Defining an Inner Class inside an Interface

Any class that is defined within an Interface is set to be 'public static' by JVM, so the modifier 'static' is not necessary. A static member class is just like another class outside the OuterClass.

Defining a class inside an Interface is purely for the code readability purpose, there is no specially techniques here. For example, if a class is only used by one Interface, it helps the code readability by defining the class inside this Interface.

------------Java-------------------

public interface FunBoy {

int NUM = 3;

class Boy{}

}

-------------Bytecode---------------

public interface FunBoy

  SourceFile: "FunBoy.java"

  InnerClasses:

       public static #14= #12 of #1; //Boy=class FunBoy$Boy of class FunBoy

  minor version: 0

  major version: 50

  flags: ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT

Constant pool:

   #1 = Class              #2             //  FunBoy

   #2 = Utf8               FunBoy

   #3 = Class              #4             //  java/lang/Object

   #4 = Utf8               java/lang/Object

   #5 = Utf8               NUM

   #6 = Utf8               I

   #7 = Utf8               ConstantValue

   #8 = Integer            3

   #9 = Utf8               SourceFile

  #10 = Utf8               FunBoy.java

  #11 = Utf8               InnerClasses

  #12 = Class              #13            //  FunBoy$Boy

  #13 = Utf8               FunBoy$Boy

  #14 = Utf8               Boy

{

  public static final int NUM;

    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL

    ConstantValue: int 3

}