I'm fazed by a nullpointererror I can't get rid of

[Martin Platt]

It's really annoying because it seems logical enough to me and I'm sure it's something really simple. I hope someone can help me. It's a bat and ball game to which I'm adding a computer opponent. The problem occurs in the Ball code:

if ((int)(x)<=10 && y>aii.b-6 && y<aii.b+35+6) {
//if (x<10) {
System.out.println(aii.b);
deltax *=-1;
hit.play();
 
}

and throws the exception because it doesn't like aii.b. But it has no problem in the next if statement:
if ((int)(x)>=490 && y>james.z-6 && y<james.z+35+6)

which is exactly the same logic.


Here's the main programme

 
import java.awt.*;
import java.applet.*;
import java.awt.Color;
import java.awt.event.*;
 

public class batball3 extends Applet implements Runnable, KeyListener, MouseListener {
 
int width, height; // these are the width and length of onscreen court
//int x, y;  // co-ordinates of centre of ball
//int z; // distance of bat from top of screen
boolean isButtonPressed = false;
Image backbuffer;
Graphics backg;
 
boolean pressedUp, pressedDown, newgame;
Thread th;
player bat;
Ball ball;
Ai ai;
boolean uppress=false;
boolean dnpress=false;
AudioClip hitwall;
AudioClip score;
AudioClip missit;
AudioClip beyonce;
private int speed;
int delay;
int canclick;
 

 
   public void init()
   {
String str = getParameter("fps");
int fps = (str != null) ? Integer.parseInt(str) : 10;
delay = (fps > 0) ? (100 / fps) : 100;
      width = getSize().width;
      height = getSize().height;
      setBackground( Color.black);
addKeyListener(this);    
addMouseListener(this);     
 
    hitwall=getAudioClip(getCodeBase(),"hitside.wav");
    score=getAudioClip(getCodeBase(),"hitbat.wav");
    missit=getAudioClip(getCodeBase(),"ballout.wav");
    beyonce=getAudioClip(getCodeBase(),"getready.wav");
    canclick=0;
 
    bat= new player(20);
 
    ball=new Ball(200,250,-5,-5,bat,hitwall,score,missit,ai);
ai=new Ai(ball);
 backbuffer = createImage( width, height );
      backg = backbuffer.getGraphics();
      if (getParameter ("speed") != null)
{
      speed=Integer.parseInt(getParameter("speed"));
}
else speed=15;
 

       }
 
public void mouseClicked(MouseEvent e)
{
        if (canclick==1){
        canclick=2;
System.out.println(canclick);
             
        }
}
 
public void mouseEntered(MouseEvent e)
{

}
 
public void mouseExited(MouseEvent e)
{
 
}
 
public void mousePressed(MouseEvent e)
{
 
}
 
public void mouseReleased(MouseEvent e)
{
 
}
 
public void mouseMoved(MouseEvent e)
{
}
 
public void mouseDragged(MouseEvent e)
{
}
 

public void keyTyped(KeyEvent e)
{
 
}
 
public void keyReleased(KeyEvent e)
{
int upCode = e.getKeyCode(); 
if (upCode == KeyEvent.VK_UP)
{
//bat.moveup();
uppress=false;
}
int dnCode = e.getKeyCode(); 
if (dnCode == KeyEvent.VK_DOWN)
{
//bat.movedown();
dnpress=false;
}
}
 
public void keyPressed(KeyEvent e)
{
int upCode = e.getKeyCode(); 
if (upCode == KeyEvent.VK_UP)
{
//bat.moveup();
uppress=true;
}
int dnCode = e.getKeyCode(); 
if (dnCode == KeyEvent.VK_DOWN)
{
//bat.movedown();
dnpress=true;
}
 
}
 

public void start()
{
th=new Thread(this);
th.start();
}
 

public void run()
   {
long tm = System.currentTimeMillis();
canclick=0;
 
getready();
while(true)
{
if (bat.getlives()>0){
 
if (uppress==true) 
{
bat.moveup();
}
if (dnpress==true) 
{
bat.movedown();
}
System.out.println(ai.intb());
//ball.intb();
ball.moveball();  //moveball only changes the co-ords and trajectory
//showStatus(ball.getdy());
backg.setColor(Color.black);
backg.fillRect(0,0,width,height);
backg.setColor(Color.white);
System.out.println(ai.intb());
backg.drawRect(0,0,510,510);
bat.drawbat(backg);
ball.drawball(backg);
ai.drawai(backg);
backg.drawString("Score:"+bat.getscore(),0,20);
backg.drawString("Lives left:"+bat.getlives(),400,20);
repaint();
}
if (bat.getlives()==0){
backg.drawString("GAME OVER",250,250);
backg.drawString("You scored "+bat.getscore(),250,260);
backg.setColor(Color.white);
backg.drawString("To play again, click the screen!",250,270);
System.out.println(canclick);
if (canclick==0)
{
canclick=1;
}
System.out.println(canclick);
if (canclick==2){
bat.goagain();
        ball.goagain();        
        getready();
canclick=0;}
repaint();
}
 
try {
tm += delay;
Thread.sleep(Math.max(0, tm - System.currentTimeMillis()));
}
catch(InterruptedException ex)
{}
 
}
}
 
public void stop ()
{
th.stop();
}
public void paint(Graphics g)
{
update(g);
}
 
public void update (Graphics g) 
{
g.drawImage( backbuffer, 0, 0, this );
}
 
public void getready(){
if (bat.getlives()==3){
//beyonce.play();
//try{
//Thread.sleep(10000);
//}
//catch(InterruptedException e){}
}
}
 

} 

And here's the Ball code...

 import java.awt.*;
import java.applet.*;
import java.awt.Color;
 
public class Ball{
 
double x;
double y;
double deltax;
double deltay;
player james;
AudioClip hit;
AudioClip scored;
AudioClip ballout;
Ai aii;
 

public Ball(double x, double y, double deltax, double deltay, player james, AudioClip hit, AudioClip scored, AudioClip miss, Ai aii)
{
this.x=x;
this.y=y;
this.deltax=deltax;
this.deltay=deltay;
this.james=james;
this.hit=hit;
this.scored=scored;
ballout=miss;
this.aii=aii;
}
 

public void drawball(Graphics g)
{
g.setColor(Color.blue);
g.fillOval(intx(),inty(),10,10);
}
 
public void moveball()
{
//ai.intb();
//System.out.println(aii.b);
if ((int)(x)<=10 && y>aii.b-6 && y<aii.b+35+6) {
//if (x<10) {
System.out.println(aii.b);
deltax *=-1;
hit.play();
 
}
 
if (y>500||y<1) {
deltay *=-1;
hit.play();
 
}
 
if ((int)(x)>=490 && y>james.z-6 && y<james.z+35+6)
{
x=490;
deltay=(y-17.5-(james.z))/7;
System.out.println("y="+y+" and z="+james.z);
System.out.println("deltay before calcs is"+deltay);
deltax=getgrad()*-1;
//deltax *=-1;
scored.play();
james.scorepoint();
}
 
if (x>500)
{
ballout.play();
System.out.println("Unlucky.  Get ready to go again!");
System.out.println("x="+x+" y="+y+" z="+james.z+" deltay="+deltay+" deltax="+deltax);
james.loselife();
try{
Thread.sleep(5000);}
catch(InterruptedException e){}
 
deltax=(int)(deltax)*-1;
deltay=(int)(deltay)*-1;
x=x-6.5;
}
else
{
x=x+deltax;
y=y+deltay;
 

}
 

 
}
 
public double getgrad()
{
double dy2=Math.pow(deltay,2);
double calc=Math.sqrt(50-dy2);
//System.out.println(deltay);
//System.out.println(dy2);
//System.out.println(calc);
return calc;
}
 
public int intx()
{      //provide an integer version of x for the draw oval method
int intx=(int)(x);
return intx;
}
public int inty()
{      //provide an integer version of y for the draw oval method
int inty=(int)(y);
return inty;
}
public int intb()
{      //provide an integer version of y for the draw oval method
int intb=aii.b;
return intb;
 

}
public void goagain()
{
//deltax=(int)(deltax)*-1;
//deltay=(int)(deltay)*-1;
x=200;
}
 
}
 
 

Here's the Player code...

 import java.awt.*;
import java.applet.*;
import java.awt.Color;
 
public class player{
 
private int lives;
private int score;
 

public int z;
 
public player(int z){
lives=3;
score=0;
}
 
public void moveup()
{
if (z>0){
z-=5;
}
 
}
 
public void movedown()
{
if (z<473){
z+=5;
}
 
}
 
public int getscore()
{
return score;
}
 
public int getlives()
{
return lives;
}
 
public void loselife()
{
--lives;
}
 
public void scorepoint()
{
score=score+10;
}
 
public void drawbat(Graphics g)
{
g.setColor(Color.red);
g.fillRect(500,z,5,35);
}
 
public void goagain()
{
lives=3;
score=0;
}
 

} 

And finally here's the AI code...

 import java.awt.*;
import java.applet.*;
import java.awt.Color;
 
public class Ai{
 
Ball ball;
public int b;
 

public Ai(Ball ball)
{
this.ball=ball;
 
}
 

public void drawai(Graphics g)
{
g.setColor(Color.blue);
if (b>ball.inty())
{b-=5;}
if (b<ball.inty())
{b+=5;}
g.fillRect(10,b,5,35);
//System.out.println(ball.inty());
 
}
 
public int intb()
{int intb=(int)(b);
return intb;
}
 
}

[Michael Dunn]

in batball3's init()

ball=new Ball(200,250,-5,-5,bat,hitwall,score,missit,ai)
ai=new Ai(ball);

ai is null - used prior to it's creation

if you change the lines around, ball will be null - same reason

[Martin Platt]

Thanks Michael, this makes sense. I see what you mean that, either way, you get the error. But I need ai to know about ball, and ball to know about ai. Is there an easy way to send this information after the object is created?
[ December 28, 2005: Message edited by: Martin Platt ]

[Michael Dunn]

you could try this (need to modify ball's constructor)

//ball=new Ball(200,250,-5,-5,bat,hitwall,score,missit,ai);//remove ai
ball=new Ball(200,250,-5,-5,bat,hitwall,score,missit);
ai=new Ai(ball);
ball.aii = ai;

[Martin Platt]

Thanks so much! It works.