Java对List使用stream流进行操作

1：java8之前List分组:

假设有个student类，有id、name、score属性，list集合中存放所有学生信息，现在要根据学生姓名进行分组。

public Map<String, List<Student>> groupList(List<Student> students) {
    Map<String, List<Student>> map = new Hash<>();
    for (Student student : students) {
        List<Student> tmpList = map.get(student.getName());
        if (tmpList == null) {
            tmpList = new ArrayList<>();
            tmpList.add(student);
            map.put(student.getName(), tmpList);
        } else {
            tmpList.add(student);
        }
    }
    return map;
}

2：其他的一些stream流转换

Java8之后常规分组

public Map<String, List<Student>> groupList(List<Student> students) {
    Map<String, List<Student>> map = students.stream().collect(Collectors.groupingBy(Student::getName));
    return map;
}

 

1：第一种： 取list中某2个字段作为Map的K,V

public Map<Long, String> getIdNameMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
}

2：第二种：将id和实体Bean做为K,V

public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}

或者这样写：

public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getId, Function.identity()));
}
account -> account是一个返回本身的lambda表达式，后面的使用Function接口中的一个默认方法代替，使整个方法更简洁优雅。

3：使用某个具体的Map类来保存，如保存时使用LinkedHashMap

public Map<String, Account> getNameAccountMap(List<Account> accounts) {
    return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new));
}

4：List<Object>转List<String,Map<String, String>>

public Map<String,List<MCode>> getCodeListMap(){
        if(CollectionUtils.isEmpty(codeListMap)){
                List<MCode> codeList = this.getCodeList();
                Set<String> keySet = codeList.stream().map(code -> code.getCodeKbn()).collect(Collectors.toSet());
                Iterator<String> it = keySet.iterator();
                while(it.hasNext()) {
                        String key = it.next();
                        codeListMap.put(key, codeList.stream().filter(code -> code.getCodeKbn().equals(key)).collect(Collectors.toList()));
                }
        }
        return codeListMap;
}

5：转成value为Set的Map

Map<String, Set<Long>> onlineTradeNoMap = tradeOrderGoodsRelateOnlineList.stream()
                .filter(tradeOrderGoodsRelateOnline -> tradeOrderGoodsRelateOnline.getSourceTradeNo() != null)
                .collect(Collectors.groupingBy(TradeOrderGoodsRelateOnline::getSourceTradeNo,
                        Collectors.mapping(TradeOrderGoodsRelateOnline::getTradeId, Collectors.toSet())));

6：获取list对象 list属性 并进行去重

List<String> collect = users.stream().map(e -> e.getUserName()).distinct().collect(Collectors.toList());

7：单独获取List对象某个属性操作

List<String> collect = users.stream().map(e -> e.getUserName()).collect(Collectors.toList());

8：单独去重操作

List<String> distinctElements = list.stream().distinct().collect(Collectors.toList());

 9: 在stream流转Map的过程中做数据类型转换

例如 Student 对象的id为 integer类型， 但我想输出Map<Long, String> 类型的数据

 

Map<Long, String> map = list..stream().collect(Collectors.toMap(student -> Long.valueOf(student.getId()), Student::getName));

 

 

各个地方整理来的，侵删

 

.stream().collect(Collectors.toMap(tradePromGroupingDto -> Long.valueOf(tradePromGroupingDto.getId()), Tra