Fundamentals - SCJP Sun Certified Programmer for Java Platform SE6 - page 41

When modifyStacks returns, names still points to the Stack containing “Kim” and
“Edward” , and grades now points to a Stack containing 95, 87, and 50. The output of
StackDemo is
Kim
Edward
95
87
50
Changing the reference one does not change the reference names . Although it is
impossible for the modifyStacks method to alter the names reference, it was quite easy for
the method to modify the object that names points to.
The concept of call by value also applies to values returned by a method, as we will see
in the next section. As discussed earlier in this chapter, you need to be able to view code
and determine when an object becomes eligible for garbage collection. When does the
object on line 9 of StackDemo become eligible for garbage collection? Because the variable
one is a parameter, it goes out of scope when the modifyStacks method returns on line 10.
Because one is the only reference pointing to the Stack object from line 9, the object is
eligible for garbage collection after line 10.
Passing References vs. Passing Objects
You need to be able to distinguish the difference between a reference and an object.
When passing arguments to a method, it is the reference that gets passed, not the object.
It is impossible to pass an object to a method. In fact, the largest amount of data that can
be copied into a parameter (or returned from a method) is a long or a double , both of
which are 64 bits.
Return values are also passed by value, meaning a copy of the data is sent to the calling
method. A method can return void , one of the eight primitive types, or a reference: there
are no other possibilities. (Of course, the reference can be of any class or interface type,
so the possible values you can return are actually endless, as long as you realize that a
reference is getting returned, never an actual object!)
Let's look at an example using primitive types. Suppose we have the following method
defi nition:
31. public int max(int a, int b) {
32. int response;
33. if( a < b) {
34. response = b;
35. } else {