Eclipse 上的 Java 不会显示 JPanel，即使我将它添加到 JFrame

Question

These are the files. I have set JFrame to be visible, and have added JPanel to it, but still, the code only shows the window without anything in it.

import java.util.List;
import java.util.ArrayList;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import java.util.Collections;


public static void main(String[] args)
{

    JFrame frame = new JFrame();
    frame.setSize(350, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setTitle("My Empty Window");
    frame.setVisible(true);

    DrawingPanel panel = new DrawingPanel();
    
    frame.add(panel);
    frame.setVisible(true);
}

-------------DRAWINGPANEL FILE-------------------

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

 public class DrawingPanel extends JPanel {

      public void painting(Graphics pen) {

         pen.drawRect(50, 50, 20, 20);
         pen.drawRect(100, 50, 40, 20);
         pen.drawOval(200,50,20,20);
         pen.drawOval(250, 50, 40, 20);
         pen.drawString("Square", 50, 90);
         pen.drawString("Rectangle", 100, 90);
         pen.drawString("Cirlce", 200, 90); 
         pen.drawString("Oval", 250, 90);
         pen.fillRect(50, 100, 20, 20);
         pen.fillRect(100, 100, 40, 20);
         pen.fillOval(250, 100, 20, 20);
         pen.fillOval(250, 100, 40, 20);
         pen.drawLine(50, 150, 300, 150);
         pen.drawArc(50, 150, 200, 100, 0, 180);
         pen.fillArc(100, 175, 200, 75, 90, 45);
     }
 }

Answer 1

Try changing the method in DrawingPanel from painting to paint , which will get called when run. paint is a method inherited from JPanel .

Edit: As mentioned by NomadMaker , use paintComponent() not paint() here. Read this for more information.

Answer 2

Here's what I get after making your code runnable, fixing your JFrame method calls and fixing your drawing JPanel .

Swing applications should always start with a call to the SwingUtilities invokeLater method. This method ensures that the Swing components are created and executed on the Event Dispatch Thread .

You pack a JFrame . You set the preferred size of your drawing JPanel . This way, you know how big your drawing JPanel is, without worrying about the JFrame decorations.

Here's the complete runnable code.

import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class DrawingPanelExample implements Runnable {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new DrawingPanelExample());
    }

    @Override
    public void run() {
        JFrame frame = new JFrame("My Empty Window");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        DrawingPanel panel = new DrawingPanel();
        frame.add(panel);

        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public class DrawingPanel extends JPanel {

        private static final long serialVersionUID = 1L;

        public DrawingPanel() {
            this.setPreferredSize(new Dimension(350, 300));
        }

        @Override
        protected void paintComponent(Graphics pen) {
            super.paintComponent(pen);

            pen.drawRect(50, 50, 20, 20);
            pen.drawRect(100, 50, 40, 20);
            pen.drawOval(200, 50, 20, 20);
            pen.drawOval(250, 50, 40, 20);
            pen.drawString("Square", 50, 90);
            pen.drawString("Rectangle", 100, 90);
            pen.drawString("Cirlce", 200, 90);
            pen.drawString("Oval", 250, 90);
            pen.fillRect(50, 100, 20, 20);
            pen.fillRect(100, 100, 40, 20);
            pen.fillOval(250, 100, 20, 20);
            pen.fillOval(250, 100, 40, 20);
            pen.drawLine(50, 150, 300, 150);
            pen.drawArc(50, 150, 200, 100, 0, 180);
            pen.fillArc(100, 175, 200, 75, 90, 45);
        }
    }

}

Answer 3

You should override paintComponent like so:

...

@Override
public void paintComponent(Graphics g)
{
    super.paintComponent(g);
    Graphics2D pen = (Graphics2D) g;
...
}

Also some suggestions:

• You can extending JComponent instead of JPanel (It should work both ways)
• You can use setSize or setPreferredSize for your panel to fit it with your frame size.
• You can only use setVisisble(true); only once after all of the configurations of your frame.
• And add it to the center of the frame like so:

...
    JFrame frame = new JFrame();
    frame.setSize(350, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setTitle("My Empty Window");

    DrawingPanel panel = new DrawingPanel();
    panel.setPreferredSize(new Dimensions(350, 300));
    
    frame.add(panel, BorderLayout.CENTER);
    frame.setVisible(true);
...

On a side note:

Adding a layout manager may not be necessary and you can also replace setPreferredSize with setBounds like so:

panel.setBounds(0, 0, 350, 300);

frame.add(panel);

Where 0s are x and y coordinates respectively.