超强的Stream流了解一下

Posted 2021-12-03 浅殇忆流年

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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