Thread: Experimenting with Graphics

I've been trying to wrap my head around how to detect and fix collision at the moment but as I'm not very familiar with drawing on a JPanel I managed to mess up somehow. At the moment my loop looks like this:

import java.awt.*;
import javax.swing.*;
 
class GameLoop implements Runnable //This is the Game Loop, it controls all rendering and that. Use the getters/setters to change sprite stuff.
{
 
	Thread thread = new Thread(this);
	Graphics g;
	private int x = 10, y = 10, a = 10, b = 10;
	private boolean hasCollided = false;
	Entity entity = new Entity(x, y, 20, 20);
	Entity entityTwo = new Entity(a, b, 20, 20);
	JPanel mainPanel;
 
	public GameLoop(JPanel mainPanel)
	{
		this.mainPanel = mainPanel;
		this.g = mainPanel.getGraphics();
	}
 
	public void startThread()
	{
		thread.start();
	}
 
	public void run()
	{
		while(true)
		{
			try
			{	
				if(entity.getBounds().intersects(entityTwo.getBounds()))
					hasCollided = true;
				else
					hasCollided = false;
 
				if( hasCollided == false)
				{
					g.setColor(Color.red);
					g.fillRect(x, x, 20, 20);
					g.setColor(Color.blue);
					g.fillRect(a, b, 20, 20);
				}
				else if( hasCollided == true )
				{
					g.setColor(Color.red);
					g.fillRect(x + 1, x + 1, 20, 20);
					entity.setX(x);
					entity.setY(y);
					g.setColor(Color.blue);
					g.fillRect(a, b, 20, 20);
				}
 
				mainPanel.repaint();
 
				Thread.sleep(30); //Anything lower than 30 bugs out a little bit. This will run at something like 30FPS or something.
			}
			catch(Exception e)
			{
				e.printStackTrace(); //Prints where the exception occurs and some other info about it if an exception happens.
			}
		}
	}
}

At line 8 I create the graphics object, then at line 18 I set it to the JPanel's graphics and then later on I use it to paint with. For some reason nothing is showing up on the JPanel.

I've removed the collision related code, here is the basic program:

Edit: Threw the program into one file, this should still work...

import javax.swing.*;
import java.awt.*;
 
class Rawr
{
	public static void main(String args[])
	{
		Window window = new Window();
	}
}
 
class Window extends JFrame
{
	private JFrame mainFrame;
	private JPanel mainPanel;
 
	public Window()
	{
		JFrame mainFrame = new JFrame();
		mainFrame.setSize(1024, 786);
		mainFrame.setLocationRelativeTo(null); //Centers the window on the users screen.
		mainFrame.setTitle("Temp Name");
		mainFrame.setDefaultCloseOperation(EXIT_ON_CLOSE); 
		mainFrame.setResizable(false);
		JPanel mainPanel = new JPanel(); //Creates a new JPanel to put everything on.
		mainFrame.getContentPane().add(mainPanel);
		mainPanel.setLayout(null); //Makes it so you can manually positon everything on the JPanel using XY coords
 
		mainFrame.setVisible(true);
		mainPanel.setVisible(true);
		GameLoop gameloop = new GameLoop(mainPanel);
	}
}
 
class GameLoop implements Runnable //This is the Game Loop, it controls all rendering and that. Use the getters/setters to change sprite stuff.
{
 
	Thread thread = new Thread(this);
	Graphics g;
	private int x = 10, y = 10;
	JPanel mainPanel;
 
	public GameLoop(JPanel mainPanel)
	{
		this.mainPanel = mainPanel;
		this.g = mainPanel.getGraphics();
	}
 
	public void startThread()
	{
		thread.start();
	}
 
	public void run()
	{
		while(true)
		{
			try
			{	
				g.setColor(Color.red);
				g.fillRect(x, x, 20, 20);
 
				mainPanel.repaint();
 
				Thread.sleep(30); //Anything lower than 30 bugs out a little bit. This will run at something like 30FPS or something.
			}
			catch(Exception e)
			{
				e.printStackTrace(); //Prints where the exception occurs and some other info about it if an exception happens.
			}
		}
	}
}

I'm going over it now, but it totally lost me when it came to this example:

import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BorderFactory;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
 
public class SwingPaintDemo2 {
 
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI(); 
            }
        });
    }
 
    private static void createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        SwingUtilities.isEventDispatchThread());
        JFrame f = new JFrame("Swing Paint Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new MyPanel());
        f.pack();
        f.setVisible(true);
    }
}
 
class MyPanel extends JPanel {
 
    public MyPanel() {
        setBorder(BorderFactory.createLineBorder(Color.black));
    }
 
    public Dimension getPreferredSize() {
        return new Dimension(250,200);
    }
 
    public void paintComponent(Graphics g) {
        super.paintComponent(g);       
 
        // Draw Text
        g.drawString("This is my custom Panel!",10,20);
    }  
}

Is f.add(new MyPanel()); starting the MyPanel constructor of the MyPanel class? How does the MyPanel class know what panel to paint to, I don't see any JPanels being created in either class.

This line:

f.add(new MyPanel());

Is the same as doing this:

MyPanel myPanel = new MyPanel();
f.add(myPanel);

It's just a little more compact is all.