java8自帶常用的函數式接口
Predicate<T> boolean test(T t)傳入一個參數返回boolean值Consumer<T> void accept(T t)傳入一個參數，無返回值Function<T,R> R apply(T t)傳入一個參數，返回另一個類型
准備數據
//計算機俱樂部 private static List<Student> computerClub = Arrays.asList( new Student("2015134001", "小明", 15, "1501"), new Student("2015134003", "小王", 14, "1503"), new Student("2015134006", "小張", 15, "1501"), new Student("2015134008", "小梁", 17, "1505") ); //籃球俱樂部 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") ); //乒乓球俱樂部 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三種創建方式
- 集合
Collection.stream() - 靜態方法
Stream.of - 數組
Arrays.stream
//1.集合 Stream<Student> stream = basketballClub.stream(); //2.靜態方法 Stream<String> stream2 = Stream.of("a", "b", "c"); //3.數組 String[] arr = {"a","b","c"}; Stream<String> stream3 = Arrays.stream(arr);
Stream的終止操作
foreach(Consumer c)遍歷操作collect(Collector)將流轉化為其他形式max(Comparator)返回流中最大值min(Comparator)返回流中最小值count返回流中元素綜述
Collectors 具體方法
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>>根據K屬性對流進行分組partitioningBy Map<boolean, List<T>>根據boolean值進行分組
//此處只是演示 此類需求直接用List構造器即可 List<Student> collect = computerClub.stream().collect(Collectors.toList()); Set<Student> collect1 = pingpongClub.stream().collect(Collectors.toSet()); //注意key必須是唯一的 如果不是唯一的會報錯而不是像普通map那樣覆蓋 Map<String, String> collect2 = pingpongClub.stream() .collect(Collectors.toMap(Student::getIdNum, Student::getName)); //分組 類似於數據庫中的group by Map<String, List<Student>> collect3 = pingpongClub.stream() .collect(Collectors.groupingBy(Student::getClassNum)); //字符串拼接 第一個參數是分隔符 第二個參數是前綴 第三個參數是后綴 String collect4 = pingpongClub.stream().map(Student::getName).collect(Collectors.joining(",", "【", "】")); //【小u,小i,小m,小n】 //三個俱樂部符合年齡要求的按照班級分組 Map<String, List<Student>> collect5 = Stream.of(basketballClub, pingpongClub, computerClub) .flatMap(e -> e.stream().filter(s -> s.getAge() < 17)) .collect(Collectors.groupingBy(Student::getClassNum)); //按照是否年齡>16進行分組 key為true和false ConcurrentMap<Boolean, List<Student>> collect6 = Stream.of(basketballClub, pingpongClub, computerClub) .flatMap(Collection::stream) .collect(Collectors.groupingByConcurrent(s -> s.getAge() > 16));
Stream的中間操作
filter(Predicate)篩選流中某些元素
//篩選1501班的學生 computerClub.stream().filter(e -> e.getClassNum().equals("1501")).forEach(System.out::println); //篩選年齡大於15的學生 List<Student> collect = computerClub.stream().filter(e -> e.getAge() > 15).collect(Collectors.toList());
map(Function f)接收流中元素，並且將其映射成為新元素，例如從student對象中取name屬性
//籃球俱樂部所有成員名 + 暫時住上商標^_^,並且獲取所有隊員名 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)將所有流中的元素並到一起連接成一個流
//獲取年齡大於15的所有俱樂部成員 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); //用雙層list獲取所有年齡大於15的俱樂部成員 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)獲取流中元素，操作流中元素，與foreach不同的是不會截斷流，可繼續操作
//籃球俱樂部所有成員名 + 贊助商商標^_^,並且獲取所有隊員詳細內容 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'}
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distinct()通過流所生成元素的equals和hashCode去重 -
limit(long val)截斷流，取流中前val個元素 -
sorted(Comparator)產生一個新流，按照比較器規則排序 -
sorted()產生一個新流，按照自然順序排序
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
//獲取list中排序后的top2 即截斷取前兩個
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);
尋找元素
findFirst——返回第一個元素findAny——返回當前流中的任意元素
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);
計數和極值
count——返回流中元素的總個數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(); }