public interface MyInterface {default String getResult(){return "hello world!";}
}

public static <T, K, D, A, M extends Map<K, D>>Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,Supplier<M> mapFactory,Collector<? super T, A, D> downstream) {Supplier<A> downstreamSupplier = downstream.supplier();  //获取downstream的容器函数BiConsumer<A, ? super T> downstreamAccumulator = downstream.accumulator();    //获取downstream的reduce操作的计算函数BiConsumer<Map<K, A>, T> accumulator = (m, t) -> {                                                      //将计算结果放入map中的函数（也是新的collector的计算函数）K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key");A container = m.computeIfAbsent(key, k -> downstreamSupplier.get());        //这步是关键，通过map的key来判断是不是同一类，如果是同一类，则返回原来的容器，否则则使用downstream的函数创建一个容器downstreamAccumulator.accept(container, t);                                   //为容器填充值};BinaryOperator<Map<K, A>> merger = Collectors.<K, A, Map<K, A>>mapMerger(downstream.combiner());  //获取合并函数@SuppressWarnings("unchecked")Supplier<Map<K, A>> mangledFactory = (Supplier<Map<K, A>>) mapFactory;  //该函数返回结果容器函数（Map类型）if (downstream.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)) {            //如果downstream的类型是IDENTITY_FINISH那么返回新的collectorreturn new CollectorImpl<>(mangledFactory, accumulator, merger, CH_ID);}else {@SuppressWarnings("unchecked")Function<A, A> downstreamFinisher = (Function<A, A>) downstream.finisher();  //否则获取计算结果的函数Function<Map<K, A>, M> finisher = intermediate -> {                             //生成新的结果函数intermediate.replaceAll((k, v) -> downstreamFinisher.apply(v));@SuppressWarnings("unchecked")M castResult = (M) intermediate;return castResult;};return new CollectorImpl<>(mangledFactory, accumulator, merger, finisher, CH_NOID);}}

Map<Department, Integer> totalByDept= employees.stream().collect(Collectors.groupingBy(Employee::getDepartment,Collectors.summingInt(Employee::getSalary)));