HashMap: Labels and Counts

A map is a labelled box

A HashMap<K, V> stores key → value pairs, like a phone book: a name (key) leads to a number (value). Each key appears once.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> price = new HashMap<String, Integer>();
        price.put("tea", 20);
        price.put("coffee", 35);
        price.put("tea", 25);                 // same key: the value is replaced
        System.out.println(price.get("tea"));
        System.out.println(price.containsKey("juice"));
        System.out.println(price.get("juice"));              // missing key gives null
        System.out.println(price.getOrDefault("juice", 0));  // or a default you choose
        System.out.println(price.size());
    }
}

Output

25
false
null
0
2

The counting pattern

The most common use of a map in DSA: how many times does each thing appear?

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        HashMap<String, Integer> votes = new HashMap<String, Integer>();
        for (int i = 0; i < n; i++) {
            String name = sc.next();
            votes.put(name, votes.getOrDefault(name, 0) + 1);
        }
        // A HashMap has no useful order, so copy into a TreeMap to print a-z
        TreeMap<String, Integer> sorted = new TreeMap<String, Integer>(votes);
        for (Map.Entry<String, Integer> e : sorted.entrySet()) {
            System.out.println(e.getKey() + " " + e.getValue());
        }
    }
}

Input (what the user types)

7
mango apple mango kiwi apple mango kiwi

Output

apple 2
kiwi 2
mango 3

Common mistake: Printing a HashMap directly and expecting an order. Its order depends on the hash buckets and can look random. When order matters, use a TreeMap (keys sorted) or sort the keys yourself.