# Question about S4P Sprites

**URL:** <https://discourse.processing.org/t/question-about-s4p-sprites/23694>\
**Category:** Libraries\
**Created:** [September 5, 2020, 3:55pm UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694 "2020-09-05T15:55:53Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![mikechr](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@mikechr](https://discourse.processing.org/u/mikechr)\
**Post date:** [September 5, 2020, 3:55pm UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/1 "2020-09-05T15:55:53Z")

</div>

Hi. Two questions about using S4P Sprites library

1. I cant make Sprite.addEventHandler() work. I am getting an Null pointer exception when I press the sprite button. I am aware there is an alternative method but I like the encapsulation offered by this approach.  
java.lang.NullPointerException  
at sprites.SMessenger.eventHandlerFailed(Unknown Source)  
at sprites.SMessenger.message(Unknown Source)  
at sprites.Sprite.fireEvent(Unknown Source)  
at sprites.Sprite.mouseEvent(Unknown Source)

2. The reason the question is so vague is the exception stack trace is missing line numbers. Is it possible to rebuild the library and run with the modified version? Is there a folder in Processing that I will place the compiled class hierarchy?

Thanks Mike

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<div class="post-metadata">

**Author:** ![quark](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/quark/32/26_2.png) [@quark](https://discourse.processing.org/u/quark)\
**Post date:** [September 6, 2020, 8:03am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/2 "2020-09-06T08:03:34Z")

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It appears that you have managed to successfully added an event handler to the sprite. 😌

In the console pane did you see this text just above the stack trace?  
`############# EXCEPTION IN EVENT HANDLER #############`

If you did please include the entire error message i.e.

`############# EXCEPTION IN EVENT HANDLER #############`  
_other error message stuff here_  
`######################################################`

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<div class="post-metadata">

**Author:** ![mikechr](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@mikechr](https://discourse.processing.org/u/mikechr)\
**Post date:** [September 7, 2020, 2:19am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/3 "2020-09-07T02:19:15Z")

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I see the exception when I try to install a handler (addEventHandler) on a sprite installed on a new class. See StartButton below. Things work when the handler is installed on a sprite hosted on the PApplet.

```auto
class StartButton 
{
  public Sprite sprite;
  
  StartButton(PApplet applet, int xpos, int ypos)
  {
    sprite = new Sprite(applet, "Start Button.png", 70);
    sprite.setXY(xpos, ypos);
    sprite.setVelXY(0, 0);
    //sprite.setScale(.5);

    sprite.respondToMouse(true);
    sprite.addEventHandler(this, "onButton");
  }
  
  void onButton(Sprite sprite)
  {
    println("ON BUTTON xxx" + sprite);
  }
}

```

Here is the full stack trace. No line numbers.

# ======================================================= GameControlPlus V1.2.2 created by Christian Riekoff and Peter Lager

java.lang.NullPointerException  
at sprites.SMessenger.eventHandlerFailed(Unknown Source)  
at sprites.SMessenger.message(Unknown Source)  
at sprites.Sprite.fireEvent(Unknown Source)  
at sprites.Sprite.mouseEvent(Unknown Source)  
at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)  
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)  
at java.lang.reflect.Method.invoke(Method.java:498)  
at processing.core.PApplet$RegisteredMethods.handle(PApplet.java:1436)  
NullPointerException  
at processing.core.PApplet.handleMethods(PApplet.java:1638)  
at processing.core.PApplet.handleMouseEvent(PApplet.java:2749)  
at processing.core.PApplet.dequeueEvents(PApplet.java:2652)  
at processing.core.PApplet.handleDraw(PApplet.java:2493)  
at processing.awt.PSurfaceAWT$12.callDraw(PSurfaceAWT.java:1547)  
at processing.core.PSurfaceNone$AnimationThread.run(PSurfaceNone.java:313)

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<div class="post-metadata">

**Author:** ![mikechr](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@mikechr](https://discourse.processing.org/u/mikechr)\
**Post date:** [September 7, 2020, 2:20am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/4 "2020-09-07T02:20:28Z")

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Oh and I am also using GameControlPlus which is the additional banner printed before the stack trace.

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<div class="post-metadata">

**Author:** ![quark](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/quark/32/26_2.png) [@quark](https://discourse.processing.org/u/quark)\
**Post date:** [September 7, 2020, 6:31am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/5 "2020-09-07T06:31:48Z")

</div>

OK we have to start somewhere so make this change

> [@mikechr](#):
>
> `public void onButton(Sprite sprite)`

and see what happens.

The event handler must have public access.

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<div class="post-metadata">

**Author:** ![mikechr](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@mikechr](https://discourse.processing.org/u/mikechr)\
**Post date:** [September 8, 2020, 11:37am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/6 "2020-09-08T11:37:31Z")

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> [@mikechr](#):
>
> ```auto
> class StartButton 
> {
> public Sprite sprite;
>   
> StartButton(PApplet applet, int xpos, int ypos)
> {
> sprite = new Sprite(applet, "Start Button.png", 70);
> sprite.setXY(xpos, ypos);
> sprite.setVelXY(0, 0);
> //sprite.setScale(.5);
> 
> sprite.respondToMouse(true);
> sprite.addEventHandler(this, "onButton");
> }
>   
> void onButton(Sprite sprite)
> {
> println("ON BUTTON xxx" + sprite);
> }
> }
> 
> ```

Yes. Making both the class and the handler method public does the trick. Thanks!.

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<div class="post-metadata">

**Author:** ![mikechr](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@mikechr](https://discourse.processing.org/u/mikechr)\
**Post date:** [September 8, 2020, 11:38am UTC](https://discourse.processing.org/t/question-about-s4p-sprites/23694/7 "2020-09-08T11:38:30Z")

</div>

Yes. Making both the class and the handler method public does the trick. Thanks!.