Once you've created your objects you can easily reference their properties (as long as they're not explicitly made private) like this:

Code:

println(sur1.xk);
println(sur1.yk);
println(sur1.zk); 

...You can do that from within your main body of code; though not obviously not from within the Surface class itself.

All those values are coming from the same Surface object: 'sur1'.

In your forma() method you run through a for loop assigning a new value to xk in each iteration.  Therefore at the end of draw() xk equals the last value defined in the for loop and that value gets printed.  Because in the next draw loop the starting value of xk is different that means the end value, after running through the for loop, will be different too; so a different value gets printed.  Each loop you get a different value and you'll get a number of different values equal to 'brivi+1', since that's the number of iterations your loop goes through because your for loop continues till j<=brivi.

Well - it's something along those lines anyway.  Here's the proof - a single surface object prints several different values for xk:

Code:

Surface surf1;
Surface surf2;
Surface surf3;


void setup() {
  size(400,300,P3D);
  surf1 = new Surface(3,50,200,150,100);
}

void draw() {
  background(0);
  surf1.forma();
}


class Surface{
 float  xk, yk, zk, x, y, z, xx, yx, zx;
 int ra, brivi;
 // ArrayList xk, yk, zk;
 float angle = 0;
 ArrayList xlist;
 Surface(){
 }

 Surface(int brojivica, int radius, float xa, float ya, float za){        
   brivi = brojivica;
   ra = radius;
   x = xa;
   y = ya;
   z = za;
 }

 void forma(){
   xlist = new ArrayList();
   stroke(2);
   beginShape();
   for(int j=0;j<brivi;j++){
     xk = cos(radians(angle))*ra;
     yk = sin(radians(angle))*ra;
     vertex(xk+x,yk+y,zk+z);
     angle+=360.0/brivi;
     xlist.add(xk);
     println(xk);
   }
   endShape(CLOSE);  
   println("-----------");
 }
}