Interface extends another interface but implements its methods

Question

In java when an interface extends another interface    Why does it implement its methods  How can it implement its methods when an interface can t contain a method body How can it implement the methods when it extends the other interface and not implement it  What is the purpose of an interface implementing another interface    This has major concepts in Java   EDIT     public interface FiresDragEvents      void addDragHandler DragHandler handler      void removeDragHandler DragHandler handler       public interface DragController extends FiresDragEvents      void addDragHandler DragHandler handler      void removeDragHandler DragHandler handler      void dragEnd       void dragMove        In eclipse there is the implement sign besides the implemented methods in DragController   And when I mouse-hover it  it says that it implements the method

User · Answer

ad 1  It does not implement its methods   ad 4  The purpose of one interface extending  not implementing another  is to build a more specific interface  For example  SortedMap is an interface that extends Map  A client not interested in the sorting aspect can code against Map and handle all the instances of for example TreeMap  which implements SortedMap  At the same time  another client interested in the sorted aspect can use those same instances through the SortedMap interface   In your example you are repeating the methods from the superinterface  While legal  it s unnecessary and doesn t change anything in the end result  The compiled code will be exactly the same whether these methods are there or not  Whatever Eclipse s hover says is irrelevant to the basic truth that an interface does not implement anything

User · Answer

Why does it implement its methods  How can it implement its methods   when an interface can t contain method body  How can it implement the   methods when it extends the other interface and not implement it  What   is the purpose of an interface implementing another interface    Interface does not implement the methods of another interface but just extends them  One example where the interface extension is needed is  consider that you have a vehicle interface with two methods moveForward and moveBack but also you need to incorporate the Aircraft which is a vehicle but with some addition methods like moveUp  moveDown so in the end you have   public interface IVehicle     bool moveForward int x     bool moveBack int x        and airplane   public interface IAirplane extends IVehicle     bool moveDown int x     bool moveUp int x

User · Answer

An interface defines behavior  For example  a Vehicle interface might define the move   method   A Car is a Vehicle  but has additional behavior  For example  the Car interface might define the startEngine   method  Since a Car is also a Vehicle  the Car interface extends the Vehicle interface  and thus defines two methods  move    inherited  and startEngine     The Car interface doesn t have any method implementation  If you create a class  Volkswagen  that implements Car  it will have to provide implementations for all the methods of its interface  move   and startEngine     An interface may not implement any other interface  It can only extend it

[java] Interface extends another interface but implements its methods

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