Java Reference
In-Depth Information
the object through which the method is invoked. In our example, that object is a
Circle
.
The bodies of the
area()
and
circumference()
methods both
use the class field
PI
. We saw earlier that class methods can
use only class fields and class methods, not instance fields or
methods. Instance methods are not restricted in this way: they
can use any member of a class, whether it is declared
static
or not.
How the this Reference Works
The implicit
this
parameter is not shown in method signatures because it is usually
not needed; whenever a Java method accesses the instance fields in its class, it is
implicit that it is accessing fields in the object referred to by the
this
parameter. The
same is true when an instance method invokes another instance method in the same
class—it's taken that this means “call the instance method on the current object.”
m
g
O
However, you can use the
this
keyword explicitly when you want to make it clear
that a method is accessing its own fields and/or methods. For example, we can
rewrite the
area()
method to use
this
explicitly to refer to instance fields:
public
double
area
()
{
return
Circle
.
PI
*
this
.
r
*
this
.
r
;
}
This code also uses the class name explicitly to refer to class field
PI
. In a method
this simple, it is not normally necessary to be quite so explicit. In more complicated
cases, however, you may sometimes find that it increases the clarity of your code to
use an explicit
this
where it is not strictly required.
In some cases, the
this
keyword
is
required, however. For example, when a method
parameter or local variable in a method has the same name as one of the fields of
the class, you must use
this
to refer to the field, because the field name used alone
refers to the method parameter or local variable.
For example, we can add the following method to the
Circle
class:
public
void
setRadius
(
double
r
)
{
this
.
r
=
r
;
// Assign the argument (r) to the field (this.r)
// Note that we cannot just say r = r
}
Some developers will deliberately choose the names of their method arguments in
such a way that they don't clash with field names, so the use of
this
can largely be
avoided.
Finally, note that while instance methods can use the
this
keyword, class methods
cannot. This is because class methods are not associated with individual objects.
