Duvida

import java.io.IOException;

import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;
import javax.microedition.lcdui.game.GameCanvas;


class Tela extends GameCanvas implements Runnable {

	public Image nave, fundo, meteoro;
	Graphics g;
	Thread t;

	public Tela() {
		super(true);
		g = getGraphics();
		try {
			fundo = Image.createImage("/fundo.png");
			meteoro = Image.createImage("/meteoro.png");
			nave = Image.createImage("/nave.png");
		} catch (IOException e) {}
		
			t = new Thread(this);
			t.start();
	}

	public void run() {
		int xM = 0; //coordenada  X do meteoro 
		int yM = 0; //coordenada Y do meteoro
		
		int xNave = 100; //coordenada X da nave
		int yNave = 200; //coordenada Y da nave
		
		int velx = 5; // velocidade horizontal
		int vely = 2; // velocidade vertical
		
		int velNaveX = 3; 
		int velNaveY = 3;
		
		int altura = getHeight();
		int largura = getWidth();
		int tecla;
		
		while (true) {
			g.drawImage(fundo, 0, 0, Graphics.TOP | Graphics.LEFT);
			g.drawImage(meteoro, xM, yM, Graphics.TOP | Graphics.LEFT);
			g.drawImage(nave, xNave, yNave, Graphics.TOP | Graphics.LEFT);
			try {
				t.sleep(1);
			} catch (InterruptedException e) {}
			
			//-------------------------------------------------
			
			tecla = getKeyStates();
			
			if((tecla & UP_PRESSED) != 0)
				yNave = Math.max(0, yNave - velNaveY);
			if((tecla & DOWN_PRESSED) != 0){
				if((yNave + nave.getHeight()) < altura)
					yNave = yNave + velNaveY;
			}
			
			if ((tecla & LEFT_PRESSED) != 0) {
				if (!(xNave <= 0)) {
					xNave -= 4;
				}
			} else if ((tecla & RIGHT_PRESSED) != 0) {
				if (!(xNave + nave.getWidth() >= getWidth())) {
					xNave += 4;
				}
			}
			
			//-------------------------------------------------
			
			xM = xM + velx;
			yM = yM + vely;
			
			if ((xM + meteoro.getWidth()) > largura || (xM < 0))
				velx = -velx;
			if ((yM + meteoro.getHeight()) > altura || (yM < 0))
				vely = -vely;
			
			//------------------------------------------------	
			
			flushGraphics();
		}
	}
}