interface vs inner classes

[srinivas bolloju]

Which of the following statements is true?
a) An interface can contain a nested top-level inner class.
b) An interface can contain a member inner class.
c) A member inner class can implement an interface.
d) A static method can contain a local class.
e) A static method can contain a nested top-level class.
i have written a small program to illustrate above, all the conditions are true according to me, pls help....

interface intera
{
class A
{
//static class B
static public void m1(){
class B
{}
}
}
}

[zarina mohammad]

All the statements are true .
as a result the following code works

 
interface abc{}
interface intera
{
   class A implements abc{
   static void method(){
      class inner{}
       }
    }
}
 

[Dan Chisholm]

Originally posted by srinivas bolloju:
Which of the following statements is true?
a) An interface can contain a nested top-level inner class.
b) An interface can contain a member inner class.
c) A member inner class can implement an interface.
d) A static method can contain a local class.
e) A static method can contain a nested top-level class.
i have written a small program to illustrate above, all the conditions are true according to me, pls help....

interface intera
{
class A
{
//static class B
static public void m1(){
class B
{}
}
}
}

srinivas,
Based on your code example your conclusions are logical, but are not correct. You have assumed that class A is non-static because you did not provide a static modifier for the declaration of class A. However, all member type declarations within an interface a implicitly public and static. It is not possible to declare a non-static class within an interface.
The following code example demonstrates that a top-level nested class can be instantiated without first creating an instance of the enclosing class. However, a non-static member class can not be instantiated without first instantiating an instance of the enclosing class.

class D {
  static class E {
    public String toString() {return "E.A";}
  }
  class F {
    public String toString() {return "F.A";}
  }
}
class E {
  public static void main (String[] args) {
D.E e = new D.E();
D.F f = new D().new F();
System.out.print(e + "," + f);
  }
}

D.E is a static nested class and therefore may be instantiated without first creating an instance of class D. However, non-static member class F can not be instantiated without first creating an instance of D.
In your code example, an instance of nested class A can be instantiated without first creating an instance of interface intera. Of course, that must be the case because it is not possible to create an instance of an interface. Therefore, nested class A must be implicitly static.

interface intera {
  class A {
    //static class B
    static public void m1(){
      class B {}
    }
    public String toString() {
      return "intera.A";
    }
  }
}
class C {
  public static void main(String[] args) {
    // Instantiate top level nested class A.
    // Note: No instance of intera exists.
    intera.A a = new intera.A();
    System.out.print(a);
  }
}

[Dan Chisholm]

I must slightly modify my previous statement. Rather than say the following.

It is not possible to declare a non-static class within an interface.

I should qualify that statement by adding the adjective "member" to the word "class".

It is not possible to declare a non-static member class within an interface.

The above modification is necessary because a local class may be declared within a code block contained within an enclosing interface, and local classes are not static.

[Jose Botella]

easy to remember: because it is not possible to create an instance of an interface, nested classes within an interface are static.

[Ron Newman]

e) A static method can contain a nested top-level class.

A method (static or otherwise) can contain only a local class or an anonymous class. By definition such a class is not "top-level".