code: public MapBuilder<KEY, VALUE> putIfAbsent(KEY key, VALUE value) {
      getMap().putIfAbsent(key, value);
      return this;
    }
docs: Puts the given {@link KEY key} mapped to the given {@link VALUE value} into the {@link Map} being built
  by this {@link MapBuilder} iff the {@link KEY key} is not already mapped to a value.
  
  @param key {@link KEY key} to put.
  @param value {@link VALUE value} to put mapped to the given {@link KEY key}.
  @return this {@link MapBuilder}.
  @see java.util.Map#putIfAbsent(Object, Object)