将JPanel添加到JLayeredPane会导致绘制和调整大小无效

Question

我正在尝试将一个JPanel(好吧,几个)添加到JLayeredPane.但是,当我这样做时,JPanel的paint组件方法似乎没有任何效果.下面是一个例子:

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

public class Example {

    public static void main(String[] args) {

        // This Works as expected
        JFrame usingPanel = new JFrame();
        JPanel p = new JPanel();
        p.add(new BluePanel());
        usingPanel.setContentPane(p);
        usingPanel.pack();
        usingPanel.setVisible(true);

        // This makes the frame but does not paint the BluePanel
        JFrame usingLayer = new JFrame();
        JLayeredPane l = new JLayeredPane();
        l.setPreferredSize(new Dimension(200,200));
        l.add(new BluePanel(), JLayeredPane.DEFAULT_LAYER);
        JPanel p2 = new JPanel();
        p2.add(l);
        usingLayer.setContentPane(p2);
        usingLayer.pack();
        usingLayer.setVisible(true);
    }


   static class BluePanel extends JPanel{

        public BluePanel(){
            setPreferredSize(new Dimension(200,200));
        }

        @Override
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            g.setColor(Color.BLUE);
            g.fillRect(0, 0, 200, 200);
        }


    }


}

为什么是这样？什么是可能的解决方案？

Answer 1

JLayeredPane没有LayoutManager,因此您需要自己设置面板的位置和大小.请参阅教程