[Java] Java8 Collectors.toMap的坑

Map<Integer, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName));

public static <T, K, U>
Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends U> valueMapper) {
  return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}
 
 
private static <T> BinaryOperator<T> throwingMerger() {
  return (u,v) -> { throw new IllegalStateException(String.format("Duplicate key %s", u)); };
}

public static <T, K, U, M extends Map<K, U>>
Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends U> valueMapper,
        BinaryOperator<U> mergeFunction,
        Supplier<M> mapSupplier) {
  BiConsumer<M, T> accumulator
    = (map, element) -> map.merge(keyMapper.apply(element),
           valueMapper.apply(element), mergeFunction);
  return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
}

if (old != null) {
  V v;
  if (old.value != null)
  v = remappingFunction.apply(old.value, value);
  else
  v = value;
  if (v != null) {
  old.value = v;
  afterNodeAccess(old);
  }
  else
  removeNode(hash, key, null, false, true);
  return v;
}

Map<Integer, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName,(oldValue, newValue) -> newValue));