CS 342 - 4/22/14

Design Patterns - use of a shared vocalubary to ease 
   communication between people

Patterns aren't invented they are discovered.

Most patterns and principles address issues of change 
    in software.

Singleton Design Pattern (Chapter 5 in HFDP)

  A design in which only a single instance of the
     object will be created/exist

Standard code features of this design pattern

public class MySingleton
{
  private static MySingleton instance = null;

  private int value;

  private MySingleton()
    {
    }

  public static MySingleton getInstance()
    {
      if (instance == null)
          instance = new MySingleton();

      return instance;
    }

   // other methods as needed
}

----------
// in method1
   MySingelton s1 = MySingleton.getInstance();
         s1.setValue (5);

// in method2
   MySingelton s2 = MySingleton.getInstance();
         int x = s2.getValue();


Assume method1 and method2 are in different threads
 and the data collision occurs, then multiple instances
 of MySingleton could be created

So we need to account for this issue

Solution 2:

   Change the static data member of instance to
    not be set to null, but invoke the constructor

public class MySingleton2
{
  private static MySingleton2 instance 
                   = new MySingleton2();

  private int value;

  private MySingleton2()
    {
    }

  public static MySingleton2 getInstance()
    {
      return instance;
    }

   // other methods as needed
}

Solution 3:

   Use the Synchronized keyword to make sure the
      constructor executes in a thread-safe manner

public class MySingleton3
{
  private volatile static MySingleton3 instance 
                   = null;

  private int value;

  private MySingleton3()
    {
    }

  public static synchronized MySingleton3 getInstance()
    {
      if (instance == null)
          instance = new MySingleton3();

      return instance;
    }

   // other methods as needed
}


Soultion 4:   

     Restrict the use of synchronized to the portion
       of getInstance() that actually calls the
        constructor

   public static MySingleton3 getInstance()
    {
      if (instance == null)
        {
          synchronized (MySinglton3.class)
             {
              if (instance == null)
                {
                  instance = new MySingleton3();
                }
              }
         }

      return instance;
    }