java8特性Stream 常见用法

最近在开发中看到同事使用stream 进行集合的数据转换List<object> ->List<String>

List<object> --> Map<Long, object>，感觉十分简单方便，上网找了一下资料学习一下

Java 8 Stream | 菜鸟教程

记录一下我们常见用法

.stream()− 为集合创建串行流

1、List<Object> --> List<String>

//List<Object> --> List<String> List<User> resultList = new ArrayList<>(); List<String> newList = resultList.stream().map(e -> e.getName()).collect(Collectors.toList()); // List<map> --> List<String> List<String> contractIds = array.stream().map(m -> MapUtils.getString(m, "contractId").toString()).collect(Collectors.toList());

2、List<User> --> Map<Long, User>

List<User> --> Map<Long, User> List<User> resultList = new ArrayList<>(); Map<Long, User> idToUser = resultList.stream().collect(Collectors.toMap(User::getId, (user -> user)));

3、List<Map<String, Object>> --> Map<String, Integer> countMap

List<user> --> Map<String, Integer> countMap

List<Map<String, Object>> reList = stdGoodsTempDao.queryCountByRegcardnm(paramMap); Map<String, Integer> countMap = reList.stream().collect(Collectors.toMap(map -> (String) map.get("regcardnm"), (map -> (Integer) map.get("goodsCount")))); // List<user> --> Map<String, Integer> countMap Map<Integer, String> countMap = list.stream().collect(Collectors.toMap(e -> e.getId(), e -> e.getBillNum()));

4、.stream().parallel().forEach轻松实现多线程

List<StdRegcardTemp> lists=new ArrayList<>(10); lists.stream().parallel().forEach(item -> { //具体处理 });

5、获取某个字段最大值的那条数据,

.stream().max(Comparator.comparing(Student::getRecord)).get();

Student student = stu.stream().max(Comparator.comparing(Student::getRecord)).get();

6、 list<object> --> list<object> 获取满足条件的返回

.filter( 获取type =1的数据

List<Cycle> cycleStream = cycleList.stream().filter( cycle -> 1.equals(cycle.getType())).collect(Collectors.toList());

7、 list<object> --> list<Integer> 获取满足条件的数，并且取出指定字段

List<Integer> projectConfigIds = cycleList.stream().filter( cycle -> "1".equals(cycle.getType())) .map(Cycle::getId) .collect(Collectors.toList());

8、修改object 属性的值,批量修改list 中对象的某个值

StdSubcodeBhs = StdSubcodeBhs.stream().peek(x -> x.setSubmitState(1)).collect(Collectors.toList());

9、list 去重

List newList = (List) list.stream().distinct().collect(Collectors.toList()); // 根据某个字段去重 //根据name进行去重 List<HospItemDO> list = addItems.stream().collect( Collectors.collectingAndThen(Collectors.toCollection( () -> new TreeSet<>(Comparator.comparing(HospItemDO::getItemcode))), ArrayList::new) );

10、字段类型转换 List<Integer> --> List<String>

List<Integer> declareTypes = new ArrayList(); List<String> declareTypes1 = declareTypes.stream().map(String::valueOf).collect(Collectors.toList());

----以下是参考扩展

filter 使用参考这个文章，写的很好

Stream filter()过滤有效数据_爆米花9958的博客-CSDN博客_stream.filter