inner classes - nested classes

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When we make the inner class static. This is commonly called a nested class. An ordinary inner class implicitly keeps a reference to the object of the enclosing class that created it. This is not the case for a static inner class.

A nested class means:

• We don’t need an outer-class object to create an object of a nested class.
• We can’t access a non- static outer-class object from an object of a nested class.

The difference between ordinary inner classes and nested classes.

• Fields and methods in ordinary inner classes can only be at the outer level of a class, so ordinary inner classes cannot have static data, static fields, or nested classes. However, nested classes can have all these.
• An ordinary (non- static) inner class can create a link to the outer-class object using a special this reference. A nested class does not have a special this reference, which makes it analogous to a static method.
• A nested class can be part of an interface.

// innerclasses/Parcel11.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.
// Nested classes (static inner classes)

public class Parcel11 {
  private static class ParcelContents implements Contents {
    private int i = 11;

    @Override
    public int value() {
      System.out.println("ParcelContents i :" + i);
      return i;
    }
  }

  protected static final class ParcelDestination implements Destination {
    private String label;

    private ParcelDestination(String whereTo) {
      System.out.println("ParcelDestination whereTo:" + whereTo);
      label = whereTo;
    }

    @Override
    public String readLabel() {
      return label;
    }
    // Nested classes can contain other static elements:
    public static void f() {}

    static int x = 10;

    static class AnotherLevel {
      public static void f() {}

      static int x = 10;
    }
  }

  public static Destination destination(String s) {
    return new ParcelDestination(s);
  }

  public static Contents contents() {
    return new ParcelContents();
  }

  public static void main(String[] args) {
    Contents c = contents();
    Destination d = destination("Tasmania");
    c.value();
  }
}
/*
My Output:
ParcelDestination whereTo:Tasmania
ParcelContents i :11
*/

  

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/innerclasses/Parcel11.java