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Interfaces

The interface as a method contract in Java: implements, several interfaces for one class, interface-typed variable, choosing between an interface and an abstract class, default method.

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  • Interface: what is it?

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    A list of method signatures that a class commits to provide, without imposing how and without requiring a family link.

    It describes what an object can do, not what it is. A variable of its type can refer to any object whose class fulfils it.

  • This class is rejected with Plane is not abstract and does not override abstract method land() in Flyer. Which fix is right?

    ```java
    interface Flyer {
    void takeOff();
    void land();
    }

    class Plane implements Flyer {
    public void takeOff() { System.out.println("runway"); }
    }
    `

    • Add public void land() { … } to Plane
    • Add void land() { … } without public to Plane
    • Add extends Flyer to the Plane class
    • Declare land() as private in Plane
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    Add public void land() { … } to Plane

    To implement is to provide every method of the contract, public as in the interface. Without public, or with private, visibility is weaker: rejected. And extends does not apply to an interface from a class.

  • A class declares that it fulfils a contract with …; it can fulfil …, separated by commas.

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    A class declares that it fulfils a contract with implements; it can fulfil several, separated by commas.

    That is how single inheritance is worked around: class Duck extends Animal implements Flyer, Swimmer takes one class and two contracts.

  • Contract: what is it?

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    A class's commitment to those who use it: the methods it provides, whatever the way it carries them out.

    Code that uses the contract depends only on it, not on the concrete classes: one can be changed without touching the other.

  • A method launch(Flyer f) must accept Bird, which already inherits from Animal, Plane, which inherits from Machine, and Drone. What should Flyer be?

    • An interface that the three classes implement
    • An abstract class that the three classes inherit from
    • A class that inherits from both Animal and Machine
    • Nothing: write three launch methods with no link between them
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    An interface that the three classes implement

    Bird and Plane already have a parent class and Java allows only one extends. An interface, which can be combined, gives the shared capability to unrelated classes, and a single launch suffices.

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