How to merge two Maps in Java 8

The Map interface has been extended with the “merge” function in Java 8. Let’s see an example on how to use it.

Consider the following Empolyee class:

public class Employee
{
    private UUID id;
    private String name;
    private int age;

    public Employee(UUID id, String name, int age)
    {
        this.id = id;
        this.name = name;
        this.age = age;
    }
        
    public UUID getId()
    {
        return id;
    }

    public void setId(UUID id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }    
    
    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }
}

Let’s say that we have the following two maps where the key is an indicator of the employees’ performance and the value is the list of employees that fall into that category:

List<Employee> averageMapOne = new ArrayList<>();
averageMapOne.add(new Employee(UUID.randomUUID(), "Elvis", 50));
averageMapOne.add(new Employee(UUID.randomUUID(), "Marylin", 18));
List<Employee> poorMapOne = new ArrayList<>();
poorMapOne.add(new Employee(UUID.randomUUID(), "Mario", 43));
poorMapOne.add(new Employee(UUID.randomUUID(), "John", 35));
List<Employee> excellentMapOne = new ArrayList<>();
excellentMapOne.add(new Employee(UUID.randomUUID(), "Julia", 55));
List<Employee> okMapOne = new ArrayList<>();
okMapOne.add(new Employee(UUID.randomUUID(), "Nick", 43));
okMapOne.add(new Employee(UUID.randomUUID(), "Richard", 61));

Map<String, List<Employee>> employeeMapOne = new HashMap<>();
employeeMapOne.put("average", averageMapOne);
employeeMapOne.put("poor", poorMapOne);
employeeMapOne.put("excellent", excellentMapOne);
employeeMapOne.put("OK", okMapOne);

List<Employee> averageMapTwo = new ArrayList<>();
averageMapTwo.add(new Employee(UUID.randomUUID(), "Lotta", 52));
averageMapTwo.add(new Employee(UUID.randomUUID(), "Eva", 42));
averageMapTwo.add(new Employee(UUID.randomUUID(), "Mark", 24));
List<Employee> poorMapTwo = new ArrayList<>();
poorMapTwo.add(new Employee(UUID.randomUUID(), "Anna", 20));
List<Employee> excellentMapTwo = new ArrayList<>();
excellentMapTwo.add(new Employee(UUID.randomUUID(), "Bertil", 28));
excellentMapTwo.add(new Employee(UUID.randomUUID(), "Cecilia", 36));
excellentMapTwo.add(new Employee(UUID.randomUUID(), "Edit", 21));

Map<String, List<Employee>> employeeMapTwo = new HashMap<>();

employeeMapTwo.put("average", averageMapTwo);
employeeMapTwo.put("poor", poorMapTwo);
employeeMapTwo.put("excellent", excellentMapTwo);

Our goal is to merge map 2 into map 1 in way that all employees that fall into the same category will have all employees in a joined list. E.g. “average” will have 5 employees: Elvis, Marylin, Lotta, Eva and Mark.

The solution requires an understanding of the Java Stream API.

Here’s an explanation of the below bit of code:

• We open a stream on the entry set of the first map
• Then we iterate through the map using the forEach method
• We pass a lambda expression into forEach where we want to do “something” with each entry in the entry set
• This “something” is the actual merge operation of map 2
• The merge operation accepts a key and a value which will be the key and value of map 1
• Merge also accepts a BiFunction which will be used in case the key already exists in map 2
• In this function we pass in the lists from map 1 and map 2, add the contents of map 1 into map 2 and return the new list

Here’s the code that will do the trick:

employeeMapOne.entrySet().stream()
        .forEach(entry -> employeeMapTwo.merge(entry.getKey(), entry.getValue(),
                (listTwo, listOne) ->
                        {
                            listOne.addAll(listTwo);
                            return listOne;
                        }));

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