Defining Classes - Beginning Java 2 SDK - page 183

Passing Objects to a Method
When you pass an object as an argument to a method, the mechanism that applies is called pass-by-
reference , because a copy of the reference contained in the variable is transferred to the method, not the
object itself. The effect of this is shown in the following diagram.

Sphere object
radius: 10.0
xCenter: 1.0
yCenter: 1.0
zCenter: 1.0

public static void main(String[] args){

...
Sphere ball = new Sphere(10.0, 1.0, 1.0, 1.0);
obj.change(ball);

...

}

This causes a copy
of ball to be made,
but not the Sphere
object

ball and the copy
both refer to
the original object

ball

copy
of ball

reference

copy made

reference

s refers to
the copy of
ball

acts on

Sphere change( Sphere s){

This statement

modifies the
original object

s.changeRadius(1.0);
return s;

through a copy of
ball

}

This illustration presumes we have defined a method, changeRadius() , in the class Sphere that will
alter the radius value for an object, and that we have a method change() in some other class that calls
changeRadius() . When the variable ball is used as an argument to the method change() , the
pass-by-reference mechanism causes a copy of ball to be made and stored in s . The variable ball just
stores a reference to the Sphere object, and the copy contains that same reference and therefore refers
to the same object. No copying of the actual object occurs. This is a major plus in term s of efficiency
when passing arguments to a method. Objects can be very complex involving a lot of instance variables.
If objects themselves were always copied when passed as arguments, it could be very time consuming
and make the code very slow.

Since the copy of ball refers to the same object as the original, when the changeRadius() method is
called the original object will be changed. You need to keep this in mind when writing methods that
have objects as parameters because this is not always what you want.

In the example shown, the method change() returns the modified object. In practice you would
probably want this to be a distinct object, in which case you would need to create a new object from s .
You will see how you can write a constructor to do this a little later in this chapter.

Remember that this only applies to objects. If you pass a variable of type int or
double to a method for example, a copy of the value is passed. You can modify the
value passed as much as you want in the method, but it won't affect the original value.