code: public static void checkArrayLength(String parameterName, int actualLength,
              int expectedLength) {
          if (actualLength != expectedLength) {
              throw Exceptions.IllegalArgument(
                      "Array %s should have %d elements, not %d", parameterName,
                      expectedLength, actualLength);
          }
      }
docs: Check that the parameter array has exactly the right number of elements.
  
  @param parameterName
  The name of the user-supplied parameter that we are validating
  so that the user can easily find the error in their code.
  @param actualLength
  The actual array length
  @param expectedLength
  The expected array length