Explain Set and Map in Java?

1. Set in Java

A Set is a collection that doesn’t allow duplicate elements. It’s ideal when you want to store unique items.

Common Types of Sets:

• HashSet (no order)
• LinkedHashSet (insertion order)
• TreeSet (sorted order)

import java.util.HashSet;

public class SetExample {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();

        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        fruits.add("Apple"); // Duplicate, won't be added

        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

Output might be:

Banana
Apple
Orange

Note: HashSet doesn’t maintain order.

✅ When to Use a Set

Use a Set when:

• You only care about unique values, not duplicates.
• You don’t need to associate elements with keys (i.e., just a list of items).
• Order doesn’t matter—or you want a specific order like sorted or insertion order.

Example Use Cases:

• Storing unique email addresses.
• Tracking visited web pages.
• Managing a list of tags or categories.

Performance Tip:

• HashSet offers constant-time performance for basic operations like add(), remove(), and contains(), assuming a good hash function.

2. Map in Java

A Map stores key-value pairs. Each key must be unique, but values can be duplicated.

Common Types of Maps:

• HashMap (no order)
• LinkedHashMap (insertion order)
• TreeMap (sorted by keys)

Example using HashMap:

import java.util.HashMap;

public class MapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> studentMap = new HashMap<>();

        studentMap.put(101, "Alice");
        studentMap.put(102, "Bob");
        studentMap.put(103, "Charlie");
        studentMap.put(101, "David"); // Key 101 updated to David

        for (Integer id : studentMap.keySet()) {
            System.out.println("ID: " + id + ", Name: " + studentMap.get(id));
        }
    }
}

Output might be:

ID: 101, Name: David
ID: 102, Name: Bob
ID: 103, Name: Charlie

✅ When to Use a Set

Use a Set when:

• You only care about unique values, not duplicates.
• You don’t need to associate elements with keys (i.e., just a list of items).
• Order doesn’t matter—or you want a specific order like sorted or insertion order.

Example Use Cases:

• Storing unique email addresses.
• Tracking visited web pages.
• Managing a list of tags or categories.

Performance Tip:

• HashSet offers constant-time performance for basic operations like add(), remove(), and contains(), assuming a good hash function.

✅ When to Use a Map

Use a Map when:

• You need to associate a key with a value (like a dictionary).
• Keys must be unique, but values can repeat.
• Fast lookups based on keys are needed.

Example Use Cases:

• Storing user information with user ID as key.
• Word counts where word is key and count is value.
• Caching with quick access by key.

Performance Tip:

• HashMap is usually very fast for get() and put() operations (again, assuming a well-distributed hash function).