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Use System time to generate unique IDTag(s): Date and Time Date and Time Varia


Since the granulaty of a PC can be as high as 55ms (down to 10ms), you can't use the System time to generate a unique ID because of the risk of getting duplicated IDs. This can be solved by using the following technique to make sure that the number returned is unique (in a single JVM). 
public class UniqueID {
  static long current= System.currentTimeMillis();
  static public synchronized long get(){
    return current++;
    }
}

See also this HowTo