java8 集合求差集、并集、交集（flatMap）

@Test
    public void test1(){
        List<Integer> list1=new ArrayList<>();
        list1.add(1);
        list1.add(2);
        list1.add(3);
        List<Integer> list2=new ArrayList<>();
        list2.add(3);
        list2.add(4);
        list2.add(5);
        System.out.println("====求交集===");
        List<Integer> list=list1.stream().filter(t->list2.contains(t)).collect(Collectors.toList());
        list.stream().forEach(System.out::println);
        System.out.println("====求差集===");
        list=list1.stream().filter(t-> !list2.contains(t)).collect(Collectors.toList());
        list.stream().forEach(System.out::println);
        System.out.println("====求并集===");
        list.addAll(list1);
        list.addAll(list2);
        list=list.stream().distinct().collect(Collectors.toList());
        list.stream().forEach(System.out::println);
    }

原文：https://www.cnblogs.com/sword-successful/archive/2017/12/05/7989035.html

求并集　（降维）

 @Test
    public  void test11(){

        // 例一
        List<Integer> list = ImmutableList.of(1, 3, 5);
        list = list.stream().flatMap(l -> {
            List<Integer> list1 = new ArrayList<>();
            list1.add(l + 1);
            list1.add(l + 2);
            return list1.stream();
        }).collect(Collectors.toList());
        System.out.println(list);// [2, 3, 4, 5, 6, 7]



        // 例二 list<map>  [{a:1},{b:1},{c:1}]  ==> {a:1,b:1,c:1}
        List<List<Map<String,Object>>> listMapMultiple = ImmutableList.of(
                ImmutableList.of(
                        ImmutableMap.of("a", 1, "b", 2), ImmutableMap.of("a", 2, "b", 3)
                ),
                ImmutableList.of(
                        ImmutableMap.of("a", 3, "b", 4), ImmutableMap.of("a", 4, "b", 5)
                ),
                ImmutableList.of(
                        ImmutableMap.of("a", 5, "b", 6), ImmutableMap.of("a", 6, "b", 7)
                )
        );
       // 将多个list合并为一个list (降维)
        List<Map<String,Object>> list11 = listMapMultiple.stream().flatMap(Collection::stream).collect(Collectors.toList());
        // [{a=1, b=2}, {a=2, b=3}, {a=3, b=4}, {a=4, b=5}, {a=5, b=6}, {a=6, b=7}]
        System.err.println("list1:"+list11);





        // 例二 [[1,2],[2,3,4],[5,6]] => [1,2,2,3,4,5,6]
        List list3 = ImmutableList.of(
                ImmutableList.of(
                        ImmutableList.of("a", 1, "b", 2), ImmutableList.of("a", 2, "b", 3)
                ),
                ImmutableList.of(
                        ImmutableList.of("a", 3, "b", 4), ImmutableList.of("a", 4, "b", 5)
                ),
                ImmutableList.of(
                        ImmutableList.of("a", 5, "b", 6), ImmutableList.of("a", 6, "b", 7)
                )
        );

        List<? extends Serializable> collect = ImmutableList.of(
                ImmutableList.of("a", 5, "b", 6), ImmutableList.of("a", 6, "b", 7)
        ).stream().flatMap(Collection::stream).collect(Collectors.toList());// [{a=1, b=2}, {a=2, b=3}, {a=3, b=4}, {a=4, b=5}, {a=5, b=6}, {a=6, b=7}]
        System.err.println(collect);
    }