java8自带常用的函数式接口
Predicate<T> boolean test(T t)传入一个参数返回boolean值Consumer<T> 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<T>把流中元素收集到ListtoSet Set<T>把流中元素收集到SettoCollection Coolection<T>把流中元素收集到Collection中groupingBy Map<K,List<T>>根据K属性对流进行分组partitioningBy Map<boolean, List<T>>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);
peek(Consumer c)获取流中元素，操作流中元素，与foreach不同的是不会截断流，可继续操作流
//篮球俱乐部所有成员名 + 赞助商商标^_^,并且获取所有队员详细内容 List<Student> collect = basketballClub.stream() .peek(e -> e.setName(e.getName() + "^_^")) .collect(Collectors.toList()); collect.forEach(System.out::println); //Student{idNum='2015134012', name='小c^_^', age=13, classNum='1503'} //Student{idNum='2015134013', name='小s^_^', age=14, classNum='1503'} //Student{idNum='2015134015', name='小d^_^', age=15, classNum='1504'} //Student{idNum='2015134018', name='小y^_^', age=16, classNum='1505'}
distinct()通过流所生成元素的equals和hashCode去重limit(long val)截断流，取流中前val个元素sorted(Comparator)产生一个新流，按照比较器规则排序sorted()产生一个新流，按照自然顺序排序
List<String> list = Arrays.asList("b","b","c","a"); list.forEach(System.out::print); //bbca List<String> collect = list.stream().distinct().sorted().collect(Collectors.toList()); collect.forEach(System.out::print);//abc //获取list中排序后的top2 即截断取前两个 List<String> collect1 = list.stream().distinct().sorted().limit(2).collect(Collectors.toList()); collect1.forEach(System.out::print);//ab
匹配
booelan allMatch(Predicate)都符合boolean anyMatch(Predicate)任一元素符合boolean noneMatch(Predicate)都不符合
boolean b = basketballClub.stream().allMatch(e -> e.getAge() < 20); boolean b1 = basketballClub.stream().anyMatch(e -> e.getAge() < 20); boolean b2 = basketballClub.stream().noneMatch(e -> e.getAge() < 20);
寻找元素
findFirst——返回第一个元素
findAny——返回当前流中的任意元素
Optional<Student> first = basketballClub.stream().findFirst(); if (first.isPresent()) { Student student = first.get(); System.out.println(student); } Optional<Student> any = basketballClub.stream().findAny(); if (any.isPresent()) { Student student2 = any.get(); System.out.println(student2); } Optional<Student> any1 = basketballClub.stream().parallel().findAny(); System.out.println(any1);
计数和极值
count——返回流中元素的总个数max——返回流中最大值min——返回流中最小值
long count = basketballClub.stream().count(); Optional<Student> max = basketballClub.stream().max(Comparator.comparing(Student::getAge)); if (max.isPresent()) { Student student = max.get(); } Optional<Student> min = basketballClub.stream().min(Comparator.comparingInt(Student::getAge)); if (min.isPresent()) { Student student = min.get(); }
