In Java arrays are treated as objects and array variable names (e.g. anull, a, b ain, bin) are oject references NOT an actaula array. This means that when you assssigning one array to another you are copying array references NOT the array contents.

So in your code below you have in line :-
1     an array of 2 floats referenced by the identifier anull
6     call the Ball constructor passing 2 copies of the reference anull
20   the array reference anull is assigned to a
21   the array reference anull is assigned to b

Lines 20 and 21 have NOT copied the array which means tha anull, a and b are all referencing the SAME array and changes to using any one of the identifers will been visible from the others.

Change lines 20 and 21 to

a = new float[ain.length];
arrayCopy(ain, a);
b= new float[bin.length];
arrayCopy(bin, b);

and check out the reference for arrayCopy

1. float[] anull= {0, 0};
2. Ball balla;
3. 
   
4. void setup()
5. {
6.   balla = new Ball(anull, anull);
7. }
8. 
   
9. void draw()
10. {
11.   balla.test();
12.   stop();
13. }
14. 
    
15. class Ball {
16.   float a[];
17.   float b[];
18. 
    
19.   Ball(float[] ain, float[] bin) {
20.     a=ain;
21.     b=bin;
22.   }
23. 
    
24.   void test() {
25.     a[0]=2;
26.     println (b[0]);
27.   }
28. }