Java Reference
In-Depth Information
Because all your classes inherit the methods defined in the
Object
class I explain the most important
ones in a little more detail.
The toString() Method
You have already made extensive use of the
toString()
method, and you know that it is used by the com-
piler to obtain a
String
representation of an object when necessary. It is obvious now why you must always
declare the
toString()
method as
public
in a class. It is declared as such in the
Object
class and you can't
declare it as anything else.
You can see what the
toString()
method that is inherited from the
Object
class outputs for an object of
one of your classes by commenting out the
toString()
method in
Animal
class in the previous example. A
typical sample of the output for an object is:
Your choice:
Spaniel@b75778b2
It's Fido the Spaniel
Woof Woof
The second line here is generated by the
toString()
method implemented in the
Object
class. This is
inherited in the
Animal
class, and it is called because you no longer override it. The hexadecimal digits fol-
lowing the
@
in the output are the hashcode of the object, which is produced by the
hashcode()
method that
is inherited from
Object
.
Obviously, knowing that
toString(
) is inherited from
Object
in the
Animal
class, it would have been
appropriate to use the
@Override
annotation for the method in the
Animal
class.
Determining the Type of an Object
The
getClass()
method that all your classes inherit from
Object
returns an object of type
Class
that iden-
tifies the class of an object. Suppose you have a variable
pet
of type
Animal
that might contain a reference
to an object of type
Dog
,
Cat
,
Duck
, or even
Spaniel
. To figure out what sort of thing it really refers to, you
could write the following statements:
Class objectType = pet.getClass(); // Get the class type
System.out.println(objectType.getName()); // Output the class name
The method
getName()
is a member of the
Class
class, and it returns the fully qualified name of the
actual class of the object for which it is called as a
String
object. Thus, the second statement outputs the
name of the class for the
pet
object. If
pet
referred to a
Duck
object, this would output:
Duck
This is the fully qualified name in this case, as the class is in the default package, which has no name. For
a class defined in a named package, the class name would be prefixed with the package name. If you just
wanted to output the class identity, you need not explicitly store the
Class
object. You can combine both
statements into one:
System.out.println(pet.getClass().getName()); // Output the class name
