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An interface is an abstraction of class with no implementation details. For example,
java.lang.Comparable is a standard interface in Java. You cannot instantiate an interface. An interface is not a class but it is written the same way. The first difference is that you do not use the
class keyword but the
interface keyword to define it. Then, there are fields and methods you cannot define here:
- A field is always a constant: it is always public, static and final, even if you do not mention it.
- A method must be public and abstract, but it is not required to write the
- Constructors are forbidden.
An interface represents a contract:
|Code listing 4.14: SimpleInterface.java
You can see that the
method1() method is abstract (unimplemented). To use an interface, you have to define a class that implements it, using the
|Code listing 4.15: ClassWithInterface.java
A class can implement several interface, separated by a comma. Java interfaces behave much like the concept of the Objective-C protocol. It is recommended to name an interface
<verb>able, to mean the type of action this interface would enable on a class. However, it is not recommended to start the name of an interface by
I as in C++. It is useless. Your IDE will help you instead.
If you have objects from different classes that do not have common superclasses, you can't call a same method on them, even if the two classes implement a method with the same signature.
|Code listing 4.16: OneClass.java
|Code listing 4.17: AnotherClass.java
|Code section 4.16: Impossible call.
The solution is to write an interface that defines the method that should be implemented in the two classes as the
SimpleInterface in the Code listing 4.14 and then the both class implement the interface as in the Code listing 4.15.
|Code section 4.17: Interface use.
You can also have this interest using a common super class but a class can only inherit from one super class whereas it can implement several interfaces. Java does not support full orthogonal multiple inheritance. Java does not allow you to create a subclass from two classes. Multiple inheritance in C++ has complicated rules to disambiguate fields and methods inherited from multiple superclasses and types inherited multiple times. By separating interface from implementation, interfaces offer much of the benefit of multiple inheritance with less complexity and ambiguity. The price of no multiple inheritance is some code redundancy; since interfaces only define the signature of a class but cannot contain any implementation, every class inheriting an interface must provide the implementation of the defined methods, unlike in pure multiple inheritance, where the implementation is also inherited. The major benefit of that is that all Java objects can have a common ancestor. That class is called
Object. When overriding methods defined in interfaces there are several rules to be followed:
- Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
- The signature of the interface method and the same return type or subtype should be maintained when implementing the methods.
- All the methods of the interface need to be defined in the class, unless the class that implements the interface is abstract.
An interface can extend several interfaces, similar to the way that a class can extend another class, using the
|Code listing 4.18: InterfaceA.java
|Code listing 4.19: InterfaceB.java
|Code listing 4.20: InterfaceAB.java
This way, a class implementing the
InterfaceAB interface has to implement the
methodB() and the
|Code listing 4.21: ClassAB.java
Doing so, a
ClassAB object can be casted into