Questions on innerclass and interface

[Niral Trivedi]

All,
I have confusion on following codes.
1).

class Outer {
  class Inner { }
}
public class InheritInner extends Outer.Inner {
  InheritInner ()  { }
  public static void main(String [ ] args) {
    Outer o = new Outer( ) ;
    InheritInner ii = new InheritInner( ) ;
  }
}

This code is giving error for InheritInner constructor saying 'no enclosing instance of type Outer is in scope'. Can somebody explain me why?
2).

  
interface Solar {
String star = "Sun" , planet1 = "Earth" , planet2 = "Mars" ; int i = 9 ,j = 5 ;
}
interface Lunar{
String star = "Sun" , planet3 = "Venus" , planet4 = "Mercury" ;
}
public class MultiImplement implements Solar, Lunar {
public static void main(String array [ ]) {
System.out.println (star);// I know about this error because of ambiguous reference..
planet1 + planet3; // i know this is also an error as this is not valid statement...
System.out.println (planet1);
planet2 += planet4;// This is giving error saying 'can not assign value to a final variable planet2'
System.out.println (planet2);
}
}

Is any variable defined in an interface automatically become a final variable??
Please help me..
TIA.
Niral

[Ron Newman]

1) You can't call a constructor for an inner class from a static method (such as main) unless you explicitly provide an instance of the outer class:
InheritInner ii = o.new InheritInner( )
2) All variables defined in an interface are public, static, and final.

[Niral Trivedi]

Ron,
Actually InheritInner is not a nested class of Outer. It is just extending Outer.Inner. So, can I do that first of all?? I've tried though and got error.
Let me explain my situation here one more time..

  
class Outer {
  class Inner { }
}
public class InheritInner extends Outer.Inner {
  InheritInner ()  { } // Line 1
  public static void main(String [ ] args) {//Line2
    Outer o = new Outer( ) ;// Line 3
    InheritInner ii = new InheritInner( );//Line4 
  }
}

If I don't do, o.new InheritInner()at line 4 then I am getting error 'no enclosing instance of type Outer is in schope' for line 1.
If I put o.new InheritInner() at line 4 then I am getting error 'Can not resolve Symbol class InheritInner' for line 4 which I think right as InheritInner is not an inner class of Outer.
Any idea??
Niral

[Vin Kris]

When you extend an inner class, the default constructors are no longer created... or they no longer work. When you try to create a class of InheritInner, you are also trying to create the sub-object Inner (which is the super class). But since, Inner is an inner class, you need a refrence to the enclosing class object. This can be acheived by passing the reference of the Outer class to the constructor of InheritInner & explicitly invoking the constructor of Outer.
For the complete explanation, refer Bruce Eckel's Thinking in Java. Thats one hell of a book.

class Outer {
        class Inner {
        }
}
class InheritInner extends Outer.Inner {
        InheritInner(Outer o) {
                o.super(); // equivalent to o.new Inner();
        }
        public static void main(String [] args) {
                Outer o = new Outer();
                InheritInner ii = new InheritInner(o);
        }
}

[ September 12, 2002: Message edited by: Vin Kris ]

[Niral Trivedi]

Thanks Ron..
Niral