What are HashMap and Set, and in which cases is it better to use them?

HashMap is a collection that stores key-value pairs. Keys are unique, while values can repeat. It uses hashing for fast access (on average O(1)) to elements by key.

Set is a collection that stores unique elements. It does not guarantee order unless a specific implementation is used (e.g., LinkedHashSet). Operations like adding, removing, and checking for an element are generally performed in O(1). There are various implementations, for example:

• HashSet: Does not guarantee order of elements.
• LinkedHashSet: Maintains insertion order.
• TreeSet: Stores elements in sorted order.

When to use:

• HashMap:
  • When efficient search, insertion, or deletion of elements by key is needed.
  • For storing associative data (e.g., a dictionary).
  • When quick existence check of a value by key is required.
• Set:
  • When only unique elements need to be stored.
  • For efficient existence checks within a collection.
  • For set operations (union, intersection).

Example of using HashMap:

// Creating a HashMap
HashMap<String, Integer> ages = new HashMap<>();

// Adding elements
ages.put("Alice", 30);
ages.put("Bob", 25);
ages.put("Charlie", 35);

// Getting value by key
int bobAge = ages.get("Bob"); // bobAge = 25

// Checking if key exists
boolean containsAlice = ages.containsKey("Alice"); // containsAlice = true

Example of using HashSet:

// Creating a HashSet
HashSet<String> names = new HashSet<>();

// Adding elements
names.add("Alice");
names.add("Bob");
names.add("Charlie");
names.add("Alice"); // Will not add, as Alice already exists

// Checking if element exists
boolean containsBob = names.contains("Bob"); // containsBob = true

// Iterating over elements
for (String name : names) {
    System.out.println(name);
}