Recently, I found that this Stream is used a lot, and I have mastered the basic grammar. However, when encountering actual scenarios, it takes a lot of time to write them out. Therefore, we summarize the writing methods of common scenarios to facilitate rapid application in real scenarios.

In addition, for basic grammar and errors encountered, you can refer to my other articles

Basic syntax reference

New features of Java8: Detailed explanation of new features such as HashMap optimization, lambda, and Stream

Problems encountered and solutions

这个文章访问量是真多，应该这个错比较常见，大家通过搜索引擎搜到了它 Java8-Stream: no instance(s) of type variable(s) R exist so that void conforms to R

1. Scenario overview

1.1, the role of Stream

We use Stream mainly to replace the bloated for loop and perform various sao operations on the collection

1.2. Usage scenarios

There are mainly the following scenarios:

1. group by

2. order by (sorting) 3. where (filtering) 4. distinct (removing duplicates) 5. appLy (performing various operations according to an attribute) 6. extracting an attribute as a list

2. Tips

2.1、group by

Group by gender

userList.stream()
	.collect(Collectors.groupingBy(User::getSex));
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2.2、order by

Sort by user age (ascending/descending) and take top3

userList.stream()
	.sorted(Comparator.comparing(User::getAge).reversed())
	.limit(3)
	.collect(Collectors.toList());
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2.3、where

2.3.1, the most value filter

Get the object with the max/min of a property

// 最小
Optional<User> min = userList.stream()
	.min(Comparator.comparing(User::getAge));
// 最大
Optional<User> max = userList.stream()
	.max(Comparator.comparing(User::getAge));

// 获得对象
User user = min.get();
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2.3.2. Conditional filtering

Filter users under the age of 30

userList.stream()
	.filter(e -> e.getAge() < 30)
	.collect(Collectors.toList());
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Select user age > 20 and gender is male (sex=1)


userList.stream()
	.filter(u -> u.getAge() > 20 && u.getSex() == 1)
	.collect(Collectors.toList());
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Query the first user named "Li Hua"

userList.stream().
	filter(u -> u.getName().equals("小明"))
	.findFirst().orElse(ll);
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2.4、distinct

Get all usernames and deduplicate

userList.stream()
	.map(User::getName)
	.distinct()
	.collect(Collectors.toList());
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Deduplication according to a field

memberListAll.stream()
	.collect(Collectors.collectingAndThen(
                        Collectors.toCollection(
				() -> new TreeSet<>(Comparator.comparing(WorkWxUserInfoVO :: getUserid))), ArrayList::new)
	);
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2.5、apply

Mass assignment to a property

userList.forEach(e -> {
            e.setName("hello");
        });
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process a field

userList.stream()
	.map(user -> {user.setName(user.getName().replaceAll("\u0000", "")); return user;})
	.collect(Collectors.toList());
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Get an object based on a field

List<User> userList = userIds.stream()
            .map(id -> {
                User user = userService.getUserById(id);
                return user;
            })
            .collect(Collectors.toList());
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2.6, extract attributes

Extract a single attribute: get all usernames and deduplicate

userList.stream()
	.map(User::getName)
	.distinct()
	.collect(Collectors.toList());
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Extract multiple attributes: combine menuId and menuName into map (menuId is unique)

userList.stream()
	.collect(Collectors.toMap(User::getMenuId, User::getMenuName)));
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Extract multiple attributes: combine menuId and menuName into map (menuId is not unique)

userList
	//去重
	.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(
                                () -> new TreeSet<>(Comparator.comparing(User :: getMenuId))), ArrayList::new))
	//转map
	.stream().collect(Collectors.toMap(User::getMenuId, User::getMenuName)));
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The above only records the most common scenarios. For more scenarios, please refer to: Java8 New Features: Detailed explanation of new features such as HashMap optimization, lambda, and Stream

Practical collection: Java8 Stream practical tips record