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

Question

Given an ArrayList with 5 Employee(id,name,location,salary) objects,write a program to extract the location details of each Employee and store it in an ArrayList, with the help of Function.

I want to use stream.map function for this question.

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());
        }

}

But I am getting an error at this line - .map(Employee:: apply). I want to use String s=f1.apply(employeeObject) in map. How to do that

@HadiJ There's no need to create a method reference when you already have a `Function` object — user, Jul 12 '20 at 17:37

score 3 · Answer 1 · edited Jul 12 '20 at 09:28

Employee has no apply method.

You should pass the f1 instance, which implements Function<Employee,String>, to map():

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

P.S. it would be better to use a getter instead of accessing the instance variable directly:

with lambda Function<Employee,String> f1 = s -> s.getLocation();
with method reference Function<Employee,String> f1 = Employee::getLocation;

Of course, you can do this without f1:

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

Eklavya · Accepted Answer · 2020-07-12T09:34:59.580

2

There is not apply method in Employee class. You can use the function directly

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

Or use lambda inside map()

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

edited Jul 12 '20 at 09:34

answered Jul 12 '20 at 09:24

Eklavya

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I know their is no apply method in Employee class. But there is one we can use with Function Interface. Like this - `String location=f1.apply(Employee e)`. Your answer helps too. We can do this without using any apply function. Thank you for helping. – Muskan Agarwal Jul 12 '20 at 09:41
1

@MuskanAgarwal That's why you can use like `f1::apply` – Eklavya Jul 12 '20 at 09:43
Yeah got it. Thanks – Muskan Agarwal Jul 12 '20 at 09:44

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

2 Answers2