经常会有童鞋把 Map 和 Foreach 用错，可能会出现如下用法：
List<Student> studentChangeList = studentList.stream()
.forEach(student -> student.setAge(99));有些编译器会直接报错，比如 IDEA。因为 ForEach 是没有返回值的，ForEach 处理过的 Stream 是无法再赋值给 studentChangeList 。
但是现在就是需要对集合进行处理，并获取处理过的集合数据，这时候可以这样做
studentList.stream().forEach(student -> student.setAge(99));studentList 中的数据就是已经处理过的数据。
下面就 Map 和 ForEach 做一些简单的说明，大体上就能对这两个方法有所理解
一、 结论
Map：返回的是一个新流，可以对这个流进一步操作
ForEach：返回void，即无返回值
二、源码
1. Map 源码说明
/**
* 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 源码说明
/**
* 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、应用
1.Map 是1对1的映射
示例：
将两位同学的年龄都加上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("张三丰");
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);
}运行结果：
117
118
2.ForEach 是对 Stream 中每一个元素进行处理。
虽然 ForEach 处理 Stream 中元素的时候没有返回值，但是 ForEach 对 Stream 中元素已经产生影响，即 ForEach 对 Stream 中元素的操作已经被保存下来。
示例：
将两位同学的年龄改为99，名字改为英俊
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("张三丰");
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);
}运行结果：
Student(studentId=1, studentName=英俊, age=99)
Student(studentId=2, studentName=英俊, age=99)
