Applet ball not moving

[R3N3G4D3]

So I wrote an applet Pong game, the paddles move fine, but the ball just stays in the center. I don't understand why, I even have a separate tread for it, and the ball's x and y should change every 100 ms, but they always stay constant (I checked by outputting coordinates to the screen, they stay at 150,100)

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;
import java.lang.*;




class Ball implements Runnable
{
	private int x=0, y=0, size=15, xdir=3, ydir=3, Ymax, Xmax;
	public int score=0;
	Paddle P1 = new Paddle();
	Paddle P2 = new Paddle();
	Thread T1 = new Thread();

	public Ball(int a, int b)
	{
		Xmax = a;
		Ymax = b;
		x = Xmax/2;
		y = Ymax/2;
		P1.setPaddle(5, (Ymax/2));
		P2.setPaddle((Xmax-5), (Ymax/2));
		T1.start();
	}

	public int getSize()
	{
		return size;
	}

	public int getX()
	{
		return x;
	}

	public int getY()
	{
		return y;
	}

	private void reset()
	{
		x=((Xmax-size)/2);
		y=((Ymax-size)/2);
	}

	public void end()
	{
		T1.stop();
	}

	public void run()
	{
		if((y <= 0) || ((y+size) >= Xmax)) //if ball hits top or bottom edge
		{
			ydir = -ydir;
		}
		if((P1.hits(x, (y+size)) == true) || (P2.hits((x+size), (y+size)) == true)) //if paddle hits
		{
			xdir = -xdir;
		}
		if((x) <= 0) //if ball hits left edge
		{
			reset();
			score=1;
		}
		else if((x+size) >= Ymax) //if ball hits right edge
		{
			reset();
			score=2;
		}
		x += xdir;
		y += ydir;
		try
		{
			T1.sleep(100);
		}
		catch(InterruptedException ie){}
	}
}

class Paddle
{
	private int x, y, xsize=3, ysize=25;

	public void setPaddle(int a, int b)
	{
		x=a;
		y=b;
	}

	public void move(int ydir)
	{
		y += ydir;
	}

	public boolean hits(int ballX, int ballY)
	{
		int tEdge = y;
		int bEdge = y+ysize;
		int lEdge = ballX-x;
		int rEdge = ballX-x+xsize;
		//if((((ballX-x) <= 1) && (bEdge < ballY) && (tEdge > ballY)) || (((ballX-x+xsize) <= 1) && (bEdge < ballY) && (tEdge > ballY))))
		if(((lEdge <= 1) && (bEdge < ballY) && (tEdge > ballY)) || ((rEdge <= 1) && (bEdge < ballY) && (tEdge > ballY)))
		{
			return true;
		}
		return false;
	}

	public int getXsize()
	{
		return xsize;
	}

	public int getYsize()
	{
		return ysize;
	}

	public int getX()
	{
		return x;
	}

	public int getY()
	{
		return y;
	}
}

class Score
{
	private int a=0, b=0;

	public int getA()
	{
		return a;
	}

	public int getB()
	{
		return b;
	}

	public void incA()
	{
		a++;
	}

	public void incB()
	{
		b++;
	}
}

public class Screen extends Applet implements KeyListener, Runnable
{
	Thread T0;
	Color ballC = Color.blue, paddleC = Color.red, C = Color.white, scoreC = Color.black;
	int Ymax = 200, Xmax = 300;
	int scoreX = Xmax/2, scoreY = 10;
	Ball ball = new Ball(Xmax, Ymax);
	Score score = new Score();

	public void stop()
	{
		if(T0!=null)
		{
			T0=null;
		}
	}

	public void start()
	{
		if(T0==null)
		{
			T0=new Thread(this);
			T0.start();
		}
	}

	public void run()
	{
		try
		{
			T0.sleep(100);
		}
		catch(InterruptedException ie){}
		if(ball.score == 1)
		{
			score.incA();
			ball.score = 0;
		}
		else if(ball.score == 2)
		{
			score.incB();
			ball.score = 0;
		}
		repaint();
	}

	public void keyPressed( KeyEvent e )
	{
		int key = e.getKeyCode();
		if (key == KeyEvent.VK_UP)
		{
			ball.P2.move(-4);
		}
		else if (key == KeyEvent.VK_DOWN)
		{
			ball.P2.move(4);
		}
		else if (key == KeyEvent.VK_Q)
		{
			ball.P1.move(-4);
		}
		else if (key == KeyEvent.VK_A)
		{
			ball.P1.move(4);
		}
		else if (key == KeyEvent.VK_0)
		{
			ball.end();
			T0.stop();
		}
		repaint();
	}

	public void keyReleased( KeyEvent e ) {}
	public void keyTyped( KeyEvent e ) {}
	
	public void init()
	{
		resize(Xmax, Ymax);
		setBackground(C);
		addKeyListener(this);
	}

	public void paint(Graphics g)
	{
		g.setColor(C);
		g.fillRect(0, 0, Xmax, Ymax);
		g.setColor(paddleC);
		g.fillRect(ball.P1.getX(), ball.P1.getY(), ball.P1.getXsize(), ball.P1.getYsize());
		g.setColor(paddleC);
		g.fillRect(ball.P2.getX(), ball.P2.getY(), ball.P2.getXsize(), ball.P2.getYsize());
		g.setColor(ballC);
		g.fillOval(ball.getX(), ball.getY(), ball.getSize(), ball.getSize());
		g.setColor(scoreC);
		g.drawString( ("Player 1: "+score.getA()+"  Player 2: "+score.getB()), scoreX, scoreY);
	}

	public int Ymax()
	{
		return Ymax;
	}

	public int Xmax()
	{
		return Xmax;
	}
}