07_java8
4/29/2021 java-basic
[toc]
# 推荐阅读
# 二、实例
# 1.筛选
# 1.1 去重
参考：分享几种 Java8 中通过 Stream 对列表进行去重的方法 (opens new window)
List<User> userList = UserMapper.findByEmailNonNull().stream()
.collect(Collectors.collectingAndThen(Collectors.toCollection(
() -> new TreeSet<>(Comparator.comparing(User::getEmail))),
ArrayList::new));
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（2）根据 Filed 去重
public class DistinctByKey {
public static <T> Predicate<T> distinctByKey(Function<? super T,Object> keyExtractor) {
Map<Object,Boolean> seen = new ConcurrentHashMap<>();
return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}
}
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用法：
List<SecurityUserLang> userLangs = userLangList.stream()
.filter(distinctByKey(userlang-> userlang.getOriginLocale() +":"+ userlang.getTargetLocale()))
.map(securityUserLang -> {
securityUserLang.setUserId(userId);
return securityUserLang;
}).collect(Collectors.toList());
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# 1.2 取最大值
Date maxReportDate = qaBagReportList.stream().map(QaBagReport::getReportDate).max(Comparator.comparing(java.util.Date::getTime)).get();
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# 2.排序
taskSetList.stream().sort((o1, o2) -> o1.getId() - o2.getId());
taskSetList.stream().sorted(Comparator.comparing(TaskSet::getId).reversed()).findFirst()
securityResourceList.stream().sorted(Comparator.comparing(securityResource -> securityResource.getResourcePath().toLowerCase())).collect(Collectors.toList());
securityResourceList.stream().sorted(Comparator.comparing(SecurityResource::getDatachangeLasttime)).collect(Collectors.toList());
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（2）自定义排序
public class Sort {
public List<User> sort(List<User> users){
return users.stream().sorted((u1, u2) -> u1.getCreatedTime().compareTo(u2.getCreatedTime())).collect(Collectors.toList());
}
}
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# 3.映射
# 3.1 map
将流中每一个元素从一种类型，映射为另一种类型
List<Long> resuourceIds = securityRoleResourceList.stream().map(SecurityRoleResource::getResourceId).collect(Collectors.toList())
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# 3.2 flatMap
将流扁平化
List<Long> bagTaskInfoIdsInTranslatingAndProjectContainTargetLocale = projectRelationList.stream().filter(projectRelation -> {
// if locale in targetLocales then return true
List<String> relationTargetLocales = Arrays.asList(projectRelation.getTargetLocales().split(","));
Optional<String> inTargetLocaleOptional = relationTargetLocales.stream().filter(targetLocaleList::contains).findAny();
if (inTargetLocaleOptional.isPresent()) {
return true;
}
return false;
}).map(projectRelation -> relationIdToBagTaskInfosMap.get(projectRelation.getId()).stream().map(ProjectRelationBagtaskInfo::getBagTaskInfoId).collect(Collectors.toList())).flatMap(Collection::stream).collect(Collectors.toList());
return bagTaskInfoIdsInTranslatingAndProjectContainTargetLocale;
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# 4.分组
分组归一化操作都可用 reducing
# 4.1 分组求最值
Map<String, HitRuleConfig> configMap = configList.parallelStream().collect(
Collectors.groupingBy(HitRuleConfig::getAppId, // 先根据appId分组
Collectors.collectingAndThen(
Collectors.reducing(( c1, c2) -> c1.getVersionSort() > c2.getVersionSort() ? c1 : c2), Optional::get)));
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