Java Reference
In-Depth Information
public static void commonName(Body bodyRef) {
bodyRef.name = "Dog Star";
bodyRef = null;
}
}
This program produces the following output:
before: 0 (Sirius)
after: 0 (Dog Star)
Notice that the contents of the object have been modified with a name
change, while the variable
sirius
still refers to the
Body
object even
though the method
commonName
changed the value of its
bodyRef
paramet-
er variable to
null
. This requires some explanation.
The following diagram shows the state of the variables just after
main
invokes
commonName
:
At this point, the two variables
sirius
(in
main
) and
bodyRef
(in
commonName
)
both refer to the same underlying object. When
commonName
changes the
field
bodyRef.name
, the name is changed in the underlying object that
the two variables share. When
commonName
changes the value of
bodyRef
to
null
, only the value of the
bodyRef
variable is changed; the value of
sirius
remains unchanged because the parameter
bodyRef
is a pass-by-
value copy of
sirius
. Inside the method
commonName
, all you are changing
is the value in the parameter variable
bodyRef
, just as all you changed in
halveIt
was the value in the parameter variable
arg
. If changing
bodyRef
