hutool工具-----CollStreamUtil

本篇博客来自hutool

Java8中Collectors详解

Java8中的新特性之一就是Stream，Hutool针对常用操作做了一些封装，本文将stream中collection和hutool工具类做了对比，同样的功能封装后是多么的简洁，之前本人都是手写工具，现在知道hutool还有这种功能，这用起来简直绝了

Student

@Data
@AllArgsConstructor
@ToString
public  class Student {
    private long termId;//学期id
    private long classId;//班级id
    private long studentId;//班级id
    private String name;//学生名称
}

    @Test
    void toIdentityMapTest() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, "张三"));
        list.add(new Student(1, 1, 2, "李四"));
        list.add(new Student(1, 1, 3, "王五"));
        //手写方法
        Map collect = list.stream().collect(Collectors.toMap(Student::getStudentId, s -> s));
        //使用封装的工具
        Map map = CollStreamUtil.toIdentityMap(list, Student::getStudentId);
    }

结果
手写方法

hutool工具

如我们可以将学生信息的id和姓名生成map：

    @Test
    void toMapTest() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, "张三"));
        list.add(new Student(1, 1, 2, "李四"));
        list.add(new Student(1, 1, 3, "王五"));
        //手写方法
        Map collect = list.stream()
                .collect(Collectors.toMap(student -> student.getStudentId(),
                        Student::getName, (s1, s2) -> s2));
        //使用封装的工具
        Map map2 = CollStreamUtil.toMap(list, Student::getStudentId, Student::getName); 
    }

结果
手写方法

工具

    @Test
    void groupByKeyTest() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, "张三"));
        list.add(new Student(1, 2, 2, "李四"));
        list.add(new Student(2, 1, 1, "擎天柱"));
        list.add(new Student(2, 2, 2, "威震天"));
        list.add(new Student(2, 3, 2, "霸天虎"));
        //手写方法
        Map> collect = list.stream().collect(Collectors.groupingBy(Student::getClassId));

        //使用封装的工具
        Map> map = CollStreamUtil.groupByKey(list, Student::getClassId);
        System.out.println("map = " + map);
    }

结果
手写方法

使用工具

第二层value是集合，将符合条件的数据放入集合中

    //将collection按照两个规则(比如有相同的年级id,班级id)分组成双层map
    @Test
    void groupBy2Key() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1,1, 1, "张三"));
        list.add(new Student(1, 1,1, 2, "李四"));
        list.add(new Student(1, 2,1, 3, "王五"));
        list.add(new Student(1, 2,1, 3, "王流"));
        list.add(new Student(1, 2, 2, 3, "汪一"));
        list.add(new Student(1, 2, 2, 3, "汪二"));
        Map>> longMapMap = CollStreamUtil.groupBy2Key(list, Student::getGradeId, Student::getClassId);
    }

结果

    @Test
    void group2Map() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, 1, "张三"));
        list.add(new Student(1, 1, 1, 2, "李四"));
        list.add(new Student(1, 2, 1, 3, "王五"));
        list.add(new Student(1, 2, 1, 3, "王流"));
        list.add(new Student(1, 2, 2, 3, "汪一"));
        list.add(new Student(1, 2, 2, 3, "汪二"));
        Map> longMapMap1 = CollStreamUtil.group2Map(list, Student::getGradeId, Student::getClassId);
    }

结果

4.作为所有groupingBy的公共方法，更接近于原生，灵活性更强

    @Test
    void groupBy() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, "张三"));
        list.add(new Student(1, 1, 2, "李四"));
        list.add(new Student(1, 2, 3, "王五"));
        //手动
        Map collect = list.stream()
                .collect(Collectors.groupingBy(Student::getClassId, Collectors.counting()));

        //统计每个班的人数
        Map longLongMap = CollStreamUtil.groupBy(list, Student::getClassId, Collectors.counting());
        System.out.println("longMapMap = " + longLongMap);
    }

结果

我们可以将学生信息列表转换提取为姓名的列表：

    @Test
    void toList() {
        List list = new ArrayList<>();
        list.add(new Student(1, 1, 1, "张三"));
        list.add(new Student(1, 1, 2, "李四"));
        list.add(new Student(1, 1, 3, "王五"));
        List list2 = CollStreamUtil.toList(list, Student::getName);
    }

结果