9

C H A P T E R

Interfaces

My apple trees will never get across

And eat the cones under his pines, I tell him.

He only says  Good Fences Make Good Neighbors. 

 Robert Frost,

Mending Wall

 (1914)

A

N interface declaration introduces a new reference type whose members are

constants and abstract methods. This type has no implementation, but otherwise

unrelated classes can implement it by providing implementations for its abstract

methods.

Java programs can use interfaces to make it unnecessary for related classes to

share a common abstract superclass or to add methods to

Object

.

An interface may be declared to be an

direct extension

of one or more other

interfaces, meaning that it implicitly specifies all the abstract methods and con 

stants of the interfaces it extends, except for any constants that it may hide.

A class may be declared to

directly implement

 one or more interfaces, mean 

ing that any instance of the class implements all the abstract methods specified by

the interface or interfaces. A class necessarily implements all the interfaces that its

direct superclasses and direct superinterfaces do. This (multiple) interface inherit 

ance allows objects to support (multiple) common behaviors without sharing any

implementation.

A variable whose declared type is an interface type may have as its value a

reference to any object that is an instance of a class declared to implement the

specified interface. It is not sufficient that the class happen to implement all the

abstract methods of the interface; the class or one of its superclasses must actually

be declared to implement the interface, or else the class is not considered to imple 

ment the interface.

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