java8 list转map,list集合中的元素的属性转set,list集合中对象的属性转list

一.使用java8对list操作

　　1.1list转map

 private Map<String, Member> getMemberMap() {
    List<Member> members = mberMapper.selectMemberList(new Member());
    if(CollectionUtils.isEmpty(members)){
       return null;
    }
    return members.stream().collect(Collectors.toMap(Member::getPhone, m -> m, (k1, k2) -> k1)); /** 去重策略,如果有多个相同的key,保留第一个*/
 }

　　1.2list中对象字段转一个list集合

 　　public static List<String> getStudentNameList(List<Student> list ){
         List<String> result = list.stream().map(student->student.getName()).collect(Collectors.toList());
         for (String name : result) {
             System.out.println("name:"+name);
         }
         return result;
     }

　　1.3list中对象字段转一个set集合

　　public static Set<String> getStudentNameSet(List<Student> list ){
        Set<String> result = list.stream().map(student->student.getName()).collect(Collectors.toSet());
        for (String name : result) {
            System.out.println("name:"+name);
        }
        return result;
    }

　 1.4 list 多字段排序,第一个字段降序,第二个字段升序

 public static void main(String[] args) {
         List<SysUserOnline> sysUserOnlines1= new ArrayList<>();
         String date = DateUtils.getDate();
         Date date1 = DateUtils.parseDate(date);
         System.out.println("date1===="+ date1.toString());
         SysUserOnline s1 = new SysUserOnline();
         s1.setDateAlloCount(3);
         s1.setLastAccessTime(date1);
         SysUserOnline s2 = new SysUserOnline();
         s2.setDateAlloCount(1);
         s2.setLastAccessTime(DateUtils.addDays(date1,-1));
         SysUserOnline s3 = new SysUserOnline();
         s3.setDateAlloCount(2);
         s3.setLastAccessTime(date1);
         SysUserOnline s4 = new SysUserOnline();
         s4.setDateAlloCount(2);
         s4.setLastAccessTime(DateUtils.addDays(date1,1));
         sysUserOnlines1.add(s1);
         sysUserOnlines1.add(s2);
         sysUserOnlines1.add(s3);
         sysUserOnlines1.add(s4);

         System.out.println(sysUserOnlines1.toString());
         List<SysUserOnline> collect = sysUserOnlines1.stream().sorted(Comparator.comparing(SysUserOnline::getLastAccessTime).reversed().thenComparing(SysUserOnline::getDateAlloCount)).collect(Collectors.toList());
         System.out.println("=======================");
         System.out.println(collect.toString());
     }

1.5list分组,根据id分组

 //List 以ID分组 Map<Integer,List<Apple>>
 Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));

 System.err.println("groupBy:"+groupBy);
 {1=[Apple{id=1, name='苹果1', money=3.25, num=10}, Apple{id=1, name='苹果2', money=1.35, num=20}], 2=[Apple{id=2, name='香蕉', money=2.89, num=30}], 3=[Apple{id=3, name='荔枝', money=9.99, num=40}]}

参照:

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