Collectors.groupingBy妙用

Collectors.groupingBy根据一个或多个属性对集合中的项目进行分组

@Data
public class Student {
    private String name;
    private String sex;
    private int age;
    private String className;

    public Student(String name,String sex,int age,String className){
        this.name = name;
        this.sex = sex;
        this.age = age;
        this.className = className;
    }
}

按照性别分组

public static void main(String[] args) {
        Student student1 = new Student("张三","1",10,"三班");
        Student student2 = new Student("张三","1",11,"三班");
        Student student3 = new Student("李四","0",12,"三班");
        Student student4 = new Student("王二","0",12,"三班");
        Student student5 = new Student("麻子","0",11,"三班");
        List<Student> studentList = Lists.newArrayList(student1, student2, student3, student4, student5);

        Map<Object, List<Student>> newList = studentList.stream().collect(Collectors.groupingBy(student -> student.getSex()));
        log.info(newList.toString());
    }

也可以多个属性分组,按照性别和班级

public static void main(String[] args) {
         Student student1 = new Student("张三","1",10,"三班");
        Student student2 = new Student("张三","1",11,"三班");
        Student student3 = new Student("李四","0",12,"三班");
        Student student4 = new Student("王二","0",12,"三班");
        Student student5 = new Student("麻子","0",11,"三班");
        List<Student> studentList = Lists.newArrayList(student1, student2, student3, student4, student5);

        Map<Object, List<Student>> newList = studentList.stream().collect(Collectors.groupingBy(student -> student.getName() + ":" + student.getSex()+ ":" + student.getAge()));
        log.info(newList.toString());

    }

另外一个用法就是判断入参数组里面是否有重复数据
这个时候就可以用

 long distinctCount = studentList.stream().collect(Collectors.groupingBy(student -> student.getName() + ":" + student.getSex()+ ":" + student.getAge()));
  if (paramList.size() != distinctCount) {
            throw new BizException("xxxxxx重复");
  }