How to debug a NullPointerException in Processing? - processing

Question

I am making an animation in Processing it is a simulation of a ball going up and coming down, I have an error saying 'NullPointerException' it keeps appearing I will give the code and show the error place:
Obj[] objs = new Obj[100];
void setup(){
size(1000, 1000);
}
void draw(){
background(250);
for(int i = 0; i < objs.length; i++){
objs[i].render();
objs[i].up();
objs[i].run();
}
}
the error is at objs[i].render();
here is the code for Obj:
class Obj{
float x;
float y;
float speed;
float pspeed;
float velocity;
void render(){
fill(0);
ellipse(x, y, 5, 5);
}
void up(){
x = random(-1000, 1000);
y = y-1010;
}
void run(){
speed = random(4, 6);
pspeed = speed;
velocity = 0.05;
while(y<random(700, 1100)){
y=y-speed;
speed=speed-velocity;
}
while(y>0){
y=y+pspeed;
pspeed = pspeed - velocity;
}
}
}

Answer 1

This line creates an array that can hold 100 instances of Obj:
Obj[] objs = new Obj[100];
But it doesn't actually create any instances. In other words, your array contains 100 null values.
Then you're looping through those null values and trying to call functions on them, which is what's causing your error.
To fix the problem, you probably just want to fill your array up with instances:
void setup(){
size(1000, 1000);
for(int i = 0; i < objs.length; i++){
objs[i] = new Obj();
}
}
Shameless self-promotion: I wrote a tutorial on arrays in Processing available here.