超强的Stream流了解一下

演示点一：

筛选年龄小于21，且薪水大于2000的数据。

第一种写法：
List<User> userList = Arrays.asList(
                new User("小明", 20, 3000),
                new User("小红", 20, 2000),
                new User("小张", 21, 3000)
        );
        Predicate<User> predicate1 = user -> user.getAge() < 21;
        Predicate<User> predicate2 = user -> user.getSalary() > 2000;

        List<User> collect = userList.stream()
                .filter(predicate1)
                .filter(predicate2)
                .collect(Collectors.toList());
        System.out.println("collect = " + collect);

第二种写法：
List<User> userList = Arrays.asList(
                new User("小明", 20, 3000),
                new User("小红", 20, 2000),
                new User("小张", 21, 3000)
        );

        List<User> collect = userList.stream()
                .filter(user -> user.getAge() < 21)
                .filter(user -> user.getSalary() > 2000)
                .collect(Collectors.toList());
        System.out.println("collect = " + collect);

第三种写法：
        List<User> userList = Arrays.asList(
                new User("小明", 20, 3000),
                new User("小红", 20, 2000),
                new User("小张", 21, 3000)
        );
        Predicate<User> predicate1 = user -> user.getAge() < 21;
        Predicate<User> predicate2 = user -> user.getSalary() > 2000;

        List<User> collect = userList.stream()
                .filter(predicate1.and(predicate2))
                .collect(Collectors.toList());
        System.out.println("collect = " + collect);

collect = [User(name=小明, age=20, salary=3000)]

演示点二：

筛选年龄小于21，或薪水大于2000的数据。

List<User> userList = Arrays.asList(
                new User("小明", 20, 3000),
                new User("小红", 20, 2000),
                new User("小张", 21, 3000)
        );
        Predicate<User> predicate1 = user -> user.getAge() < 21;
        Predicate<User> predicate2 = user -> user.getSalary() > 2000;

        List<User> collect = userList.stream()
                .filter(predicate1.or(predicate2))
                .collect(Collectors.toList());
        System.out.println("collect = " + collect);

collect = [User(name=小明, age=20, salary=3000), User(name=小红, age=20, salary=2000), User(name=小张, age=21, salary=3000)]

演示点三：

filter的使用：筛选字符串长度大于等于5的字符串。

List<String> list = Arrays.asList("Hello", "World", "Java");
        list.stream()
                .filter(str -> str.length() >= 5)
                .forEach(str -> System.out.println(str));

Hello
World

演示点四

limit 截取流中的元素

        List<String> list = Arrays.asList("Hello", "World", "Java");
        list.stream()
                .limit(2)
                .forEach(str -> System.out.println(str));

Hello
World

演示点五

sorted 排序，默认进行升序排列

List<Integer> list = Arrays.asList(1,6,9,3,8,12,0,29);
        list.stream()
                .sorted()
                .forEach(num -> System.out.println(num));

0
1
3
6
8
9
12
29

降序排列：

    List<Integer> list = Arrays.asList(1,6,9,3,8,12,0,29);
        list.stream()
                .sorted(Comparator.reverseOrder())
                .forEach(num -> System.out.println(num));

29
12
9
8
6
3
1
0

演示点六

max 求最大值，min 最小值, count 长度

List<Integer> list = Arrays.asList(1, 6, 9, 3, 8, 12, 0, 29);
        System.out.println(list.stream()
                .max(Integer::compareTo)
                .get());
        System.out.println(list.stream()
                .min(Integer::compareTo)
                .get());
        System.out.println(list.stream()
                .count());

29
0
8

演示点七

stream的map操作：对流操作，返回一个新的流。

List<Integer> list = Arrays.asList(1, 6, 9, 3, 8, 12, 0, 29);
        list.stream()
                .map(num -> num + 10)
                .forEach(num -> System.out.println(num));

演示点八

reduce 根据指定的计算模型将Stream中的值计算得到一个最终结果。

    List<Integer> list = Arrays.asList(1, 6, 9, 3, 8, 12, 0, 29);
        System.out.println(list.stream()
                .reduce((sum, num) -> sum + num).get());

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