java - 如何在单击按钮时重新绘制 JPanel？

Question

我是 swing 的新手，目前正在尝试制作一个绘图程序。这是我目前所拥有的。

这是主类:

import java.util.*;
import static java.lang.System.*;
import java.awt.*;
import javax.swing.*;
public class DrawingBoard {
    static JPanel buttonPanel;
    static JPanel drawingPanel;
    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.setSize(500, 500);
        window.setTitle("Drawing Board");
        window.getContentPane().setBackground(Color.black);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setLayout(new FlowLayout());

        buttonPanel = new JPanel();
        buttonPanel.setLayout(new BoxLayout(buttonPanel, BoxLayout.Y_AXIS));
        buttonPanel.setPreferredSize(new Dimension(80, 500));
        buttonPanel.setBackground(Color.lightGray);
        DrawingComponent drawingComp = new DrawingComponent();
        drawingComp.setButtonPanel();
        window.getContentPane().add(buttonPanel);

        drawingPanel = new JPanel();
        drawingPanel.setPreferredSize(new Dimension(420, 500));
        drawingPanel.setBackground(Color.white);
        drawingPanel.add(drawingComp);
        window.getContentPane().add(drawingPanel);

        window.pack();
        window.setVisible(true);
    }
}

这是绘图类:

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import static java.lang.System.*;
public class DrawingComponent extends JComponent implements ActionListener {
    private static final long serialVersionUID = 1L;
    public void paint(Graphics g) {
        super.paintComponents(g);
        g.setColor(Color.red);
        g.fillRect(50, 50, 150, 150);
    }
    public  void setButtonPanel() {
        JLabel colors = new JLabel("Colors", SwingConstants.CENTER);
        DrawingBoard.buttonPanel.add(colors);

        JButton red = new JButton("Red");
        DrawingBoard.buttonPanel.add(red);
        red.addActionListener(this);

        JButton blue = new JButton("Blue");
        DrawingBoard.buttonPanel.add(blue);
        blue.addActionListener(this);

        JButton green = new JButton("Green");
        DrawingBoard.buttonPanel.add(green);
        green.addActionListener(this);

        JButton yellow = new JButton("Yellow");
        DrawingBoard.buttonPanel.add(yellow);
        yellow.addActionListener(this);

        JButton orange = new JButton("Orange");
        DrawingBoard.buttonPanel.add(orange);
        orange.addActionListener(this);

        JButton pink = new JButton("Pink");
        DrawingBoard.buttonPanel.add(pink);
        pink.addActionListener(this);

        JButton purple = new JButton("Purple");
        DrawingBoard.buttonPanel.add(purple);
        purple.addActionListener(this);

        JButton black = new JButton("Black");
        DrawingBoard.buttonPanel.add(black);
        black.addActionListener(this);

        JLabel tools = new JLabel("Tools", SwingConstants.CENTER);
        DrawingBoard.buttonPanel.add(tools);

        JButton erase = new JButton("Erase");
        DrawingBoard.buttonPanel.add(erase);
        erase.addActionListener(this);

        JButton drawLine = new JButton("Lines");
        DrawingBoard.buttonPanel.add(drawLine);
        drawLine.addActionListener(this);

        JButton clear = new JButton("Clear");
        DrawingBoard.buttonPanel.add(clear);
        clear.addActionListener(this);
    }
    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand().equals("Red")) {
            out.println("red");
            repaint();
        }
        else if(e.getActionCommand().equals("Blue")) {
            out.println("blue");
            repaint();
        }
        else if(e.getActionCommand().equals("Green")) {
            out.println("green");
            repaint();
        }
        else if(e.getActionCommand().equals("Yellow")) {
            out.println("yellow");
            repaint();
        }
        else if(e.getActionCommand().equals("Orange")) {
            out.println("orange");
            repaint();
        }
        else if(e.getActionCommand().equals("Pink")) {
            out.println("pink");
            repaint();
        }
        else if(e.getActionCommand().equals("Purple")) {
            out.println("purple");
            repaint();
        }
        else if(e.getActionCommand().equals("Black")) {
            out.println("black");
            repaint();
        }
        else if(e.getActionCommand().equals("Erase")) {
            out.println("erase");
            repaint();
        }
        else if(e.getActionCommand().equals("Lines")) {
            out.println("lines");
            repaint();
        }
        else if(e.getActionCommand().equals("Clear")) {
            out.println("clear");
            repaint();
        }
    }
}

这是输出图片的链接。 /image/GEwga.png

现在，我只想让绘图板在单击按钮时显示某种图形。其他一切都很好，但是当我单击任何按钮时，绘图面板(白色区域)中没有像预期的那样显示任何图形。按钮的 println 语句仍会执行，但 repaint() 只是直接不起作用。我如何做到这一点，以便当我单击一个按钮时调用 paintComponent 方法并在 drawingPanel(窗口的白色区域)上显示一个图形？我已经在这个问题上停留了很长一段时间，所以非常感谢帮助。

Answer 1

将 drawingPanel = new JPanel(); 更改为 `drawingPanel = new JPanel(new BorderLayout());

DrawingComponent 的默认首选大小为 0x0，因此当您将它添加到 JPanel(使用 FlowLayout 默认情况下)，它的大小将变为 0x0，因此您不会看到任何绘制的内容

然后改变...

public void paint(Graphics g) {
    super.paintComponents(g);
    g.setColor(Color.red);
    g.fillRect(50, 50, 150, 150);
}

为了...

@Override
protected void paintComponent(Graphics g) {
    System.out.println("Hello");
    super.paintComponent(g); 
    g.setColor(Color.red);
    g.fillRect(10, 10, 150, 150);
}

关于你的代码还有很多其他的东西让我害怕，但我没有时间去解决所有的问题