JPanel中的Java代码绘画

Question

I just started learning Java and am now recently learning new things outside of the console. I copied a code from The Professor's board but i can't seem to get it to work. It is suppose to draw circles and rectangles for now but i just get a blank window when i try to paint. I think i may have missed a line of code or something. I am using Eclipse on Mac.

I understand there may be many ways to do this but i want to kind of keep the code exactly how he has it for now in addition with whatever fixes you think will make it work. Thanks alot.

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

public class Bunny {
    public static void main(String [] args)
    {
        MyFrame f = new MyFrame();
    f.setSize(500,400);
    f.setVisible(true);
    f.setLocation(50,100);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    }






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

public class MyFrame extends JFrame
    {
        public MyFrame()
        {   
        }
    }



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


public class MyPanel extends JPanel
{
public void paintComponent(Graphics g)
{Graphics2D g2 = (Graphics2D) g;


Ellipse2D.Double Circle = new Ellipse2D.Double(100,50,75,75);

g2.setColor(Color.RED);
g2.fill(Circle);

Rectangle box = new Rectangle(200,100,150,150);
g2.setColor(Color.RED);
g2.fill(box);

Color myColor = new Color(255,0,0);
}
}

Answer 1

I'd like to, very much, tell your professor that they need to go back to school and learn how to use Swing properly

You should start by taking a look at Painting in AWT and Swing and Performing Custom Painting for more details about how painting works in Swing

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Ellipse2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

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

    public Test() {
        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 TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g.create();
            Ellipse2D.Double Circle = new Ellipse2D.Double(100, 50, 75, 75);

            g2.setColor(Color.RED);
            g2.fill(Circle);

            Rectangle box = new Rectangle(200, 100, 150, 150);
            g2.setColor(Color.RED);
            g2.fill(box);
            g2.dispose();
        }

    }

}

Code Review...

public class Bunny {

    public static void main(String[] args) {
        MyFrame f = new MyFrame();
        f.setSize(500, 400);
        f.setVisible(true);
        f.setLocation(50, 100);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

Bad: Interacting with UI from outside the context of the Event Dispatching Thread. See Initial Threads for details

public class MyFrame extends JFrame {

    public MyFrame() {
    }
}

Bad: Extending from a top level container is (like JFrame ) is generally discourage, you're not adding any new features to the class and this is one of the areas where most problems occur. You might like to do some research into "composition over inheritance".

You also not actually adding anything to the frame, so it will appear blank.

public class MyPanel extends JPanel {

    public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g;

        Ellipse2D.Double Circle = new Ellipse2D.Double(100, 50, 75, 75);

        g2.setColor(Color.RED);
        g2.fill(Circle);

        Rectangle box = new Rectangle(200, 100, 150, 150);
        g2.setColor(Color.RED);
        g2.fill(box);

        Color myColor = new Color(255, 0, 0);
    }
}

Bad: paintComponent should never be public , there is no reason ever that any other class should be calling this method. Not calling super.paintComponent will cause no end of graphic glitches and issues, you should always call super.paintComponent first. The only times you wouldn't is when you know what you're doing and you have an extremely good reason not to...which is like <1% of the time.