Why is there no circle?

Code:

---

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;

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


public class Circle extends JFrame {
       
        public Circle(){
                super("Circle");
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                setSize(250,250);
                JPanel panel = new JPanel();
                add(panel);
                setVisible(true);
        }
       
        public void paintComponent(Graphics g){
                Graphics2D g2 = (Graphics2D)g;
                g2.setColor(Color.BLUE);
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
                g2.drawOval(25, 25, 200, 200);
                g2.dispose();
        }
       
        public static void main(String[] args){
                new Circle();
        }
       
}

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Because custom painting is done in an extended JComponent or JPanel, not in a top level window.

Adding a paintComponent method to a JFrame subclass doesn't do anything. The method is never invoked. Placing the method in an anonymous subclass of JPanel will show your circle.

Code:

---

public class Circle extends JFrame {
   
    public Circle(){
        super("Circle");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(250,250);
        JPanel panel = new JPanel() {

            public void paintComponent(Graphics g){
                Graphics2D g2 = (Graphics2D)g;
                g2.setColor(Color.BLUE);
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                        RenderingHints.VALUE_ANTIALIAS_ON);
                g2.drawOval(25, 25, 200, 200);
                g2.dispose();
            }
        };
        add(panel);
        setVisible(true);
    }
   
    public static void main(String[] args){
        SwingUtilities.invokeLater(new Runnable() {

            public void run() {
                new Circle();
            }
        });
    }
   
}

---

Also, all Swing components should be constructed and manipulated on the EDT: Lesson: Concurrency in Swing (The Java™ Tutorials > Creating a GUI With JFC/Swing)

db

Code:

---

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;

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


public class Circle extends JFrame {
       
        public Circle(){
                super("Circle");
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                setSize(250,250);
                CirclePane circle = new CirclePane();
                add(circle);
                setVisible(true);
        }
       
        public static void main(String[] args){
                new Circle();
        }
}       

class CirclePane extends JPanel {
        public void paintComponent(Graphics g){
                Graphics2D g2 = (Graphics2D)g;
                g2.setColor(Color.BLUE);
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
                Ellipse2D.Float circle = new Ellipse2D.Float(25, 10, 200, 200);
                g2.draw(circle);
                g2.dispose();
        }
}

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There are some aspects of your code that I don't understand just yet and there's only one more chapter left on swing in my textbook and it's about Java web start so I'm not sure if what you've done will be covered. It's the main method that is confusing me. I don't know anything about the SwingUtilities class.

Thanks for the link, I'll give it a read and see if that helps.