Interview questions along with their answers on HashSet and HashMap

Category : Java | Sub Category : Java Interview questions | By Prasad Bonam Last updated: 2023-08-03 13:40:33 Viewed : 319

Here are some interview questions along with their answers on HashSet and HashMap:

1. Question: What is a HashSet in Java? How does it store elements internally?

   Answer: A HashSet is a collection in Java that implements the Set interface. It stores elements in a hash table using a technique called hashing. When an element is added to the HashSet, its hash code is computed, and this hash code is used to determine the index where the element should be stored. If two elements have the same hash code (collision), they are stored as a linked list at that index.

2. Question: How do you ensure that objects in a HashSet are unique? What methods are involved?

   Answer: HashSet ensures uniqueness of objects by using the equals() and hashCode() methods. When an element is added to the HashSet, it first checks if there is an existing element with the same hash code. If there is, it uses the equals() method to compare the objects for actual equality. If equals() returns true, the element is considered a duplicate and not added to the HashSet.

3. Question: Can you have duplicate elements in a HashSet? If not, how does it handle duplicates?

   Answer: No, a HashSet does not allow duplicate elements. When an attempt is made to add a duplicate element to the HashSet, it simply ignores the addition since duplicates are not allowed. The existing element remains unchanged in the HashSet.

4. Question: How does the hashCode() method play a role in the functioning of HashSet?

   Answer: The hashCode() method plays a crucial role in HashSets functioning. It is used to compute the hash code of an element, which determines the index in the internal hash table where the element will be stored. The hashCode() method is also used to quickly identify potential collisions and reduce the number of elements to compare for equality using the equals() method.

5. Question: What is a HashMap in Java? How does it store key-value pairs internally?

   Answer: A HashMap is a collection that stores key-value pairs. It uses hashing to store and retrieve elements efficiently. Each key-value pair is stored as a Map.Entry object, and when an element is added to the HashMap, its hash code is computed to determine the bucket (index) where the corresponding entry will be stored. If multiple keys have the same hash code, they are stored as a linked list of entries at that bucket.

6. Question: Can you store duplicate keys or values in a HashMap?

   Answer: No, a HashMap does not allow duplicate keys. Each key must be unique. However, duplicate values are allowed in a HashMap. Different keys can map to the same value.

7. Question: How do you retrieve a value from a HashMap using a key?

   Answer: To retrieve a value from a HashMap, you can use the get() method and pass the key as the argument. The get() method will return the value associated with that key, or null if the key is not found in the HashMap.

   Example:

   javaCopy code
   HashMap<String, Integer> hashMap = new HashMap<>();
   hashMap.put("apple", 10);
   int value = hashMap.get("apple"); // value will be 10
   

8. Question: How do you iterate through the key-value pairs in a HashMap?

   Answer: There are multiple ways to iterate through the key-value pairs in a HashMap. One common way is to use the entrySet() method to get a set of all the key-value pairs as Map.Entry objects, and then iterate through the set using a for-each loop.

   Example:

   javaCopy code
   HashMap<String, Integer> hashMap = new HashMap<>();
   // Add key-value pairs to the HashMap
   for (Map.Entry<String, Integer> entry : hashMap.entrySet()) {
       String key = entry.getKey();
       int value = entry.getValue();
       System.out.println(key + " : " + value);
   }
   

9. Question: What are the time complexities of basic operations (put, get, remove) in a HashMap?

   Answer: The time complexities for basic operations in a HashMap are as follows:

   • put(key, value): O(1) average case, O(n) worst case (when there are hash collisions and linked lists become too long).
   • get(key): O(1) average case, O(n) worst case (similar reason as above).
   • remove(key): O(1) average case, O(n) worst case (similar reason as above).

   In the average case, these operations are very fast and constant time, but in the worst case, they can degrade to linear time when dealing with many hash collisions.

These are some common interview questions along with their answers related to HashSet and HashMap. Remember to practice implementing and using these data structures to gain a deeper understanding.