HashMap merge(key, value, BiFunction) method in Java with Examples

The merge(Key, Value, BiFunctional) method of the HashMap class is used to combine multiple mapped values for a key using the given mapping function. A bucket is actually an index of the array, that array is called a table in HashMap implementation. So table[0] is referred to as bucket0, table[1] as bucket1, and so on.

• If the key is not present or is associated with null, it simply outputs the key along with the corresponding value in the Hashmap as a new entry.
• However, if the key already holds some value, then Remapping Function merges both the old and new values with the given key
• In case the key is null, it is always mapped to bucket 0, as the hash is not calculated for null keys due to NullPointerException

Syntax

public V merge(K key, V value,
    BiFunction remappingFunction)

Parameters

This method accepts three parameters:

• Key: It is the key for which we have a particular value. If two keys have the same value they are merged.
• Value: It is the index corresponding to the particular key which is stored in the bucket.
• BiFunction: This is the function having two arguments to be used for calculating the new mapping from the old value and given value.

Return Value

• This method returns the key along with its value if the key is not present or is associated with null.
• If the key already holds any value, it merges the old value with the new value using the mapping technique.

Example of the HashMap merge()

Below programs illustrate the merge(Key, Value, BiFunctional) method:

Example 1:

Java

// Java program to demonstrate
// computeIfAbsent(Key, Function) method.

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // create a HashMap and add some values
        HashMap<Integer, String> map1 = new HashMap<>();
        map1.put(1, "L");
        map1.put(2, "M");
        map1.put(3, "N");

        HashMap<Integer, String> map2 = new HashMap<>();
        map2.put(1, "B");
        map2.put(2, "G");
        map2.put(3, "R");

        // print map details
        System.out.println("HashMap1: " + map1.toString());

        System.out.println("HashMap2: " + map2.toString());

        // provide value for new key which is absent
        // using computeIfAbsent method
        map2.forEach(
            (key, value)
                -> map1.merge(key, value,
                              (v1, v2)
                                  -> v1.equalsIgnoreCase(v2)
                                         ? v1
                                         : v1 + ", " + v2));

        // print new mapping
        System.out.println("New HashMap: " + map1);
    }
}

Output

HashMap1: {1=L, 2=M, 3=N}
HashMap2: {1=B, 2=G, 3=R}
New HashMap: {1=L, B, 2=M, G, 3=N, R}

Program 2:

Java

// Java program to demonstrate
// computeIfAbsent(Key, Function) method.
import java.util.*;

public class GFG {
    // Main method
    public static void main(String[] args)
    {

        // create a HashMap and add some values
        HashMap<Integer, String> map1 = new HashMap<>();
        map1.put(1, "Ram");
        map1.put(2, "Rohan");
        map1.put(3, "Shivam");

        HashMap<Integer, String> map2 = new HashMap<>();
        map2.put(1, "Tushar");
        map2.put(10, "Satya");
        map2.put(12, "Sundar");

        // print map details
        System.out.println("HashMap1: " + map1.toString());

        System.out.println("HashMap2: " + map2.toString());

        // provide value for new key which is absent
        // using computeIfAbsent method
        map2.forEach(
            (key, value)
                -> map1.merge(key, value,
                              (v1, v2)
                                  -> v1.equalsIgnoreCase(v2)
                                         ? v1
                                         : v1 + ", " + v2));
        // print new mapping
        System.out.println("New HashMap: " + map1);
    }
}

Output

HashMap1: {1=Ram, 2=Rohan, 3=Shivam}
HashMap2: {1=Tushar, 10=Satya, 12=Sundar}
New HashMap: {1=Ram, Tushar, 2=Rohan, 3=Shivam, 10=Satya, 12=Sundar}