Collections Framework

HashMap<K, V> key-value pairs store karta hai. put(key, value) ek entry insert ya update karta hai, aur get(key) use retrieve karta hai — ek HashMap iteration order ke bare mein koi guarantee nahi deta, isliye jo code ek particular order par inhisar karta hai use ek doosri structure chahiye.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 30);
        ages.put("Bob", 25);
        System.out.println(ages.get("Alice"));
        System.out.println(ages.get("Bob"));
    }
}

30
25

HashSet

HashSet unique values store karta hai, koi duplicates nahi — ek aisi value add karna jo pehle se present hai bas ek no-op hai, isi wajah se neeche numbers.size() 3 hai, 4 nahi, chahe add() char baar call kiya gaya ho.

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<Integer> numbers = new HashSet<>();
        numbers.add(5);
        numbers.add(3);
        numbers.add(5);
        numbers.add(1);
        System.out.println(numbers.size());
        System.out.println(numbers.contains(3));
    }
}

3
true

Aage: lambdas aur streams — Java ka modern, functional-flavored tareeqa inline behavior likhne aur collections transform karne ka.