1 获取实体类对象中某一个字段转为List

  List<Long> userIds = users.stream().map(User::getId).collect(Collectors.toList());

2 根据单个字段字段去除重复

  users.stream() .collect( Collectors.collectingAndThen(
        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(
           user - > user.getName()))), ArrayList::new)); 

3 根据多个字段去除重复

  users.stream() .collect( Collectors.collectingAndThen(
     Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(
          user ->user.getName()+";"+user.getId()))), ArrayList::new));

4 根据某个字段分组

  users.stream().collect(Collectors.groupingBy(User::getName))

5 获取所有的员工id集合

  List<Integer> employeeList = employees.stream().map(Employee::getId).collect(toList());

6 获取员工id与员工信息的Map集合

  Map<Integer, Employee> idEmployeeMap =employees.stream().collect(
                Collectors.toMap(Employee::getId, Function.identity()));

7 员工id与员工姓名的map集合

  Map<Integer, String> idNameMap = employees.stream().collect( 
                Collectors.toMap(Employee:: getId, Employee::getName));

8 部门分组，每个部门id对应多个用户信息

  Map<Integer, List<Employee>> departGroupMap = employees.stream().collect(
                Collectors.groupingBy(Employee::getDepartId));

9 部门分组，每个部门id对应多个用户名称

  Map<Integer, List<String>> departNamesMap =employees.stream().collect( Collectors.groupingBy(
                  Employee::getDepartId,Collectors.mapping(Employee::getName, Collectors.toList())) );

10 生成到新的集合中

  List<OutputRobotTestset.OutputTestset> collect = robotTestsetEntityList.stream().map(
                  e -> new OutputRobotTestset.OutputTestset(e.getUid(), e.getName())).collect(Collectors.toList());

11 list 和 String 数组互转

• String 数组转list

   String title = "\t 10月上旬\t 10月中旬\t 10月下旬"; 
   String[] arrTitles = title.split("\t");
   List list2 = Arrays.stream(arrTitles).collect(Collectors.toList());
  

• list 转 String 数组

    String[] strs1 = titleList.toArray(new String[titleList.size()]);