如果要更改组件的绘制方式（添加矩形），则需要paintComponent()在该组件中重新定义。在你的代码中，你正在使用 getGraphics()。

你不应该调用getGraphics()组件。你所做的任何绘画（Graphics退还给你的绘画）都是暂时的，并且在Swing下次确定需要重新绘画组件时将丢失。

相反，你应该覆盖paintComponent(Graphics)（``JComponent或的JPanel）方法，并使用Graphics接收到的对象作为参数在此方法中进行绘制。

检查此链接以进一步阅读。

下面是你的代码，已更正。

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

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new JPanel() {
            @Override
            public void paintComponent(Graphics g) {
                super.paintComponent(g);
                g.setColor(Color.BLUE);
                g.fillRect(0, 0, 100, 100);
            }
        };
        frame.add(panel);

        // Graphics g = panel.getGraphics();
        // g.setColor(Color.BLUE);
        // g.fillRect(0, 0, 100, 100);

        frame.validate(); // because you added panel after setVisible was called
        frame.repaint(); // because you added panel after setVisible was called
    }
}

另一个版本执行完全相同的操作，但可能对你更清楚：

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

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(600, 400);

        JPanel panel = new MyRectangleJPanel(); // changed this line
        frame.add(panel);

        // Graphics g = panel.getGraphics();
        // g.setColor(Color.BLUE);
        // g.fillRect(0, 0, 100, 100);

        frame.validate(); // because you added panel after setVisible was called
        frame.repaint(); // because you added panel after setVisible was called
    }
}

/* A JPanel that overrides the paintComponent() method and draws a rectangle */
class MyRectangleJPanel extends JPanel {
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);
    }
}