Inner classes and interfaces?

[Edwin Dalorzo]

There is a principle that says the "non-static inner classes cannot have static members". And there is another principle that says "interfaces are static by default".

That made me wonder if it was possible that an inner non-static class have an interface declaration. Strangely the code you see below compiled in JDeveloper 10g (Preview Edition), but not in the command line... at the end the JDeveloper compiler threw a compiler internal error.

But, what can I learn from all this? Is it, or is it not possible to declare an interface in a non-static inner class?

class A
{
  int aNumber = 20;
  
  public class A1
  {
    public interface A2 //Command line compiler error
    {
      double PI=Math.PI;
      void printPI();
    }
    
    public class A3
    {
      int aNumber = 30;
      public void myNumberIs()
      {
        System.out.println(A.this.aNumber);
      }
    }
    
    public void printMe()
    {
      System.out.println("I am A1");
    }
  }
}
 
public class Principal 
{
  public static void main(String args[])
  {
    A.A1 a1 = new A().new A1();
    a1.printMe();
  }
  
  public void doSample()
  {
    A.A1.A3 a3 = new A().new A1().new A3() //Jdeveloper compiler internal error
    {
    };
  }
}

[Jeroen Wenting]

interfaces are NOT static by default.
ONLY data members in interfaces are static, methods declared in interfaces are NEVER static.

So you can declare an inner class to implement any interface that doesn't have data members.

[Tony Morris]

interfaces are static by default

Your experiment is on a false premise.
Would you care to share with us where you got this idea from?
Perhaps that interface members that are not methods (fields, and inner classes) are implicitly static?

[Bilal Al-Sallakh]

Originally posted by Jeroen Wenting:
interfaces are NOT static by default.

Edwin means that memeber interfaces are implicitly static and I think this is right.

[Jeroen Wenting]

no, an interface is never static.
An interface may contain static members, but only if those members are data members and not methods.

If an interface were implicitly static you could not implement their methods in classes implementing their methods unless those methods were static...

[Jim Yingst]

Bilal is correct. From JLS2 8.5.2: "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." We're not saying that the methods in the interface are static (which is false), or that fields in an interface are static (which is true). We're saying that the nested interface itself is static. Static in the sense of "static nested class" (or interface, rather).

[Edwin]: There is a principle that says the "non-static inner classes cannot have static members".

Inner classes cannot declare static members (where members includes methods, fields, and nested types). They can however inherit static members from a superclass (or superinterface).

And there is another principle that says "interfaces are static by default".

Nested interfaces are implicitly static. It's not just the default for nested interfaces - they're static, period, and we can't possibly change that.

That made me wonder if it was possible that an inner non-static class have an interface declaration. Strangely the code you see below compiled in JDeveloper 10g (Preview Edition), but not in the command line... at the end the JDeveloper compiler threw a compiler internal error.

But, what can I learn from all this? Is it, or is it not possible to declare an interface in a non-static inner class?

No, it isn't. Sounds like this was a bug in JDeveloper.
[ January 30, 2005: Message edited by: Jim Yingst ]

[Jeroen Wenting]

ah of course, you're talking about interfaces declared as members of a class, not members declared in an interface