Question

In Effective Java inside the item “Item 22: Favor static member classes over nonstatic” Josh Bloch says:

Each instance of a nonstatic member class is implicitly associated with an
enclosing instance of its containing class. Within instance methods of a nonstatic
member class, you can invoke methods on the enclosing instance or obtain a reference
to the enclosing instance using the qualified this construct.

What does he mean by Qualified This Construct?

Answer 1

Without the qualifier, x() would recurse. With the qualifier, the enclosing instance’s x() method is invoked instead.

class Envelope {
  void x() {
    System.out.println("Hello");
  }
  class Enclosure {
    void x() {
      Envelope.this.x(); /* Qualified*/
    }
  }
}