Lambda 表达式（三）

5、Stream API

Stream 是 Java8 中处理集合的关键抽象概念，它可以指定你对集合的操作，可以执行查找、过滤和映射数据等操作。

Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。

5.1、流(Stream) 是什么呢？

数据通道，是一个来自数据源的元素队列并支持聚合操作。

注意：

Stream 自己不会存储元素。
Stream 不会改变源对象。相反，他们会返回一个持有结果的新Stream。
Stream 操作是延迟执行的。这意味着他们会等到需要结果的时候才执行。

5.2、操作流的三个步骤

创建流
中间操作
终止操作

5.3、创建Stream

//集合
List<String> list = new ArrayList<>();
Stream<String> stream = list.stream();  
//数组
IntStream stream = Arrays.stream(new int[10]);
//Stream静态方法
Stream<String> a = Stream.of("a", "2", "3");
//创建无限流
Stream<Integer> iterate = Stream.iterate(0, (x) -> x + 2);

5.4、Stream中间操作

一个Stream可以有多个中间操作，形成一个流水线，遇到终止操作依次执行中间操作。

1、filter(Predicate p)

接收 Lambda ，从流中排除某些元素

Stream<String> stream = Stream.of("abc", "122", "lemon","haha");

stream = stream.filter((e)->{
    System.out.println("filter中间操作");
    return e.length() > 3;
});

stream.forEach(System.out::println);

2、distinct()

筛选，通过流所生成元素的 hashCode() 和 equals() 去除重复元素

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream = stream.distinct();

stream.forEach(System.out::println);

3、limit

截断流，使其元素不超过给定数量。

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream = stream.distinct().limit(2);

stream.forEach(System.out::println);

4、skip(long n)

跳过元素，跳过前N个元素，与 limit(n) 互补

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream = stream.skip(2);

stream.forEach(System.out::println);

5、map(Function f)

接收一个函数作为参数，该函数会被应用到每个元素上，并将其映射成一个新的元素。

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream = stream.map((e) -> e.toUpperCase());

stream.forEach(System.out::println);

6、flatMap(Function f)

接收一个函数作为参数，将流中的每个值都换成另一个流，然后把所有流连接成一个流

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream.flatMap((e) -> {
    String[] split = e.split("");
    return Stream.of(split);
}).forEach(System.out::println);

7、sorted()

产生一个新流，其中按自然顺序排序。

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

stream.sorted().forEach(System.out::println);

8、sorted(Comparator comp)

产生一个新流，其中按比较器顺序排序（定制排序）

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

       stream.sorted(Comparator.comparingInt(String::length)).forEach(System.out::println);

5.4、Stream 的终止操作

1、count()

返回流中元素总数

Stream<String> stream = Stream.of("abc","abc", "122", "lemon","haha","abc");

long count = stream.sorted(Comparator.comparingInt(String::length)).count();

System.out.println(count);

2、max(Comparator c)

返回流中最大值

Stream<String> stream = Stream.of("123","1234", "12345", "123456","1234567","aaaaaaaaaa");

Optional<String> max = stream.max(Comparator.comparingInt(String::length));

System.out.println(max.get());

3、min(Comparator c)

返回流中最小值

Stream<String> stream = Stream.of("123","1234", "12345", "123456","1234567","aaaaaaaaaa");

Optional<String> min = stream.min(Comparator.comparingInt(String::length));

System.out.println(min.get());

4、forEach(Consumer c)

内部迭代

Stream<String> stream = Stream.of("123","1234", "12345", "123456","1234567","aaaaaaaaaa");

stream.forEach(System.out::println);

5、reduce(T iden, BinaryOperator b)

将流中元素结合起来，得到一个值，返回 T

Stream<Integer> stream = Stream.of(1,2,3,4,5);

int result = stream.reduce(0, (x, y) -> {
    return x + y;
});
System.out.println(result);

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