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);
}
}
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