OnJava8-Examples/patterns/SingletonPattern.java

// patterns/SingletonPattern.java
// Since this isn't inherited from a Cloneable
// base class and cloneability isn't added,
// making it final prevents cloneability from
// being added through inheritance:

final class Singleton {
  private static Singleton s = new Singleton(47);
  private int i;
  private Singleton(int x) { i = x; }
  public static Singleton getHandle() {
    return s;
  }
  public int getValue() { return i; }
  public void setValue(int x) { i = x; }
}

public class SingletonPattern {
  public static void main(String[] args) {
    Singleton s = Singleton.getHandle();
    System.out.println(s.getValue());
    Singleton s2 = Singleton.getHandle();
    s2.setValue(9);
    System.out.println(s.getValue());
    try {
      // Can't do this: compile-time error.
      // Singleton s3 = (Singleton)s2.clone();
    } catch(Exception e) {}
  }
}
/* Output:
47
9
*/