Java HashMap: A Comprehensive Guide

Java HashMap: A Comprehensive Guide

In Java, a HashMap stores data as key‑value pairs, enabling fast retrieval of values by unique keys. Unlike arrays, the keys can be any object (except null), and the map automatically manages hashing and collision resolution.

Key Features

• Associates a unique key with a value, allowing get() and put() operations.
• Throws NoSuchElementException when a requested key does not exist.
• Stores object references; primitive types must be wrapped (e.g., Integer, Double).

Declaring a HashMap

HashMap<String, Object> map = new HashMap<String, Object>();
HashMap x = new HashMap();

Essential Methods

• get(Object key) – returns the value for the specified key.
• put(Object key, V value) – inserts or updates the key‑value pair.
• remove(Object key) – deletes the mapping for the specified key.
• containsKey(Object key) – checks if the key exists.
• keySet() – returns a Set of all keys.
• values() – returns a Collection of all values.
• isEmpty() – tests whether the map has any entries.

Practical Example

Below is a complete, runnable snippet that demonstrates common operations:

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

public class SampleTestMaps {
    public static void main(String[] args) {
        Map<String, String> objMap = new HashMap<>();
        objMap.put("Name", "Suzuki");
        objMap.put("Power", "220");
        objMap.put("Type", "2-wheeler");
        objMap.put("Price", "85000");

        System.out.println("Elements of the Map:");
        System.out.println(objMap);
    }
}

Output:

{Type=2-wheeler, Price=85000, Power=220, Name=Suzuki}

Removing a Specific Entry

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();
        map.put(1, "Java");
        map.put(2, "Python");
        map.put(3, "PHP");
        map.put(4, "SQL");
        map.put(5, "C++");

        System.out.println("Before removal: " + map);
        map.remove(5);
        System.out.println("After removal: " + map);
    }
}

Output:

Before removal: {1=Java, 2=Python, 3=PHP, 4=SQL, 5=C++}
After removal: {1=Java, 2=Python, 3=PHP, 4=SQL}

Quick Reference

• Check existence: containsKey(key)
• List keys: keySet()
• List values: values()
• Remove entry: remove(key)
• Is empty? isEmpty()

With these fundamentals, you can confidently integrate HashMaps into any Java application for efficient data storage and retrieval.