Java HashMap merge() Method

The HashMap.merge() method in Java is used to combine a specified value with the existing value associated with a specified key using a given remapping function. If the key is not already present, the method inserts the key-value pair into the map.

Table of Contents

1. Introduction
2. merge Method Syntax
3. Examples
   • Merging Values in a HashMap
   • Real-World Use Case: Updating Product Quantities
4. Conclusion

Introduction

The HashMap.merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) method is a member of the HashMap class in Java. It allows you to combine a value associated with a specified key using a remapping function. If the key is not already present, the method inserts the key-value pair.

merge() Method Syntax

The syntax for the merge method is as follows:

public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

• The method takes three parameters:
  • key of type K, which represents the key whose associated value is to be merged.
  • value of type V, which represents the value to be merged with the existing value.
  • remappingFunction of type BiFunction<? super V, ? super V, ? extends V>, which represents the function to compute the merged value.
• The method returns the new value associated with the specified key, or null if the result of the remapping function is null.

Examples

Merging Values in a HashMap

The merge method can be used to combine values associated with a key in a HashMap.

Example with Lambda Expression

import java.util.HashMap;
import java.util.function.BiFunction;

public class MergeExample {
    public static void main(String[] args) {
        // Creating a HashMap with String keys and Integer values
        HashMap<String, Integer> people = new HashMap<>();

        // Adding entries to the HashMap
        people.put("Ravi", 25);
        people.put("Priya", 30);

        // Using merge method to update the age of "Ravi"
        people.merge("Ravi", 5, (oldValue, newValue) -> oldValue + newValue);

        // Using merge method to add a new entry for "Vijay"
        people.merge("Vijay", 35, (oldValue, newValue) -> oldValue + newValue);

        // Printing the updated HashMap
        System.out.println("Updated HashMap: " + people);
    }
}

Output:

Updated HashMap: {Ravi=30, Priya=30, Vijay=35}

Real-World Use Case: Updating Product Quantities

In a real-world scenario, you might use the merge method to update the quantities of products in an inventory system.

Example with Lambda Expression

import java.util.HashMap;
import java.util.function.BiFunction;

public class UpdateProductQuantities {
    public static void main(String[] args) {
        // Creating a HashMap with String keys (product IDs) and Integer values (quantities)
        HashMap<String, Integer> productInventory = new HashMap<>();

        // Adding entries to the HashMap
        productInventory.put("P001", 10);
        productInventory.put("P002", 15);

        // Using merge method to add more quantity to "P001"
        productInventory.merge("P001", 5, (oldValue, newValue) -> oldValue + newValue);

        // Using merge method to add a new product "P003"
        productInventory.merge("P003", 20, (oldValue, newValue) -> oldValue + newValue);

        // Printing the updated product inventory
        System.out.println("Updated Product Inventory: " + productInventory);
    }
}

Output:

Updated Product Inventory: {P001=15, P002=15, P003=20}

Conclusion

The HashMap.merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction) method in Java provides a way to combine a specified value with the existing value associated with a key using a given remapping function. By understanding how to use this method, you can efficiently manage updates and conditional modifications to the values in your map. This method is useful in various scenarios, such as updating inventory quantities, handling state transitions, and managing complex data structures in collections.