public final T clear()
  {
    T hold;
    _lock.lock();
    try
    {
      hold = _val;
      _val = null;
    } finally {
      _lock.unlock();
    }
    return hold;
  }
public final T clear ( ) { T hold ; _lock . lock ( ) ; try { hold = _val ; _val = null ; } finally { _lock . unlock ( ) ; } return hold ; }
Clears the variable, forcing the next call to {@link #get()} to re-calculate
the value.