list.stream Common operations

Java8 A series of operations can be carried out in the way of flow in the set , We will directly illustrate through some examples .

So let's define one list, And initialize some data ：

        List<String> list = new ArrayList() {
    {
    
            add(" Xiao Ming ");
            add(" Xiaohong ");
            add(" Xiao Chen ");
            add(" Xiao Zhang ");
            add(" Xiao Zhang ");
            add(" petty thief ");
            add(" Xiao Wang ");
        }};

1. Find the first element in the collection that meets the criteria

        //  Find the first object in the collection 
        Optional<String> first = list.stream().filter(a -> " petty thief ".equals(a)).findFirst();
        System.out.println(first); //  Output ： Optional[ petty thief ]
        System.out.println(first.toString()); //  Output ：Optional[ petty thief ]

2. Return the set that meets the query criteria

        //  Returns a collection that meets the criteria 
        List<String> collect = list.stream().filter(a -> " Xiao Zhang ".equals(a)).collect(Collectors.toList());
        System.out.println(collect); // [ Xiao Zhang ,  Xiao Zhang ]
        System.out.println(collect.toString()); // [ Xiao Zhang ,  Xiao Zhang ]

1. filter Filter

        // filter Filter 
        List<Integer> list1 = Arrays.asList(21,32,22,33,44);
        List<Integer> collect1 = list1.stream().filter(tmp -> tmp > 25).collect(Collectors.toList());
        System.out.println(collect1); // [32, 33, 44]

2. map Operate on each element , Will not change the original quantity

        List<String> collect2 = list.stream().map(a -> a + " Hello ").collect(Collectors.toList());
        System.out.println(collect2); // [ Hello Xiaoming ,  Hello, Xiaohong ,  Hello, Xiao Chen ,  Hello, Xiao Zhang ,  Hello, Xiao Zhang ,  Hello, Xiao Li ,  Hello, Xiao Wang ]

3. mapToInt Turn into IntStream

        // mapToInt  Turn into IntSream
        IntStream intStream = list1.stream().mapToInt(a -> a);
        List<Integer> collect3 = intStream.boxed().collect(Collectors.toList());
        System.out.println(collect3); // [21, 32, 22, 33, 44]
        // mapToDouble mapToLong  Empathy 

4. flatMap Integrate multiple sets into one

        // flatMap  Integrate multiple sets into one 
        List<List<Integer>> lists = new ArrayList() {
    {
    
           add(Arrays.asList(1,2,3));
           add(Arrays.asList(4,5,6));
        }};
        System.out.println(lists); // [[1, 2, 3], [4, 5, 6]]
        Stream<Integer> integerStream = lists.stream().flatMap(Collection::stream);
        List<Integer> collect4 = integerStream.collect(Collectors.toList());
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]

5. distinct duplicate removal

        // distinct  duplicate removal 
        List<Integer> list2 = Arrays.asList(1,1,1,2,2,2,3,3,3);
        List<Integer> collect5 = list2.stream().distinct().collect(Collectors.toList());
        System.out.println(collect5); // [1, 2, 3]

6. sorted Sort

        // sort Sort 
        System.out.println(list1);
        List<Integer> collect6 = list1.stream().sorted().collect(Collectors.toList());
        System.out.println(collect6); // [21, 22, 32, 33, 44]

7. peek Without affecting the mainstream , Extract the current stream elements

        // peek  Without affecting the mainstream , Extract the current stream elements 
        System.out.println(list); // [ Xiao Ming ,  Xiaohong ,  Xiao Chen ,  Xiao Zhang ,  Xiao Zhang ,  petty thief ,  Xiao Wang ]
        List<String> nozhang = new ArrayList<>();
        List<String> noming = new ArrayList<>();
        List<String> nohong = list.stream().filter(a -> !a.contains(" Zhang ")).peek(nozhang :: add)
                .filter(a ->  !a.contains(" bright ")).peek(noming :: add)
                .filter(a ->  !a.contains(" red ")).collect(Collectors.toList());
        System.out.println(list); // [ Xiao Ming ,  Xiaohong ,  Xiao Chen ,  Xiao Zhang ,  Xiao Zhang ,  petty thief ,  Xiao Wang ]
        System.out.println(nozhang); // [ Xiao Ming ,  Xiaohong ,  Xiao Chen ,  petty thief ,  Xiao Wang ]
        System.out.println(noming); // [ Xiaohong ,  Xiao Chen ,  petty thief ,  Xiao Wang ]
        System.out.println(nohong); // [ Xiao Chen ,  petty thief ,  Xiao Wang ]

8. limit Limit the number of flow elements

        // limit  Limit the number of flow elements 
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]
        List<Integer> collect7 = collect4.stream().limit(3).collect(Collectors.toList());
        System.out.println(collect7); // [1, 2, 3]

9. skip How many elements to skip

        // skip  How many elements to skip 
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]
        List<Integer> collect8 = collect4.stream().skip(2).collect(Collectors.toList());
        System.out.println(collect8); // [3, 4, 5, 6]

10. foreach Traverse

        // foreach  Traverse 
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]
        List<Integer> foreach = new ArrayList<>();
        List<Integer> foreach1 = new ArrayList<>();
        collect4.stream().forEach(foreach :: add);
        collect4.stream().forEachOrdered(foreach1 :: add);
        System.out.println(foreach); // [1, 2, 3, 4, 5, 6]
        System.out.println(foreach1); // [1, 2, 3, 4, 5, 6]

11. reduce Calculation

        // reduce  Calculation 
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]
        Integer reduce = collect4.stream().reduce(0, (x, y) -> x + y);
        System.out.println(reduce); // 21
        Integer integer = collect4.stream().reduce((x, y) -> x + y).orElse(-1);
        System.out.println(integer); // 21
        Integer sum = collect4.stream().reduce(0, (x, y) -> x + y, Integer::sum);
        System.out.println(sum); // 21

12. min Minimum max Maximum count total

        // min  Minimum  max  Maximum  count total 
        System.out.println(collect4); // [1, 2, 3, 4, 5, 6]
        Integer min = collect4.stream().min(Integer::compareTo).orElse(-1);
        Integer max = collect4.stream().max(Integer::compareTo).orElse(-1);
        long count = collect4.stream().count();
        System.out.println(min); // 1
        System.out.println(max); // 6
        System.out.println(count); // 6

13. findFirst first findAny Any one

        // findFirst  first  findAny  Any one 
        System.out.println(list); // [ Xiao Ming ,  Xiaohong ,  Xiao Chen ,  Xiao Zhang ,  Xiao Zhang ,  petty thief ,  Xiao Wang ]
        String s = list.stream().findFirst().toString();
        String s1 = list.stream().findAny().orElse("");
        System.out.println(s); // Optional[ Xiao Ming ]
        System.out.println(s1); //  Xiao Ming