ButtonPanel中的一个内部类，在该类中，它可以隐式访问封闭面板。

增编:正如@Andrew和@nachokk的评论中所指出的，通过使用封闭类名限定this，可以使隐式可访问性变得显式化。详情请参见this。在这个例子中，这两个调用是等价的：

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

作为一种更普遍的选择，请考虑将目标面板和颜色封装到Action中，如概述的这里。

​

​

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;
    }
}

Answer 2

ButtonPanel.setBackground()不是一个静态方法，所以不能作为一个静态方法来调用它。您需要一个ButtonPanel的具体实例来设置背景。

ButtonPanel bp = new ButtonPanel();
bp.setBackground(backgroundColor);

Answer 3

另外，改变外观和感觉也会有帮助：

//UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());