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,list 排序,list分组,条件过滤

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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 1.6list根据条件去重过滤

List<ResourceHighFile> highFiles = new ArrayList<>();

Set<ResourceHighFile> set = new TreeSet<ResourceHighFile>((file1, file2) -> file1.getFileName().compareTo(file2.getFileName()));
set.addAll(highFiles);

//根据getName去重
personList = personList.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(
                () -> new TreeSet<>(Comparator.comparing(Person::getName))), ArrayList::new));

 1.7list去除空元素

list.removeAll(Collections.singleton(null));