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class MySingleton {
  private static MySingleton _instance;

  private MySingleton() {
    // construct object . . .
    // private constructor prevents instantiation by outside callers
  }

  // lazy initialization
  // error, no synchronization on method access

  public static MySingleton getInstance() {
    if (_instance == null) {
     _instance = new MySingleton();
    }
    return _instance;
  }

  // Remainder of class definition . . .
}

// Also an error, synchronization does not prevent
// two calls of constructor.
public static MySingleton getInstance() {
 if (_instance == null) {
   synchronized (MySingleton.class) {
      _instance = new MySingleton();
   }
 }
 return _instance;
}

class MySingleton {

  private static MySingleton _instance;

  private MySingleton() {
    // construct object . . .
    // private constructor prevents instantiation by outside callers
  }

   // lazy initialization
   public static synchronized MySingleton getInstance() {
     if (_instance == null) {
       _instance = new MySingleton();
     }
     return _instance;
   }
   // Remainder of class definition . . .
}

// double-checked locking
public static MySingleton getInstance() {
 if (_instance == null) {
   synchronized (MySingleton.class) {
     if (_instance == null) {
        _instance = new MySingleton();
     }
   }
 }
}

class MySingleton {
  private static MySingleton _instance;

  private MySingleton() {
    // construct object . .
    //  private constructor prevents instantiation by outside callers
  }

  // lazy initialization
  public static synchronized MySingleton getInstance() {
    if (_instance == null) {
      _instance = new MySingleton();
    }
    return _instance;
  }

  public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }

  // Remainder of class definition . . .
}

public class ObjectPreserver implements Runnable {
  private static ObjectPreserver lifeLine = new ObjectPreserver();
  // Neither this class, nor HashSet will be garbage collected.
  // References from HashSet to other objects will also exhibit this property
  private static HashSet protectedSet = new HashSet();
  private ObjectPreserver() {
    new Thread(this).start();  // keeps the reference alive
  }
  public synchronized void run(){
  try {
    wait();
  }
  catch(InterruptedException e) { e.printStackTrace(); }
}

  // Objects passed to this method will be preserved until
  // the unpreserveObject method is called
  public static void preserveObject(Object o) {
    protectedSet.add(o);
  }

  // Unprotect the objects so that they can be garbage collected
  public static void unpreserveObject(Object o) {
     protectedSet.remove(o);
  }
}