Nested classes (static inner classes) : Nested Classes « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Nested Classes

public class MainClass {
  private static class ClassA implements A {
    private int i = 11;

    public int value() {
      return i;
    }
  }

  protected static class ClassB implements B {
    private String label;

    private ClassB(String whereTo) {
      label = whereTo;
    }

    public String readLabel() {
      return label;
    }

    // Nested classes can contain other static elements:
    public static void f() {
    }

    static int x = 10;

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

      static int x = 10;
    }
  }

  public static B dest(String s) {
    return new ClassB(s);
  }

  public static A cont() {
    return new ClassA();
  }

  public static void main(String[] args) {
    A c = cont();
    B d = dest("A");
  }
}

interface A {
  int value();
}

interface B {
  String readLabel();
}