Converting List to Map using Lambda Expression Not working

Question

I am trying to convert a List of Student objects into a map in which key is the integer(i.e. rollno field of Student object) and Value is the Student Object itself.

The following is the code that i have written:

package fibonacci;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class MApfor {


    public static void main(String[] args) {


        List<Student> maplist=new ArrayList<>();

        maplist.add(new Student(1, "pavan"));
        maplist.add(new Student(2, "Dhoni"));
        maplist.add(new Student(3, "kohli"));

        maplist.forEach(System.out::println);

        Map<Integer,Student> IS=new HashMap<>();



        IS = maplist.stream().collect(Collectors.toMap(a -> a.getRollNo,a);



    }
}

Whenever i try to write the last line, i.e.

IS = maplist.stream().collect(Collectors.toMap(a -> a.getRollNo,a);

i am unable to retrieve the rollNo field, eclipse is not showing the suggestions i.e. whenever i type a.get to assign the rollNo to key,i am not able to do so.

Please suggest the issue that i am facing.

package fibonacci;

public class Student {


    public int rollNo;
    public String name;
    public int getRollNo() {
        return rollNo;
    }
    public void setRollNo(int rollNo) {
        this.rollNo = rollNo;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Student(int rollNo, String name) {
        super();
        this.rollNo = rollNo;
        this.name = name;
    }
    @Override
    public String toString() {
        return " [rollNo=" + rollNo + ", name=" + name + "]";
    }

}

IS = maplist.stream().collect(Collectors.toMap(a -> a.getRollNo,a -> a); — Eran
– Eran, Commented Jun 28, 2018 at 9:11

pvpkiran · Accepted Answer · 2018-06-28 09:26:36Z

5

It should be like this

maplist.stream().collect(Collectors.toMap(a -> a.rollNo, Function.identity()));

or even better is to use getters with method references.

maplist.stream().collect(Collectors.toMap(Student::getRollNo, Function.identity()));

a -> a.getRollNo this is incorrect. You should use the public field or the getter. You are using neither correctly.

when you say a.getRollNo, this means there should be a public field by name getRollNo in your class, which is not true. Your field is called rollNo.
Then if you want to access the getter method of rollNo, then it should be like a.getRollNo(). (you were missing () in the end).
But you can use method references like this Student::getRollNo also

So it should be either one of these

a -> a.rollNo
a -> a.getRollNo()
Student::getRollNo

You can replace Function.identity() with a -> a as well.

edited Jun 28, 2018 at 9:26

answered Jun 28, 2018 at 9:13

pvpkiran

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2 Comments

Pavan Ghantasala Over a year ago

Thanks kiran it worked when i replaced a.getRollNo with a.rollNo..can you explain this line what i did wrong again ---"You should use the public field or the getter. You are using neither correctly."

Pavan Ghantasala Over a year ago

Now i understood,all this time i was waiting for eclipse to give the suggestion whenever i typed a.get and i am not getting this getRollNo() suggestion

azro · Accepted Answer · 2018-06-28 09:16:49Z

1

a.getRollNo does not exists, the proper getter is a.getRollNo() (or a.rollNo directly, but you'd better set your members as private)
You have to use a -> a function, map the object to itself :

IS = maplist.stream().collect(Collectors.toMap(a -> a.getRollNo(), a -> a));
// or
IS = maplist.stream().collect(Collectors.toMap(a -> a.getRollNo(), Function.identity());

Using method reference

IS = maplist.stream().collect(Collectors.toMap(Student::getRollNo, Function.identity()));

edited Jun 28, 2018 at 9:16

answered Jun 28, 2018 at 9:11

azro

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2 Comments

azro Over a year ago

@PavanGhantasala the other answers same after me but Okok play fair

Pavan Ghantasala Over a year ago

bro, i i didn't notice the time... :-(

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