HashMap.md

HashMap in Java (Beginner's Guide)

A HashMap in Java is part of the java.util package and is used to store key-value pairs. It allows efficient insertion, deletion, and lookup operations.

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Features of HashMap

1. Stores key-value pairs (like a dictionary in Python).
2. Allows one null key and multiple null values.
3. Unordered Collection – It does not maintain any order of elements.
4. Fast performance – Provides O(1) time complexity for insertion, deletion, and retrieval (on average).
5. Duplicates not allowed – Keys must be unique; values can be duplicate.

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Basic Syntax of HashMap

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // Creating a HashMap
        HashMap<Integer, String> map = new HashMap<>();

        // Adding elements (put method)
        map.put(1, "Apple");
        map.put(2, "Banana");
        map.put(3, "Cherry");

        // Accessing elements (get method)
        System.out.println("Value for key 2: " + map.get(2)); // Output: Banana

        // Removing an element
        map.remove(3);

        // Checking if a key exists
        System.out.println("Contains key 3? " + map.containsKey(3)); // Output: false

        // Iterating over HashMap
        for (Integer key : map.keySet()) {
            System.out.println("Key: " + key + ", Value: " + map.get(key));
        }
    }
}

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Common Methods of HashMap

Method	Description
put(K key, V value)	Adds a key-value pair
get(K key)	Returns the value associated with the key
remove(K key)	Removes a key-value pair
containsKey(K key)	Checks if a key exists
containsValue(V value)	Checks if a value exists
size()	Returns the number of key-value pairs
keySet()	Returns a set of all keys
values()	Returns a collection of all values
entrySet()	Returns a set of key-value pairs

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Practice Question

Question 1: Count the frequency of characters in a string using HashMap

Problem Statement:
Write a Java program that takes a string input and counts the occurrences of each character using HashMap.

Solution

import java.util.HashMap;

public class CharFrequency {
    public static void main(String[] args) {
        String str = "java hashmap example";

        HashMap<Character, Integer> charCount = new HashMap<>();

        // Loop through each character
        for (char ch : str.toCharArray()) {
            if (ch != ' ') { // Ignore spaces
                charCount.put(ch, charCount.getOrDefault(ch, 0) + 1);
            }
        }

        // Print the frequency of each character
        for (char key : charCount.keySet()) {
            System.out.println("Character: " + key + ", Count: " + charCount.get(key));
        }
    }
}

Output:

Character: j, Count: 1
Character: a, Count: 4
Character: v, Count: 1
Character: h, Count: 2
Character: s, Count: 1
Character: m, Count: 2
Character: p, Count: 1
Character: x, Count: 1
Character: e, Count: 2
Character: l, Count: 1

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Conclusion

• HashMap is a powerful data structure that allows fast lookups and insertions.
• It is widely used for problems like caching, frequency counting, and data mapping.
• The key must be unique, and the values can be duplicated.

Would you like more practice problems? 🚀