Java流操作——Stream（二）

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Java流操作——Stream（一）
Java流操作——Stream（二）

前言

本节主要介绍收集流的结果以及将流收集到映射表中

一、收集结果（collect）

 		//将流收集到集合中
        Stream<String> words= Stream.of("ad","fa",null,"gag");
        List<String> wordList =words.collect(Collectors.toList());
        Set<String> wordSet =words.collect(Collectors.toSet());
        TreeSet<String> wordTree =words.collect(Collectors.toCollection(TreeSet::new));
        //元素之间增加分隔符
        String str =words.collect(Collectors.joining(", "));

二、转化后收集结果（map.collect）

 public static void main(String[] args){
        
        User user1=new User(1,"sky",20);
        User user2=new User(2,"haha",22);
        User user3=new User(3,"xixi",19);
        List<User> res = new ArrayList<>();
        res.add(user1);
        res.add(user2);
        res.add(user3);
        //如果流中包含字符串以外的其他对象，需要通过map进行转换，获取所有id构成的集合
        List<Integer> ids=res.stream().map(User::getId).collect(Collectors.toList());
        //如果流中包含字符串以外的其他对象，需要通过map全部转化为字符串，在拼接
        String result= res.stream().map(User::toString).collect(Collectors.joining(", "));

    }

三、将流的结果约简为总和、平均值、最大值和最小值(IntSummaryStatistics)

List<String> res= Arrays.asList("adc","agsdg","aeteq","grhrh","grhrhrhd","sgddh");
        IntSummaryStatistics summaryStatistics=res.stream().collect(Collectors.summarizingInt(String::length));
        //获取字符串平均长度
        double averageWordLength= summaryStatistics.getAverage();
        //获取最长字符串长度
        double maxWordLength= summaryStatistics.getMax();
        
        List<Integer> resInt=Arrays.asList(143,33,241,342,142);
        //两种转换方法
        IntSummaryStatistics summaryStatistics2=resInt.stream().mapToInt(x->x).summaryStatistics();
        IntSummaryStatistics summaryStatistics3=resInt.stream().collect(Collectors.summarizingInt(x->x));
        //平均值
        double average=summaryStatistics2.getAverage();
        double average2=summaryStatistics3.getAverage();
        //最大值
        double max=summaryStatistics2.getMax();
        double max2=summaryStatistics3.getMax();

四、收集到映射表

1.用户类

 public class User{
        private int id;
        private String name;
        private int age;

        public User(int id, String name, int age) {
            this.id = id;
            this.name = name;
            this.age = age;
        }

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }

2.获取映射键值队(toMap)

 User user = new User(1, "hah", 20);
        User user2 = new User(2, "xix", 22);
        User user3 = new User(3, "waw", 25);
        List<User> users = Arrays.asList(user, user2, user3);
        //获取id和name的键值对映射
        Map<Integer,String> idNameMap=users.stream().collect(Collectors.toMap(User::getId,User::getName));
        //获取id和user的映射
        Map<Integer,User> idUserMap=users.stream().collect(Collectors.toMap(User::getId, Function.identity()));
        //如果多个元素具有相同的键，则会抛出IllegalStateException的异常，可以通过提供第三个函数来覆盖这种行为
        //如果存在相同元素则覆盖旧元素
        Map<Integer,String> idNamefilterMap=users.stream().collect(Collectors.toMap(User::getId,User::getName,(existingValue,newValue)->newValue));

五、 群组和分区(groupingBy和partitioningBy)

		//获取国家对应语言的映射groupby
        Stream<Locale> locale=Stream.of(Locale.getAvailableLocales());
        Map<String,List<Locale>> countryToLocales=locale.collect(Collectors.groupingBy(Locale::getCountry));
        System.out.println(countryToLocales.get("CH"));

  		Stream<Locale> locale=Stream.of(Locale.getAvailableLocales());
        //获取符合条件的元素和不符合条件的元素时使用
        Map<Boolean,List<Locale>> englishAndOtherLocales=locale.collect(Collectors.partitioningBy(l->l.getLanguage().equals("en")));
        List<Locale> englishLocales=englishAndOtherLocales.get(true);
        List<Locale> otherLocales=englishAndOtherLocales.get(false);

六、下游收集器

		Stream<Locale> locale=Stream.of(Locale.getAvailableLocales());
        //获取符合条件的元素和不符合条件的元素时使用
        Map<Boolean,List<Locale>> englishAndOtherLocales=locale.collect(Collectors.partitioningBy(l->l.getLanguage().equals("en")));
        List<Locale> englishLocales=englishAndOtherLocales.get(true);
        List<Locale> otherLocales=englishAndOtherLocales.get(false);
        //counting会产生收集到元素的个数
        Map<String,Long> localeToLocaleCounts=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.counting()));
        //summing(int/long/double)会接受一个函数作为引元将该函数应用到下游元素中，并产生他们的和
        Map<String,Integer> localeToLanguage=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.summingInt(x->x.getLanguage().length())));
        //maxBy和minBy会接受一个比较器，并产生下游元素中的最大值和最小值
        Map<String,Optional<Locale>> stateToLocales=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.maxBy(Comparator.comparing(x->x.getLanguage().length()))));
        //mapping方法会产生将函数应用到下游结果上的收集器
        Map<String,Optional<String>> stateToLocalesMapping=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.mapping(Locale::getLanguage,Collectors.maxBy(Comparator.comparing(String::length)))));
        Map<String,Set<String>> languageToLocalesMapping=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.mapping(Locale::getLanguage,Collectors.toSet())));
        //如果群组和映射函数的返回值为int、long或double，那么可以从每个分组获取这些函数值的总和、个数、平均值、最大值
        Map<String,IntSummaryStatistics> languageToCountry=locale.collect(Collectors.groupingBy(Locale::getCountry,Collectors.summarizingInt(x->x.getLanguage().length())));