map 函数的作用就是针对管道流中的每一个数据元素进行转换操作。

例如

将集合中的每一个字符串，全部转换成大写！

List<String> collect = alpha.stream().map(String::toUpperCase).collect(Collectors.toList());
//上面使用了方法引用，和下面的lambda表达式语法效果是一样的
//List<String> collect = alpha.stream().map(s -> s.toUpperCase()).collect(Collectors.toList());

直接返回各个元素的长度生成一个新的数组，也就是说不仅可以处理数据，还可以在处理数据的过程中转换数据类型

List<String> alpha = Arrays.asList("Monkey", "Lion", "Giraffe", "Lemur");
List<Integer> lengths = alpha.stream().map(String::length).collect(Collectors.toList());System.out.println(lengths); //[6, 4, 7, 5]

处理对象数据

public static void main(String[] args){Employee e1 = new Employee(1,23,"M","Rick","Beethovan");Employee e2 = new Employee(2,13,"F","Martina","Hengis");Employee e3 = new Employee(3,43,"M","Ricky","Martin");Employee e4 = new Employee(4,26,"M","Jon","Lowman");Employee e5 = new Employee(5,19,"F","Cristine","Maria");Employee e6 = new Employee(6,15,"M","David","Feezor");Employee e7 = new Employee(7,68,"F","Melissa","Roy");Employee e8 = new Employee(8,79,"M","Alex","Gussin");Employee e9 = new Employee(9,15,"F","Neetu","Singh");Employee e10 = new Employee(10,45,"M","Naveen","Jain");List<Employee> employees = Arrays.asList(e1, e2, e3, e4, e5, e6, e7, e8, e9, e10);/*List<Employee> maped = employees.stream().map(e -> {e.setAge(e.getAge() + 1);e.setGender(e.getGender().equals("M")?"male":"female");return e;}).collect(Collectors.toList());*/List<Employee> maped = employees.stream().peek(e -> {e.setAge(e.getAge() + 1);e.setGender(e.getGender().equals("M")?"male":"female");}).collect(Collectors.toList());System.out.println(maped);}