cp5 causing nullpointer exception

Contributed Library Questions

clankill3r

cp5 causing nullpointer exception

in Contributed Library Questions • 1 year ago

in draw i have:

for (Field f : fields) {
f.createField((int)random(10, 50), 20);
}

the cp5 slider does the same thing.

However, with cp5 it can happen that it triggered createField(.....) while it's also at something() in draw.

This is causing a null pointer example (just go forward and backward with the slider untill it happens).

How can i avoid this (multiple suggestions please).

I know one that is really time consuming and ugly.

import controlP5.*;
ControlP5 cp5;
ControlWindow cwindow;
ArrayList<Field> fields = new ArrayList<Field>();
void setup() {
size(400, 400);
smooth();
for (int i = 0; i < 20; i++) {
fields.add(new Field());
}
cp5 = new ControlP5(this);
cwindow = cp5.addControlWindow("stuff", 100, 100, 500, 400);
cp5.addSlider("xSteps")
.setValue(10)
.setRange(1, 50)
.moveTo(cwindow);
}
// . . . . . . . . . . . . . . . . . . .
void draw() {
background(255);
// it never crashes by this
for (Field f : fields) {
f.createField((int)random(10, 50), 20);
}
for (Field f : fields) {
text(f.something(), 20, 20);
}
}
// . . . . . . . . . . . . . . . . . . .
// but slider does
void xSteps(int xSteps) {
for (Field f : fields) {
f.createField(xSteps, 100);
}
}
// . . . . . . . . . . . . . . . . . . .
class Field {
PVector[][] field;
int xSteps, ySteps;
Field() {
createField(100, 100);
}
// . . . . . . . . . . . . . . . . . . .
void createField(int xSteps, int ySteps) {
this.xSteps = xSteps;
this.ySteps = ySteps;
field = new PVector[xSteps][ySteps];
for (int x = 0; x < xSteps; x++) {
for (int y = 0; y < ySteps; y++) {
field[x][y] = new PVector();
}
}
}
// . . . . . . . . . . . . . . . . . . .
float something() {
return field[xSteps-1][ySteps-1].x;
}
// . . . . . . . . . . . . . . . . . . .
}

Replies(4)

andreas.s..

Re: cp5 causing nullpointer exception

1 year ago

in your example you are running 2 threads, the main sketch and the controlWindow, and when the exception is thown, both threads are working on the same Array, 1 is changing it, the other is reading it, so you end up with a concurrency conflict.

an attempt to overcome this:

void draw() {
background(255);
if (n0!=n1) {
for (Field f : fields) {
f.createField(n1, 100);
}
n0 = n1;
}
for (Field f : fields) {
text(f.something(), 20, 20);
}
}
int n0;
int n1;
// instead of making changes to ArrayList fields, use a variable (here n1) to
// store updates coming from the slider.
// inside draw compare n1 with n0, if they are not equal, make changes to fields.
//By doing so you are avoiding the thread conflicts.
void xSteps(int xSteps) {
n1 = xSteps;
}

clankill3r

Re: cp5 causing nullpointer exception

1 year ago

what does cp5.setUpdate(false);

do?

I tried but it still hase the problem then.

Would be nice to set that once in setup and then in the beginning of draw use

cp5.update() so all values will be set.

rbrauer

Re: cp5 causing nullpointer exception

1 year ago

I didn't read this in depth, but maybe you could try a CopyOnWriteArrayList from the java.util.concurrent package in place of your regular ArrayList.

PhiLho

Re: cp5 causing nullpointer exception

1 year ago

clankill3r, please read or re-read https://forum.processing.org/topic/where-to-place-threads-and-other-forum-practices
controlP5 questions belong here, no Programming Questions.
Thanks.

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