Common usage of java8 feature Stream

Recently, in development, I saw my colleagues use stream for data conversion of collections List<object> ->List<String>

List<object> --> Map<Long, object>, it feels very simple and convenient. I found some information on the Internet to learn

Java 8 Stream | Novice Tutorial

Record our common usage

.stream() − Creates a serial stream for a collection

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 easily implements multithreading

List<StdRegcardTemp> lists=new ArrayList<>(10);

lists.stream().parallel().forEach(item -> { //具体处理 });

5. Get the piece of data with the maximum value of a certain field,

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

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

6. list<object> --> list<object> Get the return that meets the conditions

.filter( Get the data of type =1

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

7. list<object> --> list<Integer> Get the number that satisfies the condition, and take out the specified field

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

8. Modify the value of the object attribute, and batch modify a certain value of the object in the list

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

9. List deduplication

    List newList = (List) list.stream().distinct().collect(Collectors.toList());

10. Field type conversion List<Integer> --> List<String>

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

----The following is a reference extension

filter use refer to this article, well written

Stream filter() filters valid data_popcorn9958's blog-CSDN blog_stream.filter

Common usage of java8 feature Stream

Guess you like