Collecting Stream Items into Map in Java

Learn to collect Stream items into Map using Collectors.toMap() and Collectors.groupingBy() methods using Java Stream APIs.

1. Collectors.toMap() for Unique Key-value Pairs

If the stream items have the unique map key field then we can use Collectors.toMap() to collect items to Map in Map<keyObj, Item> format.

For example, we can collect a list of Employee objects to Map in where employee ids are unique fields and used as keys to the Map entries.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
 
public class Main 
{
    public static void main(String[] args) 
    {
        List<Employee> employeeList = new ArrayList<>(Arrays.asList(
                            new Employee(1, "A", 100),
                            new Employee(2, "A", 200),
                            new Employee(3, "B", 300),
                            new Employee(4, "B", 400),
                            new Employee(5, "C", 500),
                            new Employee(6, "C", 600)));
         
        Map<Long, Employee> employeesMap = employeeList.stream()
                                .collect( Collectors.toMap(Employee::getId, 
                                        Function.identity()) );
         
        System.out.println(employeesMap);
    }
}

Program output.

{1=Employee [id=1, name=A, salary=100.0],
2=Employee [id=2, name=A, salary=200.0],
3=Employee [id=3, name=B, salary=300.0],
4=Employee [id=4, name=B, salary=400.0],
5=Employee [id=5, name=C, salary=500.0],
6=Employee [id=6, name=C, salary=600.0]}

2. Collectors.groupingBy() when Multiple Keys have Same Value

If the stream has items where Map keys are duplicate then we can use Collectors.groupingBy() to collect elements in Map<key, List<value>> format. Here for each map key, we will store all elements in a List as the value.

For example, we can collect a list of Employee objects to map in where employee names may be duplicate fields for some stream elements. In such a case, all employees with the same name will be stored in a List, and the list will be stored as Map value field.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
 
public class Main 
{
    public static void main(String[] args) 
    {
        List<Employee> employeeList = new ArrayList<>(Arrays.asList(
                            new Employee(1, "A", 100),
                            new Employee(2, "A", 200),
                            new Employee(3, "B", 300),
                            new Employee(4, "B", 400),
                            new Employee(5, "C", 500),
                            new Employee(6, "C", 600)));
         
        Map<String, List<Employee>> employeesMap = employeeList.stream()
                                .collect(Collectors.groupingBy(Employee::getName));
         
        System.out.println(employeesMap);
    }
}

Program output.

{A=[Employee [id=1, name=A, salary=100.0], Employee [id=2, name=A, salary=200.0]],
 B=[Employee [id=3, name=B, salary=300.0], Employee [id=4, name=B, salary=400.0]],
 C=[Employee [id=5, name=C, salary=500.0], Employee [id=6, name=C, salary=600.0]]}

3. Conclusion

It is very important to know beforehand if the Stream elements will have a distinct value for the map key field or not.

If map keys are duplicate and we use Collectors.toMap() method, we will get the IllegalStateException:

Exception in thread "main" java.lang.IllegalStateException: Duplicate key Employee [id=5, name=C, salary=500.0]
	at java.util.stream.Collectors.lambda$throwingMerger$106(Collectors.java:133)
	at java.util.stream.Collectors$$Lambda$3/149928006.apply(Unknown Source)
	at java.util.HashMap.merge(HashMap.java:1245)

Happy Learning !!

Sourcecode on Github