經(jīng)常會(huì)有童鞋把 Map 和 Foreach 用錯(cuò)，可能會(huì)出現(xiàn)如下用法：

List<Student> studentChangeList = studentList.stream().forEach(student -> student.setAge(99));

有些編譯器會(huì)直接報(bào)錯(cuò)，比如 IDEA。因?yàn)?ForEach 是沒(méi)有返回值的，ForEach 處理過(guò)的 Stream 是無(wú)法再賦值給 studentChangeList 。

但是現(xiàn)在就是需要對(duì)集合進(jìn)行處理，并獲取處理過(guò)的集合數(shù)據(jù)，這時(shí)候可以這樣做

studentList.stream().forEach(student -> student.setAge(99));

studentList 中的數(shù)據(jù)就是已經(jīng)處理過(guò)的數(shù)據(jù)。

下面就 Map 和 ForEach 做一些簡(jiǎn)單的說(shuō)明，大體上就能對(duì)這兩個(gè)方法有所理解

一、 結(jié)論

Map：返回的是一個(gè)新流，可以對(duì)這個(gè)流進(jìn)一步操作

ForEach：返回void，即無(wú)返回值

二、源碼

1. Map 源碼說(shuō)明

/*** Returns a stream consisting of the results of applying the given* function to the elements of this stream.** <p>This is an <a href="package-summary.html#StreamOps">intermediate* operation</a>.** @param <R> The element type of the new stream* @param mapper a <a href="package-summary.html#NonInterference">non-interfering</a>,* <a href="package-summary.html#Statelessness">stateless</a>* function to apply to each element* @return the new stream*/<R> Stream<R> map(Function<? super T, ? extends R> mapper);

2. ForEach 源碼說(shuō)明

/*** Performs an action for each element of this stream.** <p>This is a <a href="package-summary.html#StreamOps">terminal* operation</a>.** <p>The behavior of this operation is explicitly nondeterministic.* For parallel stream pipelines, this operation does <em>not</em>* guarantee to respect the encounter order of the stream, as doing so* would sacrifice the benefit of parallelism. For any given element, the* action may be performed at whatever time and in whatever thread the* library chooses. If the action accesses shared state, it is* responsible for providing the required synchronization.** @param action a <a href="package-summary.html#NonInterference">* non-interfering</a> action to perform on the elements*/void forEach(Consumer<? super T> action);

三、應(yīng)用

1.Map 是1對(duì)1的映射

示例：
將兩位同學(xué)的年齡都加上100

public static void main(String[] args) {List<Student> studentList = new ArrayList<>();Student student1 = new Student();student1.setStudentId(1);student1.setStudentName("李毅");student1.setAge(17);Student student2 = new Student();student2.setStudentId(2);student2.setStudentName("張三豐");student2.setAge(18);studentList.add(student1);studentList.add(student2);List<Integer> ageList = studentList.stream().map(student -> student.getAge() + 100).collect(Collectors.toList());ageList.stream().forEach(System.out::println);}

運(yùn)行結(jié)果：

117 118

2.ForEach 是對(duì) Stream 中每一個(gè)元素進(jìn)行處理。

雖然 ForEach 處理 Stream 中元素的時(shí)候沒(méi)有返回值，但是 ForEach 對(duì) Stream 中元素已經(jīng)產(chǎn)生影響，即 ForEach 對(duì) Stream 中元素的操作已經(jīng)被保存下來(lái)。

示例：
將兩位同學(xué)的年齡改為99，名字改為英俊

public static void main(String[] args) {List<Student> studentList = new ArrayList<>();Student student1 = new Student();student1.setStudentId(1);student1.setStudentName("李毅");student1.setAge(17);Student student2 = new Student();student2.setStudentId(2);student2.setStudentName("張三豐");student2.setAge(18);studentList.add(student1);studentList.add(student2);studentList.stream().forEach(student -> {student.setAge(99);student.setStudentName("英俊");});studentList.stream().forEach(System.out::println);}

運(yùn)行結(jié)果：

Student(studentId=1, studentName=英俊, age=99) Student(studentId=2, studentName=英俊, age=99)

總結(jié)

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