Interfaces

Intro

• An interface defines a contract for what a class can do without specifying how.

Declaration

• All interface methods are implicitly public and abstract
• Even if some modifiers are left out, the above modifiers will be applied.
• Since they have to be overriden, methods cannot be static.
• Similarly they cannot be final, strictfp, or native.

• All variables are implicitly public, static and final
• Even if some modifiers are left out, the above modifiers will be applied.
• Since variables are final, they must be initialized

• If no access modifier is specified, the member is public, not package !
• Similarly, specifying protected/private is not allowed for methods and variables
• Watch out when classes implement interfaces and don't specify the method as public - this wont compile.
• Similarly, watch out when classes try to reassign the value of an interface.

• Note, the interface itself can be public OR package accessible, but members get default public access as described above.
• If the interface is package protected, then a class outside its package cannot access it.

public interface Mashable {
   public int COUNT; //illegal not initialized !
   int MINCOUNT = 3; //this is public, static and final implcitly.
   public static final int MAXCOUNT=10; //full correct declaration

   void foo(); //Implicity: public abstract void foo();
   static void foo(); //Illegal;
}

• An interface can extend (NOT implement) one or more interfaces.