How to convert List to Map in Java 8

In this tutorial, we are going to learn about the conversion of List to Map in Java 8.

Converting List to Map in Java 8 :

We can convert the Java List<?> of objects into Map<k,v> in Java 8 via the Collectors class. The Collectors class provides a method called toMap() to convert List to Map.

Collectors.toMap() :

toMap() is a static method. It returns the Collector instance containing results (Map) of provided mapping functions as input parameters.

To convert the List to Map in Java 8, the list should be initially converted into Stream and then only we can convert it as Map.

Example for Converting List to Map in Java 8 :

Department.java

class Department {
    private int deptId;
    private String deptName;

    public Department(int deptId, String deptName) {
        super();
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public int getDeptId() {
        return deptId;
    }
    public String getDeptName() {
        return deptName;
    }

}

Converting List To Map :

Here we are going to convert a list of Department objects into Map.

Java8_ListToMap.java

package com.onlinetutorialspoint.java8;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Java8_ListToMap {

    public static void main(String[] args) {
        List<Department> deptList = new ArrayList<Department>();    
        deptList.add(new Department(1, "IT"));
        deptList.add(new Department(2, "HR"));
        deptList.add(new Department(3, "Management"));
        deptList.add(new Department(4, "Development"));
        deptList.add(new Department(5, "Recruitment"));
        Map<Integer, String> deptMap = deptList.stream().collect(
                Collectors.toMap(Department::getDeptId, Department::getDeptName));
        
        deptMap.forEach((k,v)->System.out.println("DeptId (" + k + ") Name :" + v));
    }
    
}

Output :

Terminal

DeptId (1) Name :IT
DeptId (2) Name :HR
DeptId (3) Name :Management
DeptId (4) Name :Development
DeptId (5) Name :Recruitment

The above example runs well if the map doesn’t contain any null key or values. If anyone of the entry is null, the toMap() method throws NullPointerException.

Nullvalue.java

List<Department> deptList = new ArrayList<Department>();    
        deptList.add(new Department(1, "IT"));
        deptList.add(new Department(2, "HR"));
        deptList.add(new Department(3, null));
        deptList.add(new Department(4, "Development"));
        deptList.add(new Department(5, "Recruitment"));
        Map<Integer, String> deptMap = deptList.stream().collect(Collectors.toMap(Department::getDeptId, Department::getDeptName));
        deptMap.forEach((k,v)->System.out.println("DeptId (" + k + ") Name :" + v));

The above code will generate NullPointerException like below:

Output

Exception in thread "main" java.lang.NullPointerException
at java.util.stream.Collectors.lambda$toMap$58(Unknown Source)
at java.util.stream.ReduceOps$3ReducingSink.accept(Unknown Source)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source)
at java.util.stream.AbstractPipeline.copyInto(Unknown Source)

You can also referto know how to resolve NullPointerException in Collectors.toMap().

Happy Learning 🙂