The Singleton Design Pattern makes sure that at any point of time, there is one and only one instance of a class present and also provides a global point of access to the object.

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!Examples:

[Asser]: Here is a simple __Java__ example that illustrates the usage of
Singleton with a class fields. 
{{{
public class SingletonExample {

  /** Singleton field that gets initialized only once if it exists **/
  private String m_uniqueField;

  /**
   * Returns the content of the singleton field. The content
   * gets initialized if it is empty.
   *
   * @return Millisecond time of the previous initialization (ALWAYS not null)
   */
  public String getField() {
    // Please notice that we are using the monitor of the instance
    // itself, which means that all other methods gets synchronized
    // as well.
    synchronized (this) {
      if (m_uniqueField == null) {
         // Compute field
         m_uniqueField = String.valueOf(System.currentTimeMillis());
      }
      return m_uniqueField;
    }
  }

  /**
   * Clear the content of the singleton field.
   */
  public void clear() {
    m_uniqueField = null;
  }

}
}}}

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__question__: How does this guarantee we're getting a unique response? Nothing prevents me from doing
{{{
a = new SingletonExample();
b = new SingletonExample();
}}}
and getting different values from getField() (well, if we were looking at
anything else than the current time...). Isn't a private constructor sort
of a requirement of the singleton pattern, to guarantee one object instance?

(Delete this question if I'm off. -- [ebu])

No, I think you're correct.  You must have a private constructor, then use a static {{getInstance}}-method for getting a singleton "factory", if you will. --[Janne|JanneJalkanen]
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A few comments:
* It should be: {{public final class SingletonExample}} so I can't override its singleton behaviour adding another constructor. 
* Use ThreadLocal if you need a Singleton per thread.
* Another way to achieve lazy initialization without {{synchronized}}:
{{{public final class SingletonExample {

  private static class LazySingletonHolder {
    public static String m_uniqueField = String.valueOf(System.currentTimeMillis());
  }

  public static Singleton getField() {
    return LazySingletonHolder.m_uniqueField;
  }

  ...

} }}}
--Jano