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Thinking in Java
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Referring to the outer class object

If you need to produce the reference to the outer class object, you name the outer class followed by a dot and this. For example, in the class Sequence.SSelector, any of its methods can produce the stored reference to the outer class Sequence by saying Sequence.this. The resulting reference is automatically the correct type. (This is known and checked at compile time, so there is no run-time overhead.)

Sometimes you want to tell some other object to create an object of one of its inner classes. To do this you must provide a reference to the other outer class object in the new expression, like this:

//: c08:Parcel11.java
// Creating instances of inner classes.

public class Parcel11 {
  class Contents {
    private int i = 11;
    public int value() { return i; }
  }
  class Destination {
    private String label;
    Destination(String whereTo) { label = whereTo; }
    String readLabel() { return label; }
  }
  public static void main(String[] args) {
    Parcel11 p = new Parcel11();
    // Must use instance of outer class
    // to create an instances of the inner class:
    Parcel11.Contents c = p.new Contents();
    Parcel11.Destination d = p.new Destination("Tanzania");
  }
} ///:~


To create an object of the inner class directly, you don’t follow the same form and refer to the outer class name Parcel11 as you might expect, but instead you must use an object of the outer class to make an object of the inner class:

Parcel11.Contents c = p.new Contents();


Thus, it’s not possible to create an object of the inner class unless you already have an object of the outer class. This is because the object of the inner class is quietly connected to the object of the outer class that it was made from. However, if you make a nested class (a static inner class), then it doesn’t need a reference to the outer class object.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire