Java Reference
In-Depth Information
// Methods to operate on the instance field
public
double
area
()
{
return
PI
*
r
*
r
;
}
public
double
circumference
()
{
return
2
*
PI
*
r
;
}
}
We have defined the
Circle
class within a package named
shapes
. Because
r
is
pro
tected
, any other classes in the
shapes
package have direct access to that field and
can set it however they like. The assumption here is that all classes within the
shapes
package were written by the same author or a closely cooperating group of
authors and that the classes all trust each other not to abuse their privileged level of
access to each other's implementation details.
Finally, the code that enforces the restriction against negative radius values is itself
placed within a
protected
method,
checkRadius()
. Although users of the
Circle
class cannot call this method, subclasses of the class can call it and even override it if
they want to change the restrictions on the radius.
It is a common convention in Java that data accessor methods
begin with the prefixes “get” and “set.” But if the field being
accessed is of type
boolean
, the
get()
method may be
replaced with an equivalent method that begins with “is.” For
example, the accessor method for a
boolean
field named
read
able
is
typically
called
isReadable()
instead
of
getReadable()
.
Abstract Classes and Methods
In
Example 3-4
, we declared our
Circle
class to be part of a package named
shapes
.
Suppose we plan to implement a number of shape classes:
Rectangle
,
Square
,
Ellipse
,
Triangle
, and so on. We can give these shape classes our two basic
area()
and
circumference()
methods. Now, to make it easy to work with an array of
shapes, it would be helpful if all our shape classes had a common superclass,
Shape
.
If we structure our class hierarchy this way, every shape object, regardless of the
actual type of shape it represents, can be assigned to variables, fields, or array ele‐
ments of type
Shape
. We want the
Shape
class to encapsulate whatever features all
our shapes have in common (e.g., the
area()
and
circumference()
methods). But
our generic
Shape
class doesn't represent any real kind of shape, so it cannot define
useful implementations of the methods. Java handles this situation with
abstract
methods
.
Java lets us define a method without implementing it by declaring the method with
the
abstract
modifier. An
abstract
method has no body; it simply has a signature
