stream.map函数中函数接口的apply函数怎么写？

Question

给定一个包含 5 个 Employee(id,name,location,salary) 对象的 ArrayList，借助 Function 编写程序提取每个 Employee 的位置详细信息并将其存储在 ArrayList 中。

我想用 stream.map 函数来回答这个问题。

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

class Employee{
    int id;
    String name;
    String location;
    int salary;
    
    Employee(int id,String name,String location,int salary){
        this.id=id;
        this.name=name;
        this.location=location;
        this.salary=salary;
    }
}


public class JAVA8 {
    public static void main(String[] args){
         ArrayList<Employee> al=new ArrayList<Employee>();
         Employee e=new Employee(123,"Muskan","Rajasthan",34000);
         Employee e1=new Employee(456,"Sonali","Maharashtra",45003);
         Employee e2=new Employee(789,"Khushboo","LaxmanGarh",22222);
         Employee e3=new Employee(012,"Minakshi","USA",22);
         al.add(e);
         al.add(e1);
         al.add(e2);
         al.add(e3);
         Function<Employee,String> f1=(s)->(s.location);
         String a;
         List<String> li=al.stream()
                                .map(Employee::apply)
                                .collect(Collectors.toList());
        }

}

但是我在这一行遇到错误 - .map(Employee:: apply)。我想在地图中使用 String s=f1.apply(employeeObject)。怎么做

Answer 1

Employee 没有 apply 方法。

您应该将实现 Function<Employee,String> 的 f1 实例传递给 map():

List<String> li=al.stream()
                  .map(f1)
                  .collect(Collectors.toList());

P.S。最好使用 getter 而不是直接访问实例变量：

与 lambda Function<Employee,String> f1 = s -> s.getLocation();
附方法参考Function<Employee,String> f1 = Employee::getLocation;

当然，你可以不用f1:

List<String> li=al.stream()
                  .map(Employee::getLocation)
                  .collect(Collectors.toList());

Answer 2

Employeeclass中没有apply方法。可以直接使用函数

Function<Employee,String> f1=(s)->(s.location);
List<String> li=al.stream().map(f1).collect(Collectors.toList());

或者在 map() 中使用 lambda

List<String> li=al.stream().map(s->s.location).collect(Collectors.toList());

stream.map函数中函数接口的apply函数怎么写？

How to write apply function of function Interface in stream.map function?

java

java-stream

functional-interface