If you want to return a variable from a function, then you have to declare a return type ("void" means, that the function doesn't return anything).

For example:

1. void setup() {
     int[] myArray = getRandomColors(10);
     println(myArray);
   }
   
   int[] getRandomColors(int arrayLength) {
     int[] arr = new int[arrayLength];
     for (int i = 0; i< arrayLength; i++) {
       arr[i] = color(random(255), random(255), random(255));
     }
     return arr;
   }

well, it can't return 3 things.

1. void doStuff( float[] a, PVector[] b, PVector[c] ) {
2.       a = new float[100];
3.       b = new PVector[100];
4.       c = new PVecor[100];
5. }

But i think i make a class that holds the 3 things so i can pass and return that.

Arrays, surprisingly, are Objects (or, at least, they aren't primitives) in Java... so if you pass an array to function, and the the function directly modifies that array (or, at least, doesn't make a copy of it), then any modifications to the array will apply to it. With Objects, this is normally a problem...

The example below clearly demonstrates that fact:

1. void setup() {
2.   float[] numbers = {3, 3, 2};
3.   changeNums(numbers);
4.   println(numbers);
5. }
6. 
   
7. void changeNums(float[] nums) {
8.   for(int i = 0; i < nums.length; i ++) {
9.     nums[i] ++;
10.   }
11. }

then why do i get all nullpointer in the code from the first post in this topic?

Java passes all parameters by value. In the case of an array, the value is the reference of the array, similar to passing an object, actually. You can change the given value but the change won't be see from the caller. Now, you can change the object pointed by the value (the reference) and all variables with a reference to that object will see the change.

When you call test(val), you pass the reference (address) of val to the function. You change this reference with the assignment of the new float[]. But this change is local to the function.
Note that there is little point in passing a global variable as parameter to a function, unless you plan to call that function with other arrays.

See:

1. float[] val;
2. 
   
3. void setup() {
4.   val = new float[10];
5.   test(val);
6.   println(val.length);
7.   println(val);
8. }
9. 
   
10. void test(float[] values) {
11.  values[5] = 42;
12.  println(values);
13. }