paintComponent method not being called by repaint

Question

I'm trying to call my paintComponent method by using repaint but it is never being called. This is my first class:

public class start
{
    public static void main(String[] args){
        Frame f = new Frame();
        f.createFrame();
    }
}  

And this is the class that I want the paintComponent method to be called to but all that happens is a blank frame appears:

import javax.swing.JButton;
import javax.swing.JComponent;
import java.awt.Graphics;
import javax.swing.JFrame;
import java.awt.image.*;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import java.awt.event.*;
import java.awt.*;
import javax.swing.*;
import javax.swing.Timer;

public class Frame implements Runnable,ActionListener
{
JFrame window = new JFrame("Frame");
int i = 0;
Canvas myCanvas = new Canvas();
public void createFrame(){
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setBounds(30, 30, 700, 500);
    window.setFocusable(true);
    window.setFocusTraversalKeysEnabled(false);
    window.setVisible(true);
    (new Thread(new Frame())).start();
}

public void run(){
    Timer timer = new Timer (17,this);
    timer.start();
}

public void actionPerformed(ActionEvent e){
    myCanvas.updateGame();
    myCanvas.render();
    window.add(myCanvas);
}
}

class Canvas extends JPanel{
int x = 10;
int y = 10;
public void updateGame(){
    x++;
}

public void render(){
    repaint();
    System.out.println("repaint");
}

@Override
protected void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    g.drawString("hi",x,y);
    System.out.println("paint");
}

}

Repaint is called multiple times but paint is never called. Why isn't the paintComponent method being called by repaint?

Answer 1

Instead of creating a new frame, pass in the existing frame, I've commented the line below:

public void createFrame(){
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    window.setBounds(30, 30, 700, 500);
    window.setFocusable(true);
    window.setFocusTraversalKeysEnabled(false);
    window.setVisible(true);
    (new Thread(new Frame())).start(); <--- instead of creating a new frame, pass in the existing frame
}

You should really consider the following improvements as well:

1) Don't name your class Frame, as it collides with Frame

2) Move the body of createFrame as well as the following three lines:

JFrame window = new JFrame("Frame");
int i = 0;
Canvas myCanvas = new Canvas();

Into a constructor and make these local variables like Canvas into member fields.

3) Remove the last line that creates the thread out of the constructor, and expose it as a method (e.g 'startAnimation') and call it after you've created your object.

EDIT:

Try this:

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

    public class Frame implements Runnable, ActionListener
    {
        final JFrame window;
        final Canvas myCanvas;

        public Frame(){
            this.window = new JFrame("Frame");
            this.myCanvas = new Canvas();
            this.window.add(this.myCanvas);
            this.window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            this.window.setBounds(30, 30, 700, 500);
            this.window.setFocusable(true);
            this.window.setFocusTraversalKeysEnabled(false);
            this.window.setVisible(true);
        }

        public void startApp() {
            final Thread t = new Thread(this);
            t.start();
        }

        public void run(){
            final Timer timer = new Timer (1000,this);
            timer.start();
        }

        public void actionPerformed(ActionEvent e){
            myCanvas.updateGame();
            window.repaint();
        }

        public static void main(String[] args) {
            final Frame f = new Frame();
            f.startApp();
        }
    }

    class Canvas extends JPanel {
        int x = 10;
        int y = 10;
        public void updateGame() {
            x++;
        }

        @Override
        protected void paintComponent(Graphics g){
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g.drawString("hi", this.x, this.y);
            System.out.println("paint");
        }
    }