Can a class extend a inner class?

[Reguri Praveen]

1. class Outer {
2. class Inner { }
3. }
4.
5. public class InheritInner extends Outer.Inner {
6. InheritInner () { }
7.
8. public static void main(String [ ] args) {
9. Outer o = new Outer( ) ;
10. InheritInner ii = new InheritInner( ) ;
11. }

The above code is from http://www.akgupta.com/Java/mock_exam.htm mock exam
Qno 38

It throws a error at compile time saying that
InheritInner.java:7: an enclosing instance that contains Outer.Inner is required
InheritInner () {


My question is can we inherit a inner class? If we can then why need the concept of inner class?

Thanks in advance for the help

[Keith Lynn]

An inner class is normally only used with it's outer class. You can create instances of it in other classes by using an instance of the outer class. You can extend the inner class in other inner classes in the same outer class.

[marc weber]

An inner class (a nested class that is not static) is tied to an instance of its enclosing class.

However, note that inner classes can inherit independently from the outer class. When a base class is extended and its inner classes are re-declared in the subclass (similar to a shadowed variable), these nested classes do not automatically inherit from the derived class.

class Outer1 {
  class Inner1 {
    void meth() {
      System.out.println("Inner1 meth.");
    }
  }
}
 
class Outer2 extends Outer1 {
  class Inner2 extends Inner1 {}
}
 
class Outer3 extends Outer1 {
  class Inner1 {} //NOT the same as Outer1.Inner1
}
 
class Outer4 extends Outer1 {}
 
class TesterOuter { 
  public static void main(String[] args) {
    Outer2.Inner2 o2i2 = new Outer2().new Inner2();
    //Inner2 extends Inner1, so we have the method...
    o2i2.meth();
    
    Outer3.Inner1 o3i1 = new Outer3().new Inner1();
    //Outer3 extends Outer1, but Inner1 is not the same...
    //o3i1.meth(); //cannot find symbol
    
    Outer4.Inner1 o4i1 = new Outer4().new Inner1();
    //Inner1 NOT re-declared in Outer4, so Inner1 IS inherited...
    o4i1.meth();
  }
}

[ February 26, 2006: Message edited by: marc weber ]