NullPointerException Bug

Rosselring

Hi, I'm fairly new to coding in general and I'm working on a project for my CS class. Right now, I'm trying to make a rectangle move back on forth across the screen according to arrow-key inputs. The sketch is titled MOMM. When I run it, I get this error:

Exception in thread "Animation Thread" java.lang.NullPointerException at MOMM$Mover.(MOMM.java:50) at MOMM.setup(MOMM.java:23) at processing.core.PApplet.handleDraw(PApplet.java:2371) at processing.awt.PSurfaceAWT$12.callDraw(PSurfaceAWT.java:1499) at processing.core.PSurfaceNone$AnimationThread.run(PSurfaceNone.java:312)

Here's my code:

Mover scene;
int x;
int y;

void setup(){
  size (600,300);
  scene = new Mover();
}

void draw(){
  background(255);
  scene.update();
  scene.checkEdges();
  scene.display();
}

void keyPressed(){
  if(key == CODED){
    if (keyCode == LEFT){
      scene.moveLEFT();
    }
    if (keyCode == RIGHT){
      scene.moveRIGHT();
    }
  }
}

class Mover{
  PVector position;
  PVector velocity;
  
  Mover(){
    position = new PVector(width, height);
    velocity = new PVector(width, height);
  }
  
  {
    position.x = x;
    position.y = y;
  }
  
  void update(){
    position.add(velocity);
  }
  
  void display(){
    pushMatrix();
    rect(-450,50,900,200);
    popMatrix();
  }
  
  void checkEdges(){
    if (position.x > 0){
      position.x = 0;
    }
    if (position.x > width){
     position.x = width;
   }  
  }
  
  void moveLEFT(){
    position.x--;
  }    
   
  void moveRIGHT(){
    position.x++;
  }
}

I've searched the forum and the web but I can't seem to figure out what is happening here. I would greatly appreciate some help. Thank you so much!

GoToLoop

http://forum.Processing.org/two/discussion/8045/how-to-format-code-and-text
http://forum.Processing.org/two/discussion/8071/why-do-i-get-a-nullpointerexception

Rosselring

Thank you for the response! I'm sorry, this is probably due to me being inexperienced, but I can't identify anything that isn't initialized or has been reset. I think it needs another pair of eyes, could you please look over it (now that I've correctly formatted it)? Thank you!

Chrisir

you need to tell which line was yellow / gave the error

place lines 33&34 after line 38

Chrisir

for moveright want x++

not y

Rosselring

Thanks for the feedback! I updated it. Okay, line 38 is highlighted yellow.

Chrisir

yes

so do what I said

Chrisir

place lines 33&34 after line 38

(in the old code)

the brackets around the old lines 33 & 34 make no sense - delete them

place lines 33&34 after line 38

Chrisir

read the tutorial on oop

you need to make a new PVector before you can give its x field a value

all this takes place in the constructor Mover

GoToLoop

The NPE lies in Mover's initialization block:

{
  position.x = x;
  position.y = y;
}

B/c position is still null when that block is run! :-@
Nonetheless, you end up initializing position inside Mover's constructor.
However, Java's initialization blocks happen before constructors. @-)

Also, there are some inconsistencies in your code:

• Why do you declare "global" variables x & y? They're not used anywhere! :-\"
• Why do you use pushMatrix() & popMatrix() inside display()?
• Shouldn't rect()'s coordinates be position.x & position.y?
• Why do you initialize position & velocity w/ new PVector(width, height);?
• What have position & velocity anything to do w/ dimensions or area sizes?
• Testing for key == CODED is unnecessary btW. ;)

Rosselring

Okay, thank you! I am no longer receiving the NPE error.

GoToLoop

An online example to inspire you: :bz
http://studio.ProcessingTogether.com/sp/pad/export/ro.91tcpPtI9LrXp

Chrisir

he uses x,y ....

but not really

Chrisir

  Mover(){
    position = new PVector(width, height);
    velocity = new PVector(width, height);
       
   
     
    position.x = x;
    position.y = y;
  }
   

koogs

If nobody else has mentioned it line 53 looks wrong. It will mean that x will always be 0.

Chrisir

Oh, yes

<0

Rosselring

Thank you all so much for your help! My code is now fully functioning and I got a complete product turned in on time. You're awesome!!

Chrisir

Great!

you could post the entire code and we write a review

;-)