Java Reference
In-Depth Information
Object-oriented programming allows you to define new classes from existing classes.
This is called inheritance.
Key
Point
As discussed earlier in the topic, the procedural paradigm focuses on designing methods and
the object-oriented paradigm couples data and methods together into objects. Software design
using the object-oriented paradigm focuses on objects and operations on objects. The object-
oriented approach combines the power of the procedural paradigm with an added dimension
that integrates data with operations into objects.
Inheritance
is an important and powerful feature for reusing software. Suppose you need
to define classes to model circles, rectangles, and triangles. These classes have many common
features. What is the best way to design these classes so as to avoid redundancy and make the
system easy to comprehend and easy to maintain? The answer is to use inheritance.
inheritance
why inheritance?
Inheritance enables you to define a general class (i.e., a superclass) and later extend it
to more specialized classes (i.e., subclasses).
Key
Point
You use a class to model objects of the same type. Different classes may have some com-
mon properties and behaviors, which can be generalized in a class that can be shared by other
classes. You can define a specialized class that extends the generalized class. The specialized
classes inherit the properties and methods from the general class.
Consider geometric objects. Suppose you want to design the classes to model geometric
objects such as circles and rectangles. Geometric objects have many common properties and
behaviors. They can be drawn in a certain color and be filled or unfilled. Thus a general class
GeometricObject
can be used to model all geometric objects. This class contains the proper-
ties
color
and
filled
and their appropriate getter and setter methods. Assume that this class
also contains the
dateCreated
property and the
getDateCreated()
and
toString()
methods. The
toString()
method returns a string representation of the object. Since a circle
is a special type of geometric object, it shares common properties and methods with other
geometric objects. Thus it makes sense to define the
Circle
class that extends the
Geomet-
ricObject
class. Likewise,
Rectangle
can also be defined as a subclass of
GeometricOb-
ject
. Figure 11.1 shows the relationship among these classes. A triangular arrow pointing to
the superclass is used to denote the inheritance relationship between the two classes involved.
In Java terminology, a class
C1
extended from another class
C2
is called a
subclass
, and
C2
is called a
superclass
. A superclass is also referred to as a
parent class
or a
base class
, and a
subclass as a
child class
, an
extended class
, or a
derived class
. A subclass inherits accessible
data fields and methods from its superclass and may also add new data fields and methods.
The
Circle
class inherits all accessible data fields and methods from the
GeometricObject
class. In addition, it has a new data field,
radius
, and its associated getter and setter methods.
The
Circle
class also contains the
getArea()
,
getPerimeter()
, and
getDiameter()
methods for returning the area, perimeter, and diameter of the circle.
The
Rectangle
class inherits all accessible data fields and methods from the
Geomet-
ricObject
class. In addition, it has the data fields
width
and
height
and their associated
getter and setter methods. It also contains the
getArea()
and
getPerimeter()
methods for
returning the area and perimeter of the rectangle.
The
GeometricObject
,
Circle
, and
Rectangle
classes are shown in Listings 11.1,
11.2, and 11.3.
VideoNote
Geometric class hierarchy
subclass
superclass
Note
To avoid a naming conflict with the improved
GeometricObject
,
Circle
,
and
Rectangle
classes introduced in Chapter 13, we'll name these classes
avoid naming conflicts
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