jdk8 stream filter()的特殊用法及与map()比较

1.简单的filter和map的单独形式

List<String> newlist = list.stream().map(Person::getName).collect(toList());

list = list.stream().filter(person -> person.getAge() == 20).collect(toList());

2.map与filter的位置互换

List<String> list = entities.stream().map(Entity::getId).filter(StringUtils::isNotBlank).distinct().collect(Collectors.toList());

 List<String> list1 = entities.stream().filter(x->x.getId() != 1).map(Entity::getId).distinct().collect(Collectors.toList());

3.distinct的一般过滤形式

 List<Entity> List5 = entities.stream().distinct().collect(Collectors.toList());

4.StreamEx过滤合并多个stream

   List<String> List6 = StreamEx.of(entities).distinct(Entity::getNumber).collect(Collectors.toList());

5.distinctByKey在使用filter时需要重定义

   List<Base> obj = List.stream().filter(distinctByKey(Base::getId)).collect(Collectors.toList());

    private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }

6.list中根据多字段进行分组

 Map<String, List<VhlAlarmStatusMgtEntity>> map = list.stream().collect(
//注意，{0}#{1}#{2}#{3}#{4}必须按序，不得违反顺序
                 Collectors.groupingBy(a -> StringUtil.format("{0}#{1}#{2}#{3}#{4}", a.getCarType(),a.getVin(),a.getAlarmLevel(),a.getAlarmItem(),a.getDataType())));

自定义的分组类，注意，不要用其他依赖，否则会报错

import java.text.MessageFormat;

public class StringUtil {

    public static String format(String value, Object... paras) {
        return MessageFormat.format(value, paras);
    }
}

jdk8 stream filter()的特殊用法及与map()比较

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