# HELP! Exception in Event Handler
**URL:** https://discourse.processing.org/t/help-exception-in-event-handler/37751
**Category:** Electronics (Arduino, etc.)
**Created:** [June 28, 2022, 7:31pm UTC](https://discourse.processing.org/t/help-exception-in-event-handler/37751 "2022-06-28T19:31:25Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@charlielamus](https://discourse.processing.org/u/charlielamus)
#### Post date: [June 28, 2022, 7:31pm UTC](https://discourse.processing.org/t/help-exception-in-event-handler/37751/1 "2022-06-28T19:31:25Z")
Hello!
I know next to nothing about java, being limited to playing about with the Marlin code for my 3d printer.
I am a total beginner, so please be kind… I am also autistic so I might ask questions which sound stupid. Sorry.
I have downloaded some code called “knitty” from here:
[https://github.com/knitty/gui](https://github.com/knitty/gui)
and it runs beautifully on Processing 2.2.1, except that when I press the button marked “connect to arduino”, it produces the following error message, which has me totally baffled:
################ EXCEPTION IN EVENT HANDLER ################
An error occured during execution of the eventhandler:
CLASS: knitty\_main METHOD: handleButtonEvents
Caused by java.lang.ArrayIndexOutOfBoundsException: 1
knitty\_main.initSerialCommunication(knitty\_main.java:1987)
knitty\_main.handleButtonEvents(knitty\_main.java:1110)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
g4p\_controls.GAbstractControl.fireEvent(Unknown Source)
g4p\_controls.GButton.mouseEvent(Unknown Source)
g4p\_controls.GWindowInfo.mouseEvent(Unknown Source)
sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
processing.core.PApplet$RegisteredMethods.handle(PApplet.java:1236)
processing.core.PApplet.handleMethods(PApplet.java:1431)
processing.core.PApplet.handleMouseEvent(PApplet.java:2826)
processing.core.PApplet.dequeueEvents(PApplet.java:2725)
processing.core.PApplet.handleDraw(PApplet.java:2397)
processing.core.PGraphicsJava2D.requestDraw(PGraphicsJava2D.java:240)
processing.core.PApplet.run(PApplet.java:2256)
java.lang.Thread.run(Unknown Source)
##############################################################
The “handleButtonEvents” seems to work for all the other buttons, so I have been looking at the initSerialConnection function, which is reproduced here:
```auto
void initSerialCommunication() {
serialCommunication = new Serial(this, Serial.list()[serialport], 115200);
serialCommunicationStarted = 1;
}
```
and it is making me sweat… I don’t know what I am doing; am I doing this wrong? The code has worked for other people so what might be happening?
Thank you in advance!
Charlie
---
### Author:  [@glv](https://discourse.processing.org/u/glv)
#### Post date: [June 29, 2022, 2:12am UTC](https://discourse.processing.org/t/help-exception-in-event-handler/37751/2 "2022-06-29T02:12:14Z")
Hello @charlielamus ,
I do not use Processing 2.2.1 but did get this to run on Processing 3.5.4 and Processing 4.0b8.
Changes:
```auto
// Add this:
void settings()
{
size(GUIwidth, GUIheigth);
}
void setup() {
// size(GUIwidth, GUIheigth); // Comment this
```
```auto
import drop.*; // Added this library
import test.*;
// Don't forget to place the libraries under sketchbook/libraries/G4P and sketchbook/libraries/sDrop
import g4p_controls.*; // http://sourceforge.net/projects/g4p/
//import sojamo.drop.*; // http://www.sojamo.de/libraries/drop/ // Commented this!
```
There is firmware that comes with Knitty:
> **[Knitty Project](https://github.com/knitty)**
>
> Reverse-Engineering of an Superba/Singer 624/Memo II and Passap/Pfaff E6000 - Knitty Project
Steps:
- Uploaded one of the Knitting Machine (Arduino firmware) selections to Arduino.
- Run the Knitty Processing sketch.
- Select the Arduino COM Port that is being used.
- Select the Knitting Machine that is being used.
I selected “Knit One Row” from GUI:
`:)`
---
### Author:  [@quark](https://discourse.processing.org/u/quark)
#### Post date: [June 29, 2022, 8:31am UTC](https://discourse.processing.org/t/help-exception-in-event-handler/37751/3 "2022-06-29T08:31:12Z")
This is called a stack trace and is generated when an exception is thrown.
1. ################ EXCEPTION IN EVENT HANDLER ################
2. An error occured during execution of the eventhandler:
3. CLASS: knitty\_main METHOD: handleButtonEvents
4. Caused by java.lang.ArrayIndexOutOfBoundsException: 1
5. knitty\_main.initSerialCommunication(knitty\_main.java:1987)
6. knitty\_main.handleButtonEvents(knitty\_main.java:1110)
7. sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
8. sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
9. sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
10. java.lang.reflect.Method.invoke(Unknown Source)
11. g4p\_controls.GAbstractControl.fireEvent(Unknown Source)
12. g4p\_controls.GButton.mouseEvent(Unknown Source)
It basically lists the methods executed just before the exception was encountered. I have only shown twelve lines because that is the interesting part.
Line 12 is the method called to test whether an event occurred on a G4P button. In line 11 we see that the fireEvent method is called which will invoke (execute) the event handler. Lines 7, 8 and 9 are Java functions which locate the event handler (`handleButtonEvents`) and execute it.
Line 6 is calling the event handler where you check which button is clicked and executes your code.
Line 5 is calling your method `initSerialCommunication` and line 4 shows the cause of the exception, in this case an attempt to access an array element that doesn’t exist. So now we are back at your code
```auto
void initSerialCommunication() {
serialCommunication = new Serial(this, Serial.list()[serialport], 115200);
serialCommunicationStarted = 1;
}
```
Obviously `Serial.list()` returns an array and `serialport` indicates the element in the array you are trying to access. So change the method to
```auto
void initSerialCommunication() {
println("Array length " + Serial.list().length);
println("Element " + serialport);
// serialCommunication = new Serial(this, Serial.list()[serialport], 115200);
// serialCommunicationStarted = 1;
}
```
This exception will be thrown if
- if the array length is 0 (zero)
- if the element number is \>= array length
One last thing all G4P controls have a `tag` attribute which can be used to help debug your code. For instance immediately after creating a button called `my_button` add a statement to give a tag value e,g,
`my_button.tag = "Serial button clicked"; // or whatever you want`
If you give all your buttons different tag values then you can print the tag value when a problem occurs.