Stream API

  Saw it in a project recentlyStream,I only had a general understanding of it before,觉得写、It looks troublesome,Not as intuitive as the traditional way of dealing with collections、可读性强,But after knocking on it, I found itStreamIt turned out to be very convenient,subverted my view of it.

Stream 简介

顾名思义,Stream是“流”的意思,它将 集合 Type data is processed as a stream,Think of data as a river,举个例子,I can arrange to wash my face in order from upstream to downstream、洗衣服、洗jio三个过程,Data flows through these three processes sequentially,The last thing to do is washjio水
这里要与IOStream进行区分,不要搞混了,IOStreamis a stream for character processing,StreamIt is a collection of data processing

Stream 优势

个人认为,StreamThat's where the advantage comes from “流” data processing ideas,The return value of a method is itself,This makes it possible to chain calls to handler methods,Let the manipulation of data become a continuous process

• 化繁为简
    对 List A series of operations can be chained together、一气呵成,相比之下,传统的对Listoperation will be troublesome,比如需要将List<User>中User对象中的nameTake it out to make a new oneList,传统做法对比Stream如下：

传统:
List<User> userList = UserListFactory.get();
ArrayList<String> nameList = new ArrayList<>();
for(User user : userList) {
    
	nameList.add(user.getName());
}

Stream:
List<String> nameList = UserListFactory.get().stream()// userList转成stream
								.map(user -> user.getName())//userList中的每个userAn object corresponds to another element(这里是name)
								.collect(Collectors.toList());// Stream转List

• 可读性强
    当然,在不懂Streamusing the rules,StreamIt will be difficult to read,但是在熟悉了Stream的规则后,Stream对ListA series of operations will make you intuitively understand what I am rightListWhat operations were done and in what order
• 代码精简
    如上面的例子,传统方式在userList变nameList的过程中,The code is discontinuous,It will produce intermediate variables and intermediate results,而StreamOne sentence does it.
    If the scene is more complicated,比如要将List转换为Map再转换为List,There are also operations on properties in the middle,The traditional way would be a lot of code,And will produce more useless intermediates,而StreamOne sentence can do it

Stream 使用

  java.util.Collection接口中有stream()方法,可以得到Stream对象,List、Set、Queue实现了Collection接口,有stream方法的实现,而Map却没有这个方法,但,streamThe stream can finally be Map的形式输出

• forEach
    顾名思义,is to iterate over the elements in the collection,Elements are processed at the same time,注意,此方法无返回值.下面的例子就是将userList中每个userThe age of the elements all do+1操作

List<User> userList = UserListFactory.get();
userList.stream().forEach(user -> user.setAge(user.getAge() + 1));

• map
    此方法,是将userList中原本的userElements map to another kind of data.样例如下,注意,List转为StreamThe stream is then converted to by a mapping operationList后,是List,The data type of the elements in the collection must match the data type of the mapping result

List<User> userList = UserListFactory.get();
List<Integer> ageList = userList.stream()
						.map(user -> user.getAge() + 1)
						.collect(Collectors.toList());

• filter
    顾名思义,The function of this method is to filter,里面是一个表达式,The elements of the collection are put into the expression,结果为truethen the element passes,为falsethen the element is blocked

List<User> userList = UserListFactory.get();
List<User> userList2 = userList.stream()
						.filter(user -> user.getAge() > 18)
						.collect(Collectors.toList());

• collect
  看了上面的代码,你肯定有一个疑问,Why is there after every code examplecollect方法？Collection types are in useStream()method will be converted into Stream流类型,collect方法是将StreamThe stream type is formatted as the data type of the specified format,So far, the most commonly used in my project is to convertList或Map
  注意：The first turn belowMap的方式存在bug！None of the key values ​​are allowednull,否则会报空指针异常,If you don't know in advance that there is a mine buried here to debug the code, you will wash your face in tears..避坑！感兴趣的自行百度“Collectors.toMap空指针”或“Stream List转Map的坑”Or check the source code yourself,I won't start talking about it for the time being;Use the second turnMapWorry-free way,The second way is actually to define it yourselfCollectors.toMap方法

Stream转List:
像上面一样,stream.collect(Collectors.toList())即可

Strea转Map方式一(有bug):
List<User> userList = UserListFactory.get();
Map<String, User> userMap = userList.stream()
					.collect(Collectors.toMap(User::getName, user -> user));

Stream转Map方式二:
List<User> userList = UserListFactory.get();
HashMap<Object, Object> userMap = userList.stream()
		.collect(
				HashMap::new, 
				(map, user) -> map.put(user.getName(), user), 
				HashMap::putAll);

• limit
• sorted
• flatMap

…未完待续 2021.02.04