Java Reference
In-Depth Information
affected the value of
sirius
in
main
, the "after" line would say "
null
".
However, the variable
bodyRef
in
commonName
and the variable
sirius
in
main
both refer to the same underlying object, so the change made in-
side
commonName
is visible through the reference
sirius
.
Some people will say incorrectly that objects are passed "by reference."
In programming language design, the term
pass by reference
properly
means that when an argument is passed to a function, the invoked func-
tion gets a reference to the original value, not a copy of its value. If
the function modifies its parameter, the value in the calling code will
be changed because the argument and parameter use the same slot in
memory. If the Java programming language actually had pass-by-refer-
ence parameters, there would be a way to declare
halveIt
so that the
above code would modify the value of
one
, or so that
commonName
could
change the variable
sirius
to
null
. This is not possible. The Java pro-
gramming language does not pass objects by reference; it passes object
references by value. Because two copies of the same reference refer to
the same actual object, changes made through one reference variable
are visible through the other. There is exactly one parameter passing
modepass by valueand that helps keep things simple.
You can declare method parameters to be
final
, meaning that the value
of the parameter will not change while the method is executing. Had
bodyRef
been declared
final
, the compiler would not have allowed you to
change its value to
null
. When you do not intend to change a paramet-
er's value, you can declare it
final
so the compiler can enforce this ex-
pectation. A
final
declaration can also protect against assigning a value
to a parameter when you intended to assign to a field of the same name.
The declaration can also help the compiler or virtual machine optimize
some expressions using the parameter, because it is known to remain
the same. A
final
modifier on a parameter is an implementation de-
tail that affects only the method's code, not the invoking code, so you
can change whether a parameter is
final
without affecting any invoking
code.
