Java HashMap compute() Method

The HashMap.compute() method in Java is used to compute a new mapping for the specified key and its current mapped value (or null if there is no current mapping). This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. compute Method Syntax
3. Examples
   • Updating an Entry in a HashMap
   • Real-World Use Case: Updating Product Quantity
4. Conclusion

Introduction

The HashMap.compute() method is a member of the HashMap class in Java. It allows you to recompute the value associated with a key based on the current mapping (or lack thereof). This can be useful for performing complex updates or conditional modifications to the values in the map.

compute() Method Syntax

The syntax for the compute method is as follows:

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

• The method takes two parameters:
  • key of type K, which represents the key whose value is to be computed.
  • remappingFunction of type BiFunction<? super K, ? super V, ? extends V>, which represents the function to compute a new value for the key.
• The method returns the new value associated with the specified key, or null if the key is removed from the map.

Examples

Updating an Entry in a HashMap

The compute method can be used to update the value associated with a key based on its current value.

Example with Lambda Expression

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

public class ComputeExample {
    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);
        people.put("Vijay", 35);

        // Using compute method to update the age of "Ravi"
        people.compute("Ravi", (key, value) -> (value == null) ? 0 : value + 1);

        // Using compute method to add a new entry for "Amit"
        people.compute("Amit", (key, value) -> (value == null) ? 1 : value + 1);

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

Output:

Updated HashMap: {Ravi=26, Priya=30, Vijay=35, Amit=1}

Real-World Use Case: Updating Product Quantity

In a real-world scenario, you might use the compute method to increment the quantity of a product in a product inventory.

Example with Lambda Expression

import java.util.HashMap;

public class UpdateProductQuantity {
    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);
        productInventory.put("P003", 20);

        // Using compute method to increment the quantity of "P001"
        productInventory.compute("P001", (key, value) -> (value == null) ? 1 : value + 5);

        // Using compute method to add a new product "P004"
        productInventory.compute("P004", (key, value) -> (value == null) ? 1 : value + 5);

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

Output:

Updated Product Inventory: {P001=15, P002=15, P003=20, P004=1}

Conclusion

The HashMap.compute() method in Java provides a way to recompute the value associated with a key based on its current value. By understanding how to use this method, you can efficiently perform complex updates and conditional modifications to the values in your map. This method is useful in various scenarios, such as updating records in a database, managing inventory quantities, and handling state transitions in collections.