java中stream的一些用法

开发中对于stream有一些小的用法。进行总结一下。

将一个数组切分为多个小数组

/**
     * Description: Java8 Stream分割list集合
     * @param list 集合数据
     * @param splitSize 几个分割一组
     * @return 集合分割后的集合
     */
    public static <T> List<List<T>> splitList(List<T> list, int splitSize) {
        //判断集合是否为空
        if (CollectionUtils.isEmpty(list)) {
            return Collections.emptyList();
        }
        //计算分割后的大小
        int maxSize = (list.size() + splitSize - 1) / splitSize;
        //开始分割
        return Stream.iterate(0, n -> n + 1)
                .limit(maxSize)
                .parallel()
                .map(a -> list.parallelStream().skip(a * splitSize).limit(splitSize).collect(Collectors.toList()))
                .collect(Collectors.toList());
    }

将学生按分数进行分组，并返回各分数的学生名称

@Data
@AllArgsConstructor
class Study {
    public String name;
    public Integer scope;
}

List<Study> list = new ArrayList<Study>() {{
            add(new Study("张三", 10));
            add(new Study("李四", 10));
            add(new Study("王五", 20));
            add(new Study("赵六", 20));
            add(new Study("田七",20));
        }};

        Map<Integer, List<String>> collect = list.stream()
                .collect(
                        Collectors.groupingBy(
                                Study::getScope, 
                                Collectors.mapping(
                                        Study::getName, 
                                        Collectors.toList()
                                )
                        )
                );

        System.out.println(collect);

按指定字段进行去重

下列学生中，赵六，分数为20的重复录入了一次，通过treeset进行去重操作

@Data
@AllArgsConstructor
class Study {
    public String name;
    public Integer scope;
}

 List<Study> list = new ArrayList<Study>() {{
            add(new Study("张三", 10));
            add(new Study("李四", 10));
            add(new Study("王五", 20));
            add(new Study("赵六", 20));
            add(new Study("赵六", 20));
            add(new Study("田七",20));
        }};

        List<Study> collect = list.stream()
                .distinct()
                .collect(
                        Collectors.collectingAndThen(
                                Collectors.toCollection(
                                        () -> new TreeSet<>(Comparator.comparing(o -> o.getName() + o.getScope()))
                                ),
                                ArrayList::new
                        )
                );

        System.out.println(collect);

多字段排序

@Data
@AllArgsConstructor
@NoArgsConstructor
class demoEnt{
    public Integer age;
    public Integer index;
}

List<demoEnt> list = new ArrayList<>();
        list.add(new demoEnt(11, 2));
        list.add(new demoEnt(11,3));
        list.add(new demoEnt(12,1));

        //按年龄倒序并按index正序
        List<demoEnt> collect = list.stream().sorted(
                Comparator.comparing(demoEnt::getAge)
                        .reversed()
                        .thenComparing(demoEnt::getIndex)
        ).collect(Collectors.toList());

        System.out.println(collect);