How can I assemble bits into a long to create a unique ID?

long getUniqueID()
{
    long timestamp = System.currentTimeMillis();
    long random = some random long

    ...

    return id;
}

return java.util.UUID.randomUUID().getLeastSignificantBits();

public class UniqueID {
  static long current= System.currentTimeMillis();
  static public synchronized long get(){
    return current++;
    }
}