Java Reference
In-Depth Information
An interface can inherit other interfaces using the
extends
keyword. Such an interface is
called a
subinterface
. For example,
NewInterface
in the following code is a subinterface of
Interface1
, . . . , and
InterfaceN
.
subinterface
public
interface
NewInterface
extends
Interface1, . . . , InterfaceN {
// constants and abstract methods
}
A class implementing
NewInterface
must implement the abstract methods defined in
NewInterface
,
Interface1
, . . . , and
InterfaceN
. An interface can extend other inter-
faces but not classes. A class can extend its superclass and implement multiple interfaces.
All classes share a single root, the
Object
class, but there is no single root for interfaces.
Like a class, an interface also defines a type. A variable of an interface type can reference any
instance of the class that implements the interface. If a class implements an interface, the
interface is like a superclass for the class. You can use an interface as a data type and cast a
variable of an interface type to its subclass, and vice versa. For example, suppose that
c
is an
instance of
Class2
in Figure 15.7.
c
is also an instance of
Object
,
Class1
,
Interface1
,
Interface1_1
,
Interface1_2
,
Interface2_1
, and
Interface2_2
.
Interface
1_2
Interface
2_2
Interface
1_1
Interface
1
Interface
2_1
Object
Class1
Class2
F
IGURE
15.7
Class1
implements
Interface1
;
Interface1
extends
Interface1_1
and
Interface1_2
.
Class2
extends
Class1
and implements
Interface2_1
and
Interface2_2
.
Note
Class names are nouns. Interface names may be adjectives or nouns.
naming convention
Design Guide
Abstract classes and interfaces can both be used to specify common behavior of objects.
How do you decide whether to use an interface or a class? In general, a
strong is-a rela-
tionship
that clearly describes a parent-child relationship should be modeled using
classes. For example, Gregorian calendar is a calendar, so the relationship between the
class
java.util.GregorianCalendar
and
java.util.Calendar
is modeled
using class inheritance. A
weak is-a relationship
, also known as an
is-kind-of relation-
ship
, indicates that an object possesses a certain property. A weak is-a relationship can
be modeled using interfaces. For example, all strings are comparable, so the
String
class implements the
Comparable
interface.
is-a relationship
is-kind-of relationship
In general, interfaces are preferred over abstract classes because an interface can
define a common supertype for unrelated classes. Interfaces are more flexible than
interface preferred
