Java 8 Stream convert List to Map

Below example shows how to convert a List to Map in Java 8 using Stream. Suppose we have a List of students and we want to construct a Map with key as student rollnumber and value as student name, you can do it as below:

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
	
class Student {
    private int rollNumber;
    private String name;
    private int age;
	
    Student (int rollNumber, String name, int age) {
        this.rollNumber = rollNumber;
        this.name = name;
        this.age = age;
    }
    
    // removed getters and setter for brevity    
   
    @Override
    public String toString() {
        return "Student [rollNumber=" + rollNumber + ", name=" + name + ", age="+ age + "]";
    }
}

// Converts List to Map using Java 8 Collectors toMap() method
public class ListToMapConverter { 

    public static void main(String[] args) {
	
        List<Student> students = new ArrayList<Student>();
        students.add(new Student(1, "John", 13));
        students.add(new Student(2, "Mark", 14));
        students.add(new Student(3, "Henry", 13));
			
        Stream<Student> studentStream = students.stream();
        Map<Integer, String> rollNumberNameMap = 
        studentStream.collect(Collectors.toMap(Student::getRollNumber, Student::getName));
        System.out.println(rollNumberNameMap);  
    }
}	

Output :

{1=John, 2=Mark, 3=Henry}        

Suppose you have duplicate keys and you create a Map from the List of Student, you will get IllegalStateException (Duplicate Key).

// Throws IllegalStateException
public class ListToMapConverter { 

    public static void main(String[] args) {
			
        List<Student> students = new ArrayList<Student>();
        students.add(new Student(1, "John", 13));
        students.add(new Student(2, "Mark", 14));
        students.add(new Student(3, "Henry", 13));
        students.add(new Student(1, "James", 15)); //oops student with same rollNumber
			
        Map<Integer, String> rollNumberNameMap = 
            students.stream().collect(Collectors.toMap(Student::getRollNumber, Student::getName));
        System.out.println(rollNumberNameMap); 
    }
}	

Output :

Exception in thread "main" java.lang.IllegalStateException: Duplicate key John
	at java.util.stream.Collectors.lambda$throwingMerger$0(Unknown Source)
	at java.util.HashMap.merge(Unknown Source)	

public class ListToMapConverter { 

    public static void main(String[] args) {
        // create students list
			
        Stream<Student> studentStream = students.stream();
        Map<Integer, String> rollNumberNameMap = 
            studentStream.collect(Collectors.toMap(Student::getRollNumber,
            Student::getName, (k1, k2) -> {return k1;}));
        System.out.println(rollNumberNameMap); 
    }
}	

Output :

{1=John, 2=Mark, 3=Henry}	

Another example that shows how to convert List to Map with key as rollNumber and value as List of Students.

public class ListToMapConverter { 

    public static void main(String[] args) {
        // create students list
			
        Map<Integer, List<Student>> rollNumberNameMap = 
            students.stream().collect(Collectors.toMap(Student::getRollNumber, Function.identity()));
        System.out.println(rollNumberNameMap); 
        
        rollNumberNameMap = students.stream().collect(Collectors.toMap(Student::getRollNumber,
            p -> p));
        System.out.println(rollNumberNameMap); 
    }
}	

Console Output :

{1=Student [rollNumber=1, name=John, age=13], 2=Student [rollNumber=2, name=Mark, age=14], 3=Student [rollNumber=3, name=Henry, age=13]}
{1=Student [rollNumber=1, name=John, age=13], 2=Student [rollNumber=2, name=Mark, age=14], 3=Student [rollNumber=3, name=Henry, age=13]}