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list map 处理方式

java 2025/6/30 6:37:40

List 实体中获取摸个字段的集合

Set<Long> taskIds = orderList.stream().map(EDeliverOrderDO::getTaskId).collect(Collectors.toSet());

List里面的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在一起：

// list
List<Apple> appleList = new ArrayList<>();//存放apple对象集合
Apple apple1 =  new Apple(1,"苹果1",new BigDecimal("3.25"),10);
Apple apple12 = new Apple(1,"苹果2",new BigDecimal("1.35"),20);
Apple apple2 =  new Apple(2,"香蕉",new BigDecimal("2.89"),30);
Apple apple3 =  new Apple(3,"荔枝",new BigDecimal("9.99"),40);appleList.add(apple1);
appleList.add(apple12);
appleList.add(apple2);
appleList.add(apple3);

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

将list转换map(key和Bean的形式)

Map<String, BazTgTO> tgMap = tgList.stream().collect(Collectors.toMap(BazTgTO::getTgId, n -> n));

将list转换map(key和value是实体的属性)

Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName));

将list转换map(key和value是实体的属性) - 忽略重复key

假设你想在遇到重复键时保留已有的值

Map<String, String> map = list.stream().collect(Collectors.toMap(Person::getId,  // Key mapperPerson::getName,  // Value mapper(existingValue, newValue) -> existingValue  // Merge function: 保留已有的值));

将list 根据字段移除对象

// A code block
List list = new ArrayList<>();
list.add(new User(1,"张三"));
list.add(new User(4,"赵六"));
//条件删除
list.removeIf(user -> user.getUserId() == 4);

将list 排序

Collections.sort(personVOList, new Comparator<personVO>() {public int compare(personVO arg0, personVO arg1) {return arg0.getUnitCountBegin().compareTo(arg1.getUnitCountBegin());}});

list 过滤

// 过滤查询 level 层级为1的// 第一种写法List<ArticleClassDO> firstArticleClassList = articleClassDOList.stream().filter(p -> 1 == p.getTreeLevel()).collect(Collectors.toList());// 第二种写法List<ArticleClassDO> firstArticleClassList = articleClassDOList.stream().filter(new Predicate<ArticleClassDO>() {public boolean test(ArticleClassDO articleClassDO) {return articleClassDO.getTreeLevel() == 1;}}).collect(Collectors.toList());

list 过滤 -其中的字段


List<String> levelOneNames = articleList.stream() // 将列表转换为流.filter(article -> article.getLevel() == 1) // 筛选level为1的元素.map(ArticleClassDO::getName) // 提取name字段.collect(Collectors.toList()); // 收集结果为列表

List 实体中获取摸个字段匹配的一个实体

ImesEventResultVO resultVO = resultVOList.stream().filter(vo -> "aaa".equals(vo.getName())).findFirst().orElse(null);

List 实体中- 根据字段去重

// 首先自定义去重方法，方法公共类方法中
public class CollectionDistinctUtil {public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {ConcurrentHashMap<Object, Boolean> map = new ConcurrentHashMap<>(16);return t -> map.putIfAbsent(keyExtractor.apply(t),Boolean.TRUE) == null;}
}

// 调用去重方法处理数据
// 下面用两个字段拼装来去重
personVOList = personVOList.stream().filter(CollectionDistinctUtil.distinctByKey(personVO -> (personVO.getSex()+ personVO.getName()))).collect(Collectors.toList());

List 实体中- 获取字段拼装字符串

如: 获取姓名，拼装成 ‘姓名1,姓名2,姓名3’

// A code block
List list = new ArrayList<>();
list.add(new User(1,"张三"));
list.add(new User(4,"赵六"));
//拼装字符串
String name = list.stream().map(QwContactDO::getName).collect(Collectors.joining(","));