Top 20+ HashMap Interview Questions With Answers

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Did you know that HashMap is one of the most commonly used data structures in Java programming? In fact, it’s widely used in software development. That’s why understanding HashMap is essential for many programming roles, especially in Java-based positions. So, to help you with your interview preparation, we’ve put together a list of over 15 HashMap interview questions with simple answers.  

This guide includes commonly asked questions suitable for beginners as well as those at intermediate and advanced levels. 

Let’s start your preparation!

Basic HashMap Interview Questions for Freshers

Here are some important HashMap in java interview questions and answers for freshers.

1. What is a HashMap in Java?

 A HashMap in Java is a data structure that stores key-value pairs. It allows efficient retrieval of values based on unique keys. 

When you add an entry to a HashMap using put(key, value), the HashMap computes a hash code for the key to determine where to store the corresponding value. 

HashMaps are widely used for fast lookup and mapping operations in Java applications.

2. Can a HashMap contain duplicate values?

Yes, a HashMap can contain duplicate values but not duplicate keys. Each key in a HashMap must be unique, but different keys can map to the same value. 

3. What happens if you try to add a duplicate key to a HashMap?

If you attempt to add a duplicate key to a HashMap, the existing value associated with that key will be overwritten by the new value.

4. Why is the initial capacity of a HashMap important?

The initial capacity of a HashMap is important because it affects its performance. Setting an appropriate initial capacity can reduce the number of rehashing operations, which can be costly in terms of time and memory.

5. How do you add elements to a HashMap?

Elements can be added to a HashMap using the put() method. 

For example:

HashMap<String, Integer> map = new HashMap<>();

map.put(“apple”, 10);

map.put(“orange”, 15);

HashMap in Java Interview Questions for Experienced

These are some commonly asked HashMap interview questions and answers for experienced candidates. 

6. How does HashMap handle collisions? 

HashMap handles collisions by using a technique called chaining. If multiple keys hash to the same index, their entries are stored as a linked list at that index.

7. What is the maximum number of entries you can store in HashMap?

HashMap does not have a fixed maximum limit on entries. When a bucket is full due to hash collisions, HashMap uses a linked list to manage the keys. This linked list can grow indefinitely, utilizing available memory effectively without a predefined entry limit.

8. What is the significance of the load factor in a HashMap?

The load factor determines when the HashMap will resize itself. When the number of elements exceeds the product of the load factor and the current capacity, the HashMap will rehash its contents to a larger capacity, improving performance.

9. How can you make a custom object usable as a key in a HashMap?

To use a custom object as a key in a HashMap, you must override the hashCode() and equals() methods in the object class. This ensures that HashMap can compute the hash code correctly and check for equality between keys.

10. What factors determine the performance of a HashMap?

The performance efficiency of a HashMap depends on two key factors:

• Capacity: Capacity refers to the total number of buckets in a HashMap. By default, a HashMap starts with a capacity of sixteen buckets. As more key-value pairs are added to the HashMap, its capacity can be increased dynamically.
• Load Factor: The load factor determines when the HashMap should increase its capacity. The default load factor is 75%. When the number of entries exceeds 75% of the current capacity, the HashMap automatically increases its capacity to maintain performance efficiency. 

Advanced HashMap and HashTable Interview Questions

These are some advanced HashMap interview questions, along with answers. 

11. What is the difference between HashMap and HashTable in Java?

HashMap is not synchronized and is not thread-safe, whereas HashTable is synchronized and is thread-safe. Additionally, HashMap allows null keys and values, whereas HashTable does not allow null keys or values.

12. How does HashMap handle concurrency issues, and why might ConcurrentHashMap be preferred in concurrent environments?

HashMap is not thread-safe and can lead to concurrent modification exceptions in a multi-threaded environment. ConcurrentHashMap, on the other hand, uses finer-grained locking mechanisms to achieve thread safety, making it more suitable for concurrent applications.

13. What happens if two different keys have the same hash code in a HashMap?

If two different keys have the same hash code (hash collision), HashMap uses a linked list (or a balanced tree in Java 8+) to store these keys and their corresponding values within the same bucket.

14. How can you improve the performance of HashMap in terms of reducing collisions? 

To reduce collisions in HashMap, you can provide custom implementations of hashCode() and equals() methods in key objects to distribute keys more evenly across buckets, thus improving performance.

Also Read - Top 25 Exception Handling Questions In Java Interview

Internal Working of HashMap Interview Questions

Here are some important HashMap in java interview questions and answers. 

15. How does HashMap store key-value pairs internally? 

HashMap uses an array of buckets (or slots) to store key-value pairs. Each bucket can hold multiple entries due to potential hash collisions.

16. What is hashing, and why is it important in HashMap? 

Hashing is a technique used by HashMap to convert a key into an index (hash code) for efficient storage and retrieval. It’s important because it allows HashMap to quickly locate the bucket where a key-value pair should be stored or retrieved.

17. What is the role of the hashCode() method in HashMap? 

The hashCode() method is used by HashMap to compute the hash code of keys. This hash code determines the index (bucket) where the key-value pair will be stored and helps in efficient retrieval.

18. Why are keys in a HashMap typically immutable objects?

Keys in HashMap are typically immutable to ensure that their hash code remains consistent throughout their lifetime. This prevents unexpected behaviour, such as not being able to retrieve values due to a change in hash code.

HashMap Coding Questions      

Here are some common HashMap programming interview questions and answers. 

19. How do you check if a key exists in a HashMap? 

You can use the containsKey() method to check if a specific key exists in a HashMap. 

For example:

HashMap<String, Integer> map = new HashMap<>();

map.put(“apple”, 10);

if (map.containsKey(“apple”)) {

   System.out.println(“Key ‘apple’ exists in the HashMap!”);

}

20. How do you retrieve a value from a HashMap given a key?

Use the get() method to retrieve the value associated with a given key in a HashMap. 

For example:

int value = map.get(“apple”);

System.out.println(“Value associated with ‘apple’: ” + value);

21. How do you remove a key-value pair from a HashMap?

You can use the remove() method to remove a specific key-value pair from a HashMap. 

For example:

map.remove(“apple”);

22. How do you check if a HashMap is empty?

You can use the isEmpty() method to check if a HashMap contains any key-value pairs. 

For example:

if (map.isEmpty()) {

   System.out.println(“HashMap is empty!”);

} else {

   System.out.println(“HashMap is not empty!”);

}

Wrapping Up

And that’s a wrap! 

These are the 15+ most commonly asked HashMap interview questions and answers. Understanding HashMaps is essential for Java developers, as they are widely used in programming. And, if you’re searching for latest tech job opportunities in India, visit Hirist. It is an IT job portal where you can easily find the best tech jobs with lucrative packages.