Why an interface can't be defined in an inner class

According to JLS: 8.1.3 Inner Classes and Enclosing Instances, inner classes may not declare static initializers or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields.

According to 8.5.2 Static Member Type Declarations, "Member interfaces are always implicitly static. It is permitted but not required for the declaration of a member interface to explicitly list the static modifier". They are always top-level, not inner.

Therefore, an interface can't be defined in an inner class. The member interface can only be defined in inside a top-level class or interface. For example,

interface OuterInterface { // Top-level interface

  interface NestedInterface {// top-level nested interface 
  }

}

class OuterClass {
  
  /*Nested top-level classes are always defined with a static keyword*/
  static class NestedClass { 
    interface NestedInterface { } //OK
  }

  /*Inner class */
  class InnerClass { 

  /* This will cause an error. Because, you can not define a static modifier
      inside an inner class. InnerClass is an inner class and NestedInterface 
      is implicitly static.  An compile time error will occurs. 
  */ 

    interface NestedInterface { //compile time error, 
    }
  }
}

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