Java Stream API 的使用

java8引入的java.util.stream.Stream流操作，使得访问和操作数组（Array）、集合（Collection）变得非常方便和优雅。
1、过滤元素和转化元素类型

java">    private static void filterMapToInt() {List<String> list = new ArrayList<>();list.add("12");list.add("ab");list.add("34");int[] array = list.stream().filter(str -> {Pattern pattern = Pattern.compile("\\d+");return pattern.matcher(str).find();}).mapToInt(Integer::parseInt).toArray();System.out.println(Arrays.toString(array)); // [12, 34]}

2、平铺流元素和转化

java">    private static void flatMapStreamOfMap() {List<String> list = new ArrayList<>();list.add("12,ab,34");list.add("11,22,bb");Stream<String> stream = list.stream();Stream<String> stringStream = stream.flatMap(str -> Stream.of(str.split(",")).map(String::toUpperCase));System.out.println(stringStream.toList()); // [12, AB, 34, 11, 22, BB]}

3、嵌套类型转为列表

java">private static void mapMulti() {List<Object> objects = List.of(1, List.of(2, List.of(3, 4)), 5);Stream<Object> objectStream = objects.stream().mapMulti(StreamTest::expandIterable);List<Integer> list = objectStream.map(o -> Integer.parseInt(o.toString())).toList();System.out.println(list); // [1, 2, 3, 4, 5]}static void expandIterable(Object e, Consumer<Object> c) {if (e instanceof Iterable<?> elements) {for (Object ie : elements) {expandIterable(ie, c);}} else if (e != null) {c.accept(e);}}

4、多个数据类型转为单一类型

java">private static void mapMulti2() {Map<String, Object> map = new HashMap<>();map.put("aa", new Object());map.put("23.3", new Object());List<Object> objects = List.of(1, map, 1.345f, List.of("bb", 2.45d));// 整数不变，浮点数转化为整数，字符串丢弃，map只需要keyStream<Object> objectStream = objects.stream().mapMulti(StreamTest::handleMapMulti);System.out.println(objectStream.map(object -> Integer.parseInt(object.toString())).toList()); // [1, 23, 1, 2]}static void handleMapMulti(Object e, Consumer<Object> c) {if (e instanceof Iterable<?> elements) {for (Object ie : elements) {handleMapMulti(ie, c);}} else if (e != null) {Object intTarget = e;if (e instanceof Map<?, ?> map) {handleMapMulti(map.keySet(), c);}if (intTarget instanceof Float f) {intTarget = Math.round(f);}if (intTarget instanceof Double d) {intTarget = (int) Math.round(d);}if (intTarget instanceof String str) {try {intTarget = (int) Float.parseFloat(str);} catch (NumberFormatException _) {}}if (intTarget instanceof Integer integer) {c.accept(integer);}}}

5、元素去重

java">    private static void distinct() {List<String> list = new ArrayList<>();list.add("aa");list.add("aa");list.add("bb");System.out.println(list.stream().distinct().toList()); // [aa, bb]}

6、无参数排序（元素自己实现了Comparable）

java">    private static void sortedByItself() {List<Integer> list = new ArrayList<>();list.add(1);list.add(3);list.add(2);System.out.println(list.stream().sorted().toList()); // [1, 2, 3]System.out.println(list.stream().sorted(Comparator.reverseOrder()).toList()); // [3, 2, 1] 倒序}

7、有参数排序（传入Comparator）

java">    private static void sortedByComparator() {List<String> list = new ArrayList<>();list.add("xiaoming,96");list.add("xiaohong,80");list.add("xiaozhang,100");List<String> list1 = list.stream().sorted((s1, s2) -> {String[] info1 = s1.split(",");String[] info2 = s2.split(",");int score1 = Integer.parseInt(info1[1]);int score2 = Integer.parseInt(info2[1]);return score2 - score1;}).toList();System.out.println(list1); // [xiaozhang,100, xiaoming,96, xiaohong,80]}

8、用于debug程序的peek

java">    private static void debugByPeek() {List<String> list = Stream.of("one", "two", "three", "four").filter(e -> e.length() > 3).peek(e -> System.out.println("Filtered value: " + e)).map(String::toUpperCase).peek(e -> System.out.println("Mapped value: " + e)).toList();System.out.println(list);}

打印：
Filtered value: three
Mapped value: THREE
Filtered value: four
Mapped value: FOUR
[THREE, FOUR]

9、限制返回个数

java">    private static void limit() {List<Integer> list = new ArrayList<>();list.add(1);list.add(3);list.add(2);System.out.println(list.stream().limit(2).toList()); // [1, 3]}

10、跳过若干个元素

java">    private static void skip() {List<Integer> list = new ArrayList<>();list.add(1);list.add(3);list.add(2);System.out.println(list.stream().skip(2).toList()); // [2]}

11、有条件的获取元素（java9）

java">    private static void taskWhile() {Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);// 使用 takeWhile 提取元素，直到遇到第一个大于 5 的元素Stream<Integer> takenStream = stream.takeWhile(n -> n <= 5);// 输出提取的元素takenStream.forEach(System.out::print); // 12345}

12、有条件的丢弃元素（java9）

java">    private static void dropWhile() {Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);// 使用 dropWhile 提取元素，丢弃小于等于 5 的元素Stream<Integer> takenStream = stream.dropWhile(n -> n <= 5);// 输出提取的元素takenStream.forEach(System.out::print); // 678910}

13、按顺序执行（并行流（多线程）也能按顺序）

java">    private static void foreachOrdered() {// 在并行流中，forEachOrdered 需要额外的同步机制来保证顺序，因此性能可能不如 forEach。如果顺序不重要，优先使用 forEach// 如果流本身是无序的（例如 HashSet 的流），forEachOrdered 的行为可能与 forEach 相同// 在串行流中，forEach 和 forEachOrdered 的行为几乎一致Stream<Integer> parallelStream = Stream.of(1, 2, 3, 4, 5).parallel();parallelStream.forEach(System.out::print); // 34152 顺序不保证Stream<Integer> parallelStream2 = Stream.of(1, 2, 3, 4, 5).parallel();parallelStream2.forEachOrdered(System.out::print); // 12345 顺序保证}

14、转为list或者转为数组

java">    private static void toListToArray() {String s = "1,2,3";List<String> list = Stream.of(s.split(",")).toList();Object[] objectArr = Stream.of(s.split(",")).toArray();String[] strArr = Stream.of(s.split(",")).toArray(String[]::new);Integer[] intArr = Stream.of(s.split(",")).map(Integer::parseInt).toArray(Integer[]::new);System.out.println("stream.toList:" + list);System.out.println("stream.toArray:" + Arrays.toString(objectArr));System.out.println("stream.toArray(String[]::new):" + Arrays.toString(strArr));System.out.println("stream.toArray(Integer[]::new):" + Arrays.toString(intArr));}

打印：
stream.toList:[1, 2, 3]
stream.toArray:[1, 2, 3]
stream.toArray(String[]::new):[1, 2, 3]
stream.toArray(Integer[]::new):[1, 2, 3]

15、将列表转个单个元素（累积函数，根据某个规则将多个元素合并）

java">    private static void reduce() {List<Integer> numbers = Arrays.asList(1, 2, 3);Optional<Integer> sum = numbers.stream().reduce(Integer::sum); // 求和System.out.println(sum.orElse(0)); // 输出 6Optional<Integer> max = numbers.stream().reduce(Integer::max);// 最大值System.out.println(max.orElse(0)); // 输出 3List<String> list = Arrays.asList("Hello ", "world!");String concat = list.stream().reduce("", String::concat); // 字符调拼接System.out.println(concat); // Hello world!// 初始值（identity）// 必须是累积函数的单位元，即满足：// accumulator.apply(identity, t) == t// 例如：加法初始值为0（0 + t = t），乘法初始值为1（1 * t = t)List<Integer> intList = Arrays.asList(1, 2, 3);int product = intList.stream().reduce(1, (a, b) -> a * b);System.out.println(product); // 输出 6}

16、把流转化为集合、Set或者对流分组和分区

java">  private static void collect() {// 转为 List (等价于：Stream.of("1", "2", "3").toList())List<String> list = Stream.of("1", "2", "3").collect(Collectors.toList());// 转为 Set（自动去重）Set<String> set = Stream.of("1", "2", "3").collect(Collectors.toSet());// 转为特定集合类型TreeSet<String> treeSet = Stream.of("1", "2", "3").collect(Collectors.toCollection(TreeSet::new));// 直接拼接 （等价于： String.join("", "1", "2", "3");）String joined = Stream.of("1", "2", "3").collect(Collectors.joining());// 带分隔符（等价于：String.join(", ", "1", "2", "3");）String joinedWithComma = Stream.of("1", "2", "3").collect(Collectors.joining(", "));// 带前缀和后缀String wrapped = Stream.of("1", "2", "3").collect(Collectors.joining(", ", "[", "]"));// 平均值Double avg = Stream.of("1", "2", "3").collect(Collectors.averagingInt(Integer::parseInt));// 总和 （等价于：Stream.of("1", "2", "3").mapToInt(Integer::parseInt).sum();）Integer sum = Stream.of("1", "2", "3").collect(Collectors.summingInt(Integer::parseInt));// 基础用法（键重复会抛出异常）Map<String, Integer> map = Stream.of(new Student(93, "xiaoming")).collect(Collectors.toMap(Student::getName,  // 键提取函数Student::getScore // 值提取函数));// 处理键冲突（第三个参数定义合并策略）Map<String, Integer> safeMap = Stream.of(new Student(93, "xiaoming"), new Student(94, "xiaohong"), new Student(6, "xiaohong")).collect(Collectors.toMap(Student::getName,Student::getScore,Integer::sum // 合并重复键的值));// 自定义 Map 类型（如 LinkedHashMap）Map<String, Integer> linkedMap = Stream.of(new Student(93, "xiaoming"), new Student(94, "xiaoming")).collect(Collectors.toMap(Student::getName,Student::getScore,(oldVal, _) -> oldVal,LinkedHashMap::new));// 示例：自定义收集到逗号分隔的字符串Collector<Student, StringJoiner, String> customCollector =Collector.of(() -> new StringJoiner(","), // Supplier（初始化容器）(j, e) -> j.add(e.getName()), // Accumulator（累加器：添加元素）StringJoiner::merge,          // Combiner（多线程：合并并行结果）StringJoiner::toString        // Finisher（最终转换）);String names = Stream.of(new Student(93, "xiaoming"), new Student(94, "xiaohong")).collect(customCollector);System.out.println(names); // xiaoming, xiaohong// 分区（按布尔条件分为两组）Map<Boolean, List<Student>> partition = Stream.of(new Student(93, "xiaoming"), new Student(94, "xiaohong")).collect(Collectors.partitioningBy(e -> e.getScore() > 93));System.out.println(partition);  // {false=[score=93, name='xiaoming], true=[score=94, name='xiaohong]}// 按学科分组（默认返回 Map<K, List<T>>）Map<String, List<Student>> groupByDept = Stream.of(new Student(93, "xiaoming", "English")).collect(Collectors.groupingBy(Student::getSubject));// 分组后统计（下游收集器，统计每个学科有多少人）Map<String, Long> countByDept = Stream.of(new Student(93, "xiaoming", "English")).collect(Collectors.groupingBy(Student::getSubject,Collectors.counting() // 统计每组的元素数量));}static class Student {int score;String name;String subject;}

代码中Student省略getter、setter和构造函数

17、求最大值最小值

java">    private static void minMax() {Optional<Integer> min = Stream.of(1, 2, 3).min(Integer::compareTo);System.out.println(min.orElse(0)); // 1Optional<Integer> max = Stream.of(1, 2, 3).max(Integer::compareTo);System.out.println(max.orElse(4)); // 3}

18、计算流包含的元素个数

java">    private static void count() {System.out.println(Stream.of(1,2,3).count()); // 3}

19、执行条件匹配的判断

java">    private static void anyMatchAllMatchNoneMatch() {System.out.println(Stream.of(1, 2, 3).anyMatch(num -> num > 1));  // trueSystem.out.println(Stream.of(1, 2, 3).allMatch(num -> num > 2));  // false，不是全都大于2System.out.println(Stream.of(1, 2, 3).noneMatch(num -> num > 1)); // false，存在匹配的数据}

20、查找流的第一个元素或者任意一个元素

java">    private static void findFirstFindAny() {Optional<Integer> first = Stream.of(null, 2, 3).findFirst(); // java.lang.NullPointerExceptionOptional<Integer> one = Stream.of(1, 2, 3).findFirst(); // 1System.out.println(Stream.of(1, 2, 3).findAny().get()); // 其中一个，也会报空指针异常的可能}

21、通过构造器生成流

java">    private static void builder() {// add(T t)	添加元素到流，返回 Builder 自身（支持链式调用）// accept(T t)	与 add 功能相同，但返回 void（实现 Consumer 接口）// build()	构建最终的 Stream，之后不能再修改Stream.Builder<String> builder = Stream.builder();builder.add("Apple");builder.add("Banana").add("Cherry"); // 链式调用Stream<String> stream = builder.build();stream.forEach(System.out::println); // 输出：Apple Banana Cherry}

22、无限流的构造以及无限流的终止

java">    private static void iterate() {// static <T> Stream<T> iterate(T seed, UnaryOperator<T> f)// iterate是一个用于生成无限流或有限流的静态方法， 生成一个无限流，从初始值seed开始，依次应用UnaryOperator生成后续元素// 在java8中结合limit来限制个数，java9中允许通过条件（predicate）来终止迭代// 生成偶数序列：0, 2, 4, 6, ...Stream<Integer> evenNumbers = Stream.iterate(0, n -> n + 2);// 通常需要结合 limit() 截断无限流(java8)evenNumbers.limit(5).forEach(System.out::println); // 输出 0 2 4 6 8// 生成数字直到小于 10：0, 1, 2, ..., 9 (java9)Stream<Integer> numbers = Stream.iterate(0,n -> n < 10, // 条件：当 n < 10 时继续n -> n + 1   // 迭代函数);numbers.forEach(System.out::println); // 输出 0 到 9}

Java Stream API 的使用

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