【JDK1.8新特性】集合stream()常见用法介绍

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已于 2024-12-09 16:36:38 修改

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分类专栏： Java进阶文章标签： java 性能优化

于 2022-08-02 16:29:40 首次发布

本文链接：https://blog.youkuaiyun.com/weixin_42979278/article/details/126124754

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集合转map(key唯一)

List<Entity> entitys;
Map<String, Entity> entityMaps = entitys.stream().collect(Collectors.toMap(e -> "" + e.getId(), e -> e, (entity1, entity2) -> entity1));

集合转map，取某一字段作为value(key唯一)

List<Entity> entitys;
Map<String, Integer> sIMap = entitys.stream().collect(Collectors.toMap(Entity::getId, Entity::getTotalCount, (entity1, entity2) -> entity1));

集合转map，分组统计

List<Entity> entitys;
Map<String, Long> map = entitys.stream().collect(Collectors.groupingBy(Entity::getId, Collectors.counting()));

集合过滤filter

List<Entity> entitys;
entitys = entitys.stream().filter(s -> Constant.id.equals(s.getId())).collect(Collectors.toList());

集合分组groupingBy

List<Entity> entitys;
Map<String, List<Entity>> entitysMap = entitys.stream().collect(Collectors.groupingBy(Entity::getId));

Map<String, List<Entity>> entitysMap = entitys.stream().collect(Collectors.groupingBy(i -> i.getA() + "_" + i.getB()));


Map<String, List<Entity>> entitysMap = entitys.stream().collect(Collectors.groupingBy(
    i -> {
    if(i.getA() < 3) {
        return "3";
    }else {
        return "other";
    }
}
));

多级集合分组

Map<String, Map<String, List<Entity>>> entitysMap = entitys.stream().collect(Collectors.groupingBy(Entity::getId, Collectors.groupingBy(i -> {
    if(i.getA() < 3) {
        return "3";
    }else {
        return "other";
    }
})));

集合取某一字段重组集合

List<Entity> entitys;
List<Long> ids = entitys.stream().flatMap(Collection::stream)
        .map(Entity::getId).collect(Collectors.toList());
List<Long> ids = entitys.stream().map(s -> s.getId()).collect(Collectors.toList());

List根据某个字段求和

int sum = entitys.stream().collect(Collectors.summingInt(Entity::getNumber));

int ageSum = entitys.stream().mapToInt().sum(Entity::getNumber);

集合根据某一字段排序

List<Entity> entitys;
entitys = entitys.stream().sorted(Comparator.comparing(entitys::getId)).collect(Collectors.toList());

集合-Map根据排序

 List<Entity> entitys;
entitys = entitys.stream().sorted(Comparator.comparing((Map m) -> (new BigDecimal(m.get("age").toString())))).collect(Collectors.toList());

List 转字符串

第一种：使用谷歌Joiner方法
String result = Joiner.on(",").join(list);

第二种：stream流
String result = list.stream().map(String::valueOf).collect(Collectors.joining(","));