Inner class - Using .this and .new

To produce the reference to the outer-class object, we name the outer class followed by a dot and this. The resulting reference is automatically the correct type, known and checked at compile time, so there is no runtime overhead. Here’s how to use .this:

// innerclasses/DotThis.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Accessing the outer-class object

public class DotThis {
  void f() {
    System.out.println("DotThis.f()");
  }

  public class Inner {
    public DotThis outer() {
      return DotThis.this;
      // A plain "this" would be Inner's "this"
    }
  }

  public Inner inner() {
    return new Inner();
  }

  public static void main(String[] args) {
    DotThis dt = new DotThis();
    System.out.println("dt:" + dt);
    DotThis.Inner dti = dt.inner();
    System.out.println("dti.outer():" + dti.outer()); // output the same dt
    dti.outer().f();
  }
}
/* My Ouput:
dt:DotThis@6d06d69c
dti.outer():DotThis@6d06d69c
DotThis.f()
*/

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

// innerclasses/Parcel3.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Using .new to create instances of inner classes

public class Parcel3 {
  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) {
    Parcel3 p = new Parcel3();
    // Must use instance of outer class
    // to create an instance of the inner class:
    Parcel3.Contents c = p.new Contents();
    Parcel3.Destination d = p.new Destination("Tasmania");
  }
}

It’s not possible to create an object of the inner class unless we 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 it was made from. However, if we make a nested class (a static inner class), it doesn’t need a reference to the outer-class object.

for better understanding this, please read here.

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/innerclasses/DotThis.java

3. https://github.com/wangbingfeng/OnJava8-Examples/tree/master/innerclasses