You can run this and see.

1. class circle
2. {
3.   void make(int x,int y,int dia){
4.     ellipse(x,y,dia,dia);
5.   }
6. }
7. void setup(){
8.   size(300,300);
9. }
10. void draw(){
11.   circle spoon = new circle();
12.   spoon.make(20,20,50);
13. println(spoon);
14. }

I am not 100% sure what Java does with the unused objects. I know it has an automated garbage collection that takes over.

Actually I thought that the garbage collector's purpose was to deallocate memory when objects are not used anymore. But this interesting article from Sun says that the JVM wouldn't be efficient enough if it had to care about all situations when it's happening (i.e. variables falling out of scope, objects that are not referenced anymore, etc).

So when you're not sure, explicitly null your reference and you'll be fine :

1. void draw() {
   
2.   ball = new Circle(x, y);
3.   ...
   
4.   ball = null;
5. }
   

This book dates back to 2001! To my knowledge, such issue is probably obsolete by now, the garbage collector(s) of Java have made great progresses since then...
At least, I hope such issue (and need to nullify reference) is a thing of the past!