java8 stream api流式编程

java8自带常用的函数式接口

Predicate boolean test(T t) 传入一个参数返回boolean值

Consumer void accept(T t) 传入一个参数，无返回值

Function<T,R> R apply(T t) 传入一个参数，返回另一个类型

准备数据

//计算机俱乐部 private static List<Student> computerClub = Arrays.asList( new Student("2015134001", "小明", 15, "1501"), new Student("2015134003", "小王", 14, "1503"), new Student("2015134006", "小张", 15, "1501"), new Student("2015134008", "小梁", 17, "1505") ); //篮球俱乐部 private static List<Student> basketballClub = Arrays.asList( new Student("2015134012", "小c", 13, "1503"), new Student("2015134013", "小s", 14, "1503"), new Student("2015134015", "小d", 15, "1504"), new Student("2015134018", "小y", 16, "1505") ); //乒乓球俱乐部 private static List<Student> pingpongClub = Arrays.asList( new Student("2015134022", "小u", 16, "1502"), new Student("2015134021", "小i", 14, "1502"), new Student("2015134026", "小m", 17, "1504"), new Student("2015134027", "小n", 16, "1504") ); private static List<List<Student>> allClubStu = new ArrayList<>(); allClubStu.add(computerClub); allClubStu.add(basketballClub); allClubStu.add(pingpongClub); 常用的stream三种创建方式

集合 Collection.stream()

静态方法 Stream.of

数组 Arrays.stream

//1.集合 Stream<Student> stream = basketballClub.stream(); //2.静态方法 Stream<String> stream2 = Stream.of("a", "b", "c"); //3.数组 String[] arr = {"a","b","c"}; Stream<String> stream3 = Arrays.stream(arr); Stream的终止操作

foreach(Consumer c) 遍历操作

collect(Collector) 将流转化为其他形式

max(Comparator) 返回流中最大值

min(Comparator) 返回流中最小值

count 返回流中元素综述

Collectors 具体方法

toList List 把流中元素收集到List

toSet Set 把流中元素收集到Set

toCollection Coolection 把流中元素收集到Collection中

groupingBy Map<K,List> 根据K属性对流进行分组

partitioningBy Map<boolean, List> 根据boolean值进行分组

//此处只是演示 此类需求直接用List构造器即可 List<Student> collect = computerClub.stream().collect(Collectors.toList()); Set<Student> collect1 = pingpongClub.stream().collect(Collectors.toSet()); //注意key必须是唯一的 如果不是唯一的会报错而不是像普通map那样覆盖 Map<String, String> collect2 = pingpongClub.stream() .collect(Collectors.toMap(Student::getIdNum, Student::getName)); //分组 类似于数据库中的group by Map<String, List<Student>> collect3 = pingpongClub.stream() .collect(Collectors.groupingBy(Student::getClassNum)); //字符串拼接 第一个参数是分隔符 第二个参数是前缀 第三个参数是后缀 String collect4 = pingpongClub.stream().map(Student::getName).collect(Collectors.joining(",", "【", "】")); //【小u,小i,小m,小n】 //三个俱乐部符合年龄要求的按照班级分组 Map<String, List<Student>> collect5 = Stream.of(basketballClub, pingpongClub, computerClub) .flatMap(e -> e.stream().filter(s -> s.getAge() < 17)) .collect(Collectors.groupingBy(Student::getClassNum)); //按照是否年龄>16进行分组 key为true和false ConcurrentMap<Boolean, List<Student>> collect6 = Stream.of(basketballClub, pingpongClub, computerClub) .flatMap(Collection::stream) .collect(Collectors.groupingByConcurrent(s -> s.getAge() > 16)); Stream的中间操作

filter(Predicate) 筛选流中某些元素!

//筛选1501班的学生 computerClub.stream().filter(e -> e.getClassNum().equals("1501")).forEach(System.out::println); //筛选年龄大于15的学生 List<Student> collect = computerClub.stream().filter(e -> e.getAge() > 15).collect(Collectors.toList());

map(Function f) 接收流中元素，并且将其映射成为新元素，例如从student对象中取name属性

//篮球俱乐部所有成员名 + 暂时住上商标^_^,并且获取所有队员名 List<String> collect1 = basketballClub.stream() .map(e -> e.getName() + "^_^") .collect(Collectors.toList()); collect1.forEach(System.out::println); //小c^_^^_^ //小s^_^^_^ //小d^_^^_^ //小y^_^^_^

flatMap(Function f) 将所有流中的元素并到一起连接成一个流

//获取年龄大于15的所有俱乐部成员 List<Student> collect2 = Stream.of(basketballClub, computerClub, pingpongClub) .flatMap(e -> e.stream().filter(s -> s.getAge() > 15)) .collect(Collectors.toList()); collect2.forEach(System.out::println); //用双层list获取所有年龄大于15的俱乐部成员 List<Student> collect3 = allClubStu.stream() .flatMap(e -> e.stream().filter(s -> s.getAge() > 15)) .collect(Collectors.toList()); collect3.forEach(System.out::println);