Merge two maps in Java

This post will discuss how to merge two maps of the same type in Java. The solution returns a new map, containing all mappings in both maps. The merge operation will handle common keys present in both maps.

1. Using `putAll()` method

A simple solution is to use the Map.putAll() method to copy all mappings from the original map to another map.

import java.util.HashMap;

import java.util.Map;

public class Main

{

public static void main(String[] args)

{

Map<Integer, Character> hm1 = new HashMap<>();

hm1.put(1, 'A');

hm1.put(2, 'B');

Map<Integer, Character> hm2 = new HashMap<>();

hm2.put(3, 'C');

hm2.put(4, 'D');

Map<Integer, Character> map = new HashMap<>();

map.putAll(hm1);

map.putAll(hm2);

System.out.println(map);

}

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Output:

{1=A, 2=B, 3=C, 4=D}

Note that if a key k exists in both maps, the value in hm2 will overwrite the value in hm1. i.e., map[k] = hm2[k]. For example,

import java.util.HashMap;

import java.util.Map;

public class Main

{

public static void main(String[] args)

{

Map<Integer, Character> hm1 = new HashMap<>();

hm1.put(1, 'A');

hm1.put(2, 'B');

Map<Integer, Character> hm2 = new HashMap<>();

hm2.put(3, 'C');

hm2.put(2, 'D');

Map<Integer, Character> map = new HashMap<>();

map.putAll(hm1);

map.putAll(hm2);

System.out.println(map);

}

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Output:

{1=A, 2=D, 3=C}

We can avoid making an extra call to the putAll() method by passing the first map to the HashMap constructor.

1 2	Map<Integer, Character> map = new HashMap<>(hm1); map.putAll(hm2);

If you need to add contents of a map hm1 to map hm2, you can just do:

1	hm1.putAll(hm2);

2. Using Guava

Guava also provides a builder for creating immutable map instances using the putAll() method, whose successive method invocations can be chained:

import com.google.common.collect.ImmutableMap;

import java.util.HashMap;

import java.util.Map;

public class Main

{

public static void main(String[] args)

{

Map<Integer, Character> hm1 = new HashMap<>();

hm1.put(1, 'A');

hm1.put(2, 'B');

Map<Integer, Character> hm2 = new HashMap<>();

hm2.put(3, 'C');

hm2.put(4, 'D');

Map<Integer, Character> map = ImmutableMap.<Integer, Character>builder()

.putAll(hm1)

.putAll(hm2)

.build();

System.out.println(map);

}

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Output:

{1=A, 2=B, 3=C, 4=D}

3. Using Java 8

In Java 8 and above, you can get the stream of elements of both maps, and call the Collectors.toMap() to collect all the map elements into a new map by applying the specified mapping functions to it, to help identify the keys and values. This is demonstrated below:

import java.util.HashMap;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class Main

{

public static void main(String[] args)

{

Map<Integer, Character> hm1 = new HashMap<>();

hm1.put(1, 'A');

hm1.put(2, 'B');

Map<Integer, Character> hm2 = new HashMap<>();

hm2.put(3, 'C');

hm2.put(4, 'D');

Map<Integer, Character> map = Stream.of(hm1, hm2)

.flatMap(m -> m.entrySet().stream())

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

System.out.println(map);

}

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Output:

{1=A, 2=B, 3=C, 4=D}

The above code throws java.lang.IllegalStateException if a duplicate key is present in both maps. To handle this, provide a merge function to resolve collisions between values associated with the same key.

The following code provides a BinaryOperator to merge values for duplicate keys, where the old value takes precedence over the new value in the stream.

import java.util.HashMap;

import java.util.Map;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class Main

{

public static void main(String[] args)

{

Map<Integer, Character> hm1 = new HashMap<>();

hm1.put(1, 'A');

hm1.put(2, 'B');

Map<Integer, Character> hm2 = new HashMap<>();

hm2.put(2, 'C');

hm2.put(3, 'D');

Map<Integer, Character> map = Stream.of(hm1, hm2)

.flatMap(m -> m.entrySet().stream())

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,

(oldValue, newValue) -> oldValue));

System.out.println(map);

}

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Output:

{1=A, 2=B, 3=D}

That’s all about merging two maps of the same types in Java.

1. Using putAll() method

2. Using Guava

3. Using Java 8

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1. Using `putAll()` method