Inheriting from inner classes

Because the inner class constructor must attach to a reference of the enclosing class object, things are slightly complicated when you inherit from an inner class. The problem is that the “secret” reference to the enclosing class object must be initialized, and yet in the derived class there’s no longer a default object to attach to. The answer is to use a syntax provided to make the association explicit:

//: c08:InheritInner.java
// Inheriting an inner class.

class WithInner {
  class Inner {}
}

public class InheritInner extends WithInner.Inner {
  //! InheritInner() {} // Won't compile
  InheritInner(WithInner wi) {
    wi.super();
  }
  public static void main(String[] args) {
    WithInner wi = new WithInner();
    InheritInner ii = new InheritInner(wi);
  }
} ///:~

You can see that InheritInner is extending only the inner class, not the outer one. But when it comes time to create a constructor, the default one is no good, and you can’t just pass a reference to an enclosing object. In addition, you must use the syntax

enclosingClassReference.super();

inside the constructor. This provides the necessary reference, and the program will then compile.

Thinking in Java
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