/**
 * Public constructor singleton
 */
public class Singleton {
  private static Singleton instance = null;

  public static Singleton getInstance() {
    if (null == instance) {
      instance = new Singleton();
    }
    return instance;
  }

  public Singleton() {
    if (instance != null) {
      throw new RuntimeException("Sorry this is a singleton");
    }
    instance = this;
  }
}