Set and Map

Set stores unique elements. Map stores key-value pairs.

import java.util.HashSet;
import java.util.HashMap;

HashSet<Integer> set = new HashSet<>();
set.add(1);
set.add(2);
set.add(1); // Duplicate, won't be added

HashMap<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);
System.out.println(map.get("a")); // 1

Use Set when uniqueness matters. Use Map when you need to associate keys to values.