JPanel未显示

Question

我直接从书上抄了这些例子。代码应该在JFrame上绘制一些东西，但没有显示任何东西（除了JFrame)，这里是带有main方法的类

import java.awt.BorderLayout;
import javax.swing.JFrame;

public class JavaApplication24 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
    JFrame frame = new JFrame("Window");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.setSize(480,270);

    frame.setVisible(true);
    NewClass panel = new NewClass();

    frame.add(BorderLayout.CENTER, panel);

    }

下面是JPanel的一个子类

import java.awt.Graphics;
import java.awt.Color;
import javax.swing.JPanel;


public class NewClass extends JPanel {

    @Override
    public void paintComponent(Graphics g){
        g.setColor(Color.BLUE);
        g.fillRect(0,0,100,100);

        g.setColor(Color.GREEN);
        g.drawRect(50,50,100,100);

        g.setColor(Color.RED);
        g.drawString("Hello",200,200);

        g.setColor(Color.RED);
        g.fillOval(240,40,100,30);

    }
}

Answer 1

您的newclass应该提供大小调整提示，布局管理器（在本例中为borderlayout)可以决定如何最好地布局组件。

在进行任何自定义绘制之前，还应该调用super.paintcomponent，否则将导致呈现工件的结束

public class NewClass extends JPanel {

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 400);
    }
    
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLUE);
        g.fillRect(0, 0, 100, 100);

        g.setColor(Color.GREEN);
        g.drawRect(50, 50, 100, 100);

        g.setColor(Color.RED);
        g.drawString("Hello", 200, 200);

        g.setColor(Color.RED);
        g.fillOval(240, 40, 100, 30);

    }
}

我直接从书上抄了例子

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                ex.printStackTrace();
            }
            
            JFrame frame = new JFrame("Testing");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(new NewClass());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    });
}

frame.setVisible(true);
NewClass panel = new NewClass();

frame.add(BorderLayout.CENTER, panel);

NewClass panel = new NewClass();
frame.add(BorderLayout.CENTER, panel);    
frame.setVisible(true);