Java Reference
In-Depth Information
private String type;
}
Directory "TestDerived"
This has a member,
type
, to identify the type of animal, and its value is set by the constructor. It is
private
and is therefore not inherited in a class derived from
Animal
. You also have a
toString()
method
for the class to generate a string representation of an object of the class.
You can now define another class, based on the class
Animal
, to define dogs. You can do this immedi-
ately, without affecting the definition of the class
Animal
. You could write the basic definition of the class
Dog
as:
public class Dog extends Animal {
// constructors for a Dog object
protected String name; // Name of a Dog
protected String breed; // Dog breed
}
You use the keyword
extends
in the definition of a subclass to identify the name of the direct superclass.
The class
Dog
inherits only the method
toString()
from the class
Animal
, because the
private
data mem-
ber and the constructor cannot be inherited. Of course, a
Dog
object has a
type
data member that needs to
be set to
"Dog"
; it just can't be accessed by methods that you define in the
Dog
class. You have added two
new instance variables in the derived class. The
name
member holds the name of the particular dog, and the
breed
member records the kind of dog it is. These are both
protected
and therefore are accessible in any
class derived from
Dog
. All you need to add is the means of creating
Dog
class objects.
Derived Class Constructors
You can define two constructors for the subclass
Dog
, one that just accepts an argument for the name of a
dog and another that accepts both a name and the breed of the
Dog
object. For any derived class object, you
need to make sure that the
private
base class member,
type
, is properly initialized. You do this by calling
a base class constructor from the derived class constructor:
public class Dog extends Animal {
public Dog(String aName) {
super("Dog");
// Call the base constructor
name = aName;
// Supplied name
breed = "Unknown";
// Default breed value
}
public Dog(String aName, String aBreed) {
super("Dog");
// Call the base constructor
name = aName;
// Supplied name
breed = aBreed;
// Supplied breed
