java - 为什么点击按钮时会出现相同的按钮？

Question

当我运行此代码时，“颜色”按钮将出现在左上角。当我拖动窗口时，该按钮消失了。该代码基本上可以工作，但我不知道会发生这种情况。每次单击该按钮时都会出现该按钮。

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

public class TrafficLightPanel extends JPanel implements ActionListener{
private JLabel changeColor;
private Color color;

public TrafficLightPanel(){
    setSize(new Dimension(300,200));    
    setPreferredSize (new Dimension(300, 200));
    color = color.RED;

}
public void paintComponent(Graphics g){
    g.setColor(color);
    g.fillOval(100,100,100,100);
}
public void setColor (Color shade)
{
    color = shade;
}
public void  createButton(){}
public void actionPerformed(ActionEvent e) {
    // TODO Auto-generated method stub

}
public static void main(String[] args){
    JFrame frame = new JFrame("Color Circle");
    JButton[] buttons = new JButton[3];
    String[] colors = new String[]{"RED", "GREEN", "BLUE"};
    TrafficLightPanel panel = new TrafficLightPanel()
    {
        @Override
        public void  createButton(){
            for(int i = 0; i < 3; i++)
            {
                // make new button name 

                buttons[i] = new JButton("" + colors[i]);
                if(i == 0)
                    buttons[i].addActionListener(this);
                else if(i == 1)
                    buttons[i].addActionListener(this);
                else if(i == 2)
                    buttons[i].addActionListener(this);

                add(buttons[i]);
                //System.out.println(buttons[i]);
            }
        }
        @Override
        public void actionPerformed(ActionEvent e) {
               if (e.getSource() == buttons[0]) {
                   setColor(Color.RED);
                    repaint();
               } else if (e.getSource() == buttons[1]) {
                   setColor(Color.GREEN);
                    repaint();
               }
               else if (e.getSource() == buttons[2]) {
                   setColor(Color.BLUE);
                    repaint();
                   }
            }

            };
            panel.createButton();   
    frame.getContentPane().add(panel);
    frame.pack();
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


}

}

Answer 1

您的paintComponent 方法应该首先调用super 的方法，以便JPanel 执行其内务图形操作，其中包括删除所谓的“脏”像素。所以:

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);  // *** add this ***
    g.setColor(color);
    g.fillOval(100, 100, 100, 100);
}

就我个人而言，我会做一些不同的事情，并提出一些您可以考虑或不考虑的建议:

• 创建一个仅用于绘图的 JPanel，没有其他任何内容，没有按钮，什么都没有。
• 将 JButton 放置在不同的 JPanel 中。
• 使用枚举，可能称为 LightColor，用于将 Color 与字符串组合，并在创建按钮时使用。
• 在paintComponent方法中，将Graphics对象转换为Graphics2D对象，以便我们可以使用RenderingHints来绘制更平滑的圆形。
• 避免设置任何尺寸。相反，在需要时重写 setPreferredSize，并在创建 JFrame 之后但在显示之前调用 JFrame 上的 pack() 来让应用程序的组件和布局管理器自行调整大小。

例如:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;

import javax.swing.*;

// main GUI JPanel that holds the light drawing JPanel as well as the buttons
@SuppressWarnings("serial")
public class TrafficLightPanel2 extends JPanel {
    private static final int LIGHT_SIZE = 200; // size of the circle
    private static final int GAP = 10; // border gap around the jpanel

    // the Jlight drawing JPanel that draws the circles
    private LightDrawingPanel lightDrawingPanel = new LightDrawingPanel(null, LIGHT_SIZE);

    public TrafficLightPanel2() {
        // JPanel to hold the buttons, 1 row, variable number of columns, gap between buttons
        JPanel buttonPanel = new JPanel(new GridLayout(1, 0, GAP, 0));
        for (LightColor lightColor : LightColor.values()) {
            // create each button within the loop, giving it an Action -- an ActionListener "on steroids"
            buttonPanel.add(new JButton(new LightColorAction(lightColor, lightDrawingPanel)));
        }

        setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
        setLayout(new BorderLayout(GAP, GAP));
        add(lightDrawingPanel, BorderLayout.CENTER);  // add the light drawer to the center
        add(buttonPanel, BorderLayout.PAGE_END);  // and the buttons to the bottom
    }

    private static void createAndShowGui() {
        TrafficLightPanel2 mainPanel = new TrafficLightPanel2();

        JFrame frame = new JFrame("Traffic Light");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();  // have layout managers do their thing
        frame.setLocationRelativeTo(null); // center
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

// just acts as a class that connects a Color with a String
enum LightColor {
    RED(Color.RED, "Red"), YELLOW(Color.YELLOW, "Yellow"), GREEN(Color.GREEN, "Green");

    private LightColor(Color color, String text) {
        this.color = color;
        this.text = text;
    }
    private Color color;
    private String text;

    public Color getColor() {
        return color;
    }

    public String getText() {
        return text;
    }

    @Override
    public String toString() {
        return text;
    }
}

// create a class that only draws the circle, and that's it
@SuppressWarnings("serial")
class LightDrawingPanel extends JPanel {
    private Color color;
    private int size;

    public LightDrawingPanel(Color color, int size) {
        this.color = color;
        this.size = size;
    }

    public void setColor(Color color) {
        this.color = color;
        repaint();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);  // again, the super must be called

        // if no color defined, get out of here
        if (color == null) {
            return;
        }

        g.setColor(color);

        // make for smooth rendering 
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        // center the circle
        int x = (getWidth() - size) / 2;
        int y = (getHeight() - size) / 2;
        g2.fillOval(x, y, size, size);
    }

    // make our JPanel at least as large as the circle
    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet()) {
            return super.getPreferredSize();
        }
        return new Dimension(size, size);
    }
}

// AbstractAction for our buttons
// like an ActionListener "on steroids"
@SuppressWarnings("serial")
class LightColorAction extends AbstractAction {
    private LightColor lightColor;
    private LightDrawingPanel lightDrawingPanel;

    public LightColorAction(LightColor lightColor, LightDrawingPanel lightDrawingPanel) {
        super(lightColor.getText());  // text for the button to show

        // initialize our fields
        this.lightColor = lightColor;
        this.lightDrawingPanel = lightDrawingPanel;

        // alt-key mnemonic derived from the text String
        int mnemonic = (int) lightColor.getText().charAt(0);
        putValue(MNEMONIC_KEY, mnemonic);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // set the color of the drawing panel
        lightDrawingPanel.setColor(lightColor.getColor());
    }
}