code: @NonNull
      public static <T> T checkNotNull(T value, @NonNull String errorMessage) {
          if (value == null) {
              throw new NullPointerException(errorMessage);
          }
          return value;
      }
docs: Ensures the value is not null.
  
  @param value value to be validated.
  @param errorMessage error message thrown if value is null.
  @param <T> type of value.
  @return value passed in if not null.