Java8 Stream 使用技巧

• 排序

list.stream()
            .sorted(Comparator.comparing(x -> MapUtils.getString((Map) x, "order"))   //排序字段
            .reversed())   //反转
            .collect(Collectors.toList());

• List 分组

Map<String, List<Map<String, Object>>> groupList =
            list.stream().collect(Collectors.groupingBy(x -> MapUtils.getString(x, "code"))); //分组字段

• List to Map

Map<String, OrganizationVo> orgMap =
                orgList.stream().
                collect(Collectors.toMap(OrganizationVo::getOrgId, Function.identity(), (a, b) -> a));
                // OrganizationVo::getOrgId 映射成 key
                // Function.identity() 即 e -> e 映射成 value
                // (a, b) -> a 是防止key重复的选择策略 

• List 去重

// 普通简单写法
Set<Object> set = Sets.newConcurrentHashSet();
List<OrganizationVo> resultList = 
list.stream().filter(vo -> set.add(vo.getOrgId())).collect(Collectors.toList());

// 函数式接口写法
public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
    Set<Object> set = Sets.newConcurrentHashSet();
    return t -> set.add(keyExtractor.apply(t));
}

List<OrganizationVo> resultList= 
list.stream().filter(distinctByKey(vo -> vo.getOrgId())).collect(Collectors.toList());