CHAPTER 9
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)
An 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 constants 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, meaning 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 inheritance 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 implement the interface.