code: private static void checkPreconditions(final String trueValue, final String falseValue) {
  		if( trueValue == null ) {
  			throw new NullPointerException("trueValue should not be null");
  		}
  		if( falseValue == null ) {
  			throw new NullPointerException("falseValue should not be null");
  		}
  	}
docs: Checks the preconditions for constructing a new ParseBool processor.
  
  @param trueValue
  the String which represents true
  @param falseValue
  the String which represents false
  @throws NullPointerException
  if trueValue or falseValue is null