Java 使用 Graphics 在 JFrame 上显示形状

Question

I am learning how to program a graphical user interface in Java. I pretty much know some basics but in this program, I am trying to draw onto a JFrame with a black background, but as soon as I run the program the JFrame only displays a white line on a white background. I would appreciate it very much if anyone knew how to fix this, I have been trying myself but I can't seem to figure it out. Thanks for your attention. I'm looking forward to a reply.

public class test1 {
public static void main (String[] args)
{
    JFrame frame = new JFrame();
    frame.setSize(1835,1019);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.setResizable(false);
    frame.getContentPane().setBackground(Color.BLACK);

    
    JPanel raum = new JPanel()
    {
        public void paint(Graphics g)
        {
            Graphics2D g2 = (Graphics2D) g;
            g2.setPaint(Color.WHITE);
            
            g2.drawLine(500,500,500,800);
        }
        
    };
    frame.add(raum);
}

}

Answer 1

The raum Panel is the one occupying the frame, so the background of the frame is the same background color of the JPanel. So you have to paint the background before drawing the other shapes.

JPanel raum = new JPanel()
{
    public void paint(Graphics g)
    {
        Graphics2D g2 = (Graphics2D) g;
        g2.setColor(Color.BLACK);
        g2.fillRect(0, 0, this.getWidth(), this.getHeight());

        g2.setPaint(Color.WHITE);

        g2.drawLine(500,500,500,800);
    }

};

Answer 2

There a number of issues which are going to cause you endless amount of problems going into the future.

The obvious one is the fact that the background color of the panel is very close to WHITE , so it makes it very difficult to see the line. You could change the background color of the panel or the line and it should solve the immediate issue.

You really need to take a look at Performing Custom Painting and Painting in AWT and Swing to get a better understanding of how painting works in Swing.

It is generally recommended to override paintComponent and avoid overriding paint . paint does a lot work and unless you're willing to take over ALL it's workload, you're better off avoiding it.

As a general rule, you should also call the super.paintXxx method before you do any custom painting. Again, painting is generally a complex workflow, best to just let the parent class do its job.

A component should also provide sizing hints back to the parent container, the parent container can then make better decisions (via the LayoutManager ) as to how all the components should be laid out. Because different platforms (and even same platforms with different settings) can generate different size window decorations, you're better off managing the size of the "content" over the size of the "window". Again, this is going to save you no end of headaches into the future.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main {

    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            setBackground(Color.BLACK);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(1080, 1920);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setPaint(Color.WHITE);
            g2d.drawLine(500, 500, 500, 800);
            g2d.dispose();
        }

    }
}