code: public static <K, V> void validateNotNull(K key, V value) {
          checkNotNull(key, NULL_KEY_IS_NOT_ALLOWED);
          checkNotNull(value, NULL_VALUE_IS_NOT_ALLOWED);
      }
docs: Validates that key, value pair are both not null.
  
  @param key   the key to be validated.
  @param <K>   the type of key.
  @param value the value to be validated.
  @param <V>   the type of value.
  @throws java.lang.NullPointerException if key or value is null.