项目中使用到的集合

一、List 集合截取部分内容

userList = userList .subList(5,10);

截取 userList [5,10) 条数据

二、将有序的集合打乱（变为乱序）

Collections.shuffle(userList);

将userList 内容的元素打乱

三、将两个List 集合合并为一个，并去重

list01.removeAll(list02);
list01.addAll(list02);

 四、将集合，按照某个属性进行合并

  List<Student> list = new ArrayList<>();
        list.add(new Student(1,"张三",10,50));
        list.add(new Student(2,"李四",20,20));
        list.add(new Student(3,"张三",30,30));
        list.add(new Student(1,"李四",40,40));


        List<Student> studentList = new ArrayList<>();

        list.stream().collect(Collectors.groupingBy(Student::getId,Collectors.toList()))
                .forEach((id,transfer)->{
                    transfer.stream().reduce((a,b)->new Student(a.getId(),a.getName(),a.getAge()+b.getAge(),a.getAge2()+b.getAge2())).ifPresent(studentList::add);
                });
        studentList.forEach(e->System.out.println(e));

五、对集合中某一列属性求和

方法一

Integer totalStarCnt = starCntList.stream()
        .collect(Collectors.summingInt(CommentStarCntVO::getNumber));

方法二

   Integer totalStarCnt = starCntList.stream()
                .mapToInt(CommentStarCntVO::getNumber).sum();

六、求集合中某俩列乘积再求和

Double ss = orderList.stream().reduce(0.0, (x, y) -> x + (y.getNum() * y.getPrice()), Double::sum);

 七、集合提取某个唯一属性作为key,转换为Map

   Map<Long,FxUserSimpleDTO> fxMap = fxUserSimpleDTOList.stream()
                    .collect(Collectors.toMap(FxUserSimpleDTO::getUid,Function.identity()));

八、将List 集合中实体的2个属性作为 key-value ，转换为 Map

 List<Student>  list = Arrays.asList(
                new Student(1,"",20),
                new Student(2,"",30),
                new Student(3,"",40),
                new Student(4,"",50)
        );

        Map<Integer,Integer>  studentMap = list.stream()
                .collect(Collectors.toMap(Student::getId,Student::getAge));


        for(Map.Entry<Integer,Integer> entry : studentMap.entrySet()){
            System.out.println(entry);
        }

九、将 List 数组中对象，按照某个属性进行分组。

  List<Student> list = Arrays.asList(
                new Student(1,"1.jpg",20),
                new Student(2,"2.jpg",30),
                new Student(2,"3.jpg",40),
                new Student(1,"4.jpg",50)
        );

        Map<Integer, List<Student>> listMap = list.stream()
                .collect(Collectors.groupingBy(Student::getId,Collectors.toList()));


        for(Map.Entry<Integer, List<Student>> entry: listMap.entrySet()){
                System.out.println(entry.getKey());
                System.out.println(entry.getValue());
        }

集合转换参考：https://blog.csdn.net/codeissodifficulty/article/details/88864358

集合合并参考：https://blog.csdn.net/qq_36850813/article/details/106117908