Java Reference
In-Depth Information
The first line declares
minivan
as a reference to an object of type
Vehicle
. Thus,
minivan
is a variable that can refer to an object, but it is not an object itself. At this point,
minivan
does not refer to an object. The next line creates a new
Vehicle
object and assigns a refer-
ence to it to
minivan
. Now,
minivan
is linked with an object.
Reference Variables and Assignment
In an assignment operation, object reference variables act differently than do variables of
a primitive type, such as
int
. When you assign one primitive-type variable to another, the
situation is straightforward. The variable on the left receives a
copy
of the
value
of the vari-
able on the right. When you assign one object reference variable to another, the situation
is a bit more complicated because you are changing the object that the reference variable
refers to. The effect of this difference can cause some counterintuitive results. For example,
consider the following fragment:
At first glance, it is easy to think that
car1
and
car2
refer to different objects, but this is not
the case. Instead,
car1
and
car2
will both refer to the same object. The assignment of
car1
to
car2
simply makes
car2
refer to the same object as does
car1
. Thus, the object can be
acted upon by either
car1
or
car2
. For example, after the assignment
executes, both of these
println( )
statements
display the same value: 26.
Although
car1
and
car2
both refer to the same object, they are not linked in any other
way. For example, a subsequent assignment to
car2
simply changes the object to which
car2
refers. For example:
