java - Why should I use an interface when there is only one implementation class? -

i'm new @ programming , i'm learning java. wondering why should use interface when there 1 implementation class?

you prevent others accessing implementing type. example, hide implementing type inside library, give type package access, , return instance of interface users of library:

// users of library know class // work: public interface someinterface {     void dosomethinguseful();     void dosomethingelse(); }  // class itself, hidden clients class myimplementation implements someinterface {     private somedependency dependency = new somedependency();     public void dosomethinguseful() {         ...     }     public void dosomethingelse() {         ...     } } 

your clients obtain objects this:

public class myfactory {     static someinterface make() {         // myfactory can see myimplementation         return new myimplementation();     } } 

this trick becomes useful when implementation uses lots of libraries. efficiently decouple interface of library implementation, user wouldn't have know dependencies internal library.