一个人行走的范围，就是他的世界。——北岛
我们在使用Collectors.groupingBy时会遇到这种情况：
Map<String, List<User>> map = Arrays.asList(new User(), null).stream().collect(Collectors.groupingBy(User::getName));
为了避免这种情况，于是我自己实现了一个：
@SafeVarargs @SuppressWarnings("unchecked") public static <T, K, D, A, M extends Map<K, D>> M listGroupBy(List<T> list, Function<T, K> sFunction, Collector<? super T, A, D> downstream, boolean isParallel, Consumer<T>... peeks) { boolean hasFinished = downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH); return peekStream(list, isParallel, peeks).collect(new Collector<T, HashMap<K, A>, M>() { @Override public Supplier<HashMap<K, A>> supplier() { return HashMap::new; } @Override public BiConsumer<HashMap<K, A>, T> accumulator() { return (m, t) -> { K key = Optional.ofNullable(t).map(sFunction).orElse(null); A container = m.computeIfAbsent(key, k -> downstream.supplier().get()); downstream.accumulator().accept(container, t); }; } @Override public BinaryOperator<HashMap<K, A>> combiner() { return (m1, m2) -> { for (Map.Entry<K, A> e : m2.entrySet()) { m1.merge(e.getKey(), e.getValue(), downstream.combiner()); } return m1; }; } @Override public Function<HashMap<K, A>, M> finisher() { return hasFinished ? i -> (M) i : intermediate -> { // a-> a[0] intermediate.replaceAll((k, v) -> (A) downstream.finisher().apply(v)); @SuppressWarnings("unchecked") M castResult = (M) intermediate; return castResult; }; } @Override public Set<Characteristics> characteristics() { return hasFinished ? Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH)) : Collections.emptySet(); } }); }
使用方式：
Map<String, List<User>> map = listGroupBy(Arrays.asList(new User(), null), User::getName, Collectors.toList(), false)
这样避免了抛出异常，返回了对null值友好的结果(map里包含一个key为null的结果)
我稍作修改放到MP的SimpleQuery和hutool中的CollStreamUtil以及CollectorUtil中去了
