code: public static <KEY, VALUE> SimpleKeyValue<KEY, VALUE> newKeyValue(KEY key) {
      return new SimpleKeyValue<>(key);
    }
docs: Factory method used to construct a new instance of {@link SimpleKeyValue} initialized with the given {@code key}
  and no value.
  
  @param <KEY> {@link Class type} of the key.
  @param <VALUE> {@link Class type} of the value.
  @param key key in the key/value mapping; must not be {@literal null}.
  @return a new instance of {@link SimpleKeyValue} initialized with the given {@code key} with no value.
  @throws IllegalArgumentException if {@code key} is {@literal null}.
  @see #SimpleKeyValue(Object)