Java HashMap put() Method

The HashMap.put() method in Java is used to insert key-value pairs into a HashMap.

Table of Contents

1. Introduction
2. put Method Syntax
3. Examples
   • Adding Entries to a HashMap
   • Updating Values in a HashMap
   • Handling Null Values
   • Real-World Use Case: Storing Country Codes
4. Conclusion

Introduction

The HashMap.put() method is a member of the HashMap class in Java. It allows you to add or update key-value pairs in a HashMap. If the key is not already present in the map, the method inserts the new key-value pair and returns null. If the key is already present, the method updates the value associated with the key and returns the previous value.

put() Method Syntax

The syntax for the put method is as follows:

public V put(K key, V value)

• The method takes two parameters:
  • key of type K, which represents the key to be inserted or updated.
  • value of type V, which represents the value to be associated with the key.
• The method returns the previous value associated with the key, or null if there was no mapping for the key.

Examples

Adding Entries to a HashMap

The put method can be used to add key-value pairs to a HashMap.

Example

import java.util.HashMap;
import java.util.Map;

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

        // Adding entries to the HashMap
        countries.put("India", "IN");
        countries.put("United States", "US");
        countries.put("Canada", "CA");

        // Printing the HashMap
        System.out.println("HashMap: " + countries);
    }
}

Output:

HashMap: {India=IN, United States=US, Canada=CA}

Updating Values in a HashMap

The put method can be used to update the value associated with an existing key in a HashMap.

Example

import java.util.HashMap;
import java.util.Map;

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

        // Adding entries to the HashMap
        countries.put("India", "IN");
        countries.put("United States", "US");

        // Updating the value for the key "India"
        String oldValue = countries.put("India", "IND");

        // Printing the old value and the updated HashMap
        System.out.println("Old value for India: " + oldValue);
        System.out.println("Updated HashMap: " + countries);
    }
}

Output:

Old value for India: IN
Updated HashMap: {India=IND, United States=US}

Handling Null Values

The put method can handle null keys and values.

Example

import java.util.HashMap;
import java.util.Map;

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

        // Adding entries with null key and value
        countries.put(null, "Unknown");
        countries.put("United States", null);

        // Printing the HashMap
        System.out.println("HashMap with null values: " + countries);
    }
}

Output:

HashMap with null values: {null=Unknown, United States=null}

Real-World Use Case: Storing Country Codes

In a real-world scenario, you might use the put method to store and update country codes in a HashMap.

Example

import java.util.HashMap;
import java.util.Map;

public class CountryCodes {
    public static void main(String[] args) {
        // Creating a Map with String keys (country names) and String values (country codes)
        Map<String, String> countryCodes = new HashMap<>();

        // Adding country codes
        countryCodes.put("India", "IN");
        countryCodes.put("United States", "US");
        countryCodes.put("Canada", "CA");

        // Updating country code for India
        String oldCode = countryCodes.put("India", "IND");

        // Printing the old code and the updated country codes
        System.out.println("Old code for India: " + oldCode);
        System.out.println("Updated Country Codes: " + countryCodes);
    }
}

Output:

Old code for India: IN
Updated Country Codes: {India=IND, United States=US, Canada=CA}

Conclusion

The HashMap.put() method in Java provides a way to add or update key-value pairs in a HashMap. By understanding how to use this method, you can efficiently manage collections of key-value pairs in your Java applications. This method is useful in various scenarios, such as storing and updating data, handling null values, and managing complex data structures.