How to merge two Maps in Java

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:

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Insert a non-existent value into a Map in Java 8

Consider the following Employee 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 put some Employee objects into a hash map:

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Return a default value from a Map in Java 8

Consider the following Employee 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 put some Employee objects into a hash map:

Read more of this post

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:

Read more of this post

Insert a non-existent value into a Map in Java 8

Consider the following Employee 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 put some Employee objects into a hash map:

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Replacing a value in a Map in Java 8

The Java 8 SDK has a couple of interesting new default “replace” methods available on the Map interface.

Consider the following HashMap:

Map<String, String> sizes = new HashMap<>();
sizes.put("XS", "Extra small");
sizes.put("S", "Small");
sizes.put("M", "Medium");
sizes.put("L", "Large");
sizes.put("XL", "Extra large");
sizes.put("XXL", "Extra extra large");

Say we’d like to replace the value of key “S”:

String replacedValue = sizes.replace("S", "Small size");

The replace method returns the value of the replaced string. In the above case the key “S” will have a new value “Small size” and “replace” returns “Small” as it was the value of “S” before the replace operation.

Read more of this post

Return a default value from a Map in Java 8

Consider the following Employee 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 put some Employee objects into a hash map:

Map<Integer, Employee> employeeMap = new HashMap<>();
employeeMap.put(1, new Employee(UUID.randomUUID(), "Elvis", 50));
employeeMap.put(2, new Employee(UUID.randomUUID(), "Marylin", 18));
employeeMap.put(3, new Employee(UUID.randomUUID(), "Freddie", 25));
employeeMap.put(4, null);
employeeMap.put(5, new Employee(UUID.randomUUID(), "Mario", 43));
employeeMap.put(6, new Employee(UUID.randomUUID(), "John", 35));
employeeMap.put(7, new Employee(UUID.randomUUID(), "Julia", 55));

Note the null value for key 4. Let’s also define a default Employee object:

Employee defaultEmployee = new Employee(UUID.fromString("00000000-0000-0000-0000-000000000000"), "", -1);

Java 8 includes a new method called “getOrDefault” on the Map interface. It accepts a key, like the “get” method on the Map, but it also accepts a default object that will be returned if the key does not exist.

Can you guess what the below code will return?

Employee employee = employeeMap.getOrDefault(4, defaultEmployee);

“employee” will be null of course, as key 4 exists and its value is null. However, if you simply call the “get” method with 4 as the key input then you don’t know exactly how to interpret the null result: does 4 exist as key in the map and its value is null or does the key 4 not exist at all in the map? With getOrDefault returning 0 in this case you can be 100% sure that a null response is unambiguous.

Let’s see what the below bit of code returns:

Employee employee = employeeMap.getOrDefault(12, defaultEmployee);

This time it returns the default employee as the key 12 does not exist in the map.

View all posts related to Java here.

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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