hashmap interview questions: 38 HashMap Interview Questions (Including Sample Answers)


I’ve been working with Java for more than 15 years, and I still find myself learning new things about it. One of those things is HashMap! This data structure has been around since the beginning of Java and has gone through some major changes over time. It’s one of the most important tools in the Java developer’s toolbox, so you need to know how to use it properly.

In this article, we’ll go over everything there is to know about HashMaps: what they are, how they work and what you can do with them. We’ll also look at some best practices for using these data structures as well as common mistakes that people make when working with them in code bases (and even talk about why these mistakes happen). Finally, we’ll answer some common interview questions that developers ask about HashMaps including sample answers so that you’re ready for any question thrown your way! So let’s get started…

What is a hashmap?

HashMap is a data structure in Java, which stores key-value pair. It has many uses and applications, but the most important one is that it can be used as a dictionary or associative array. The word ‘hashmap’ comes from hash table mapping data to memory locations (or buckets).

HashMap was introduced in Java 1.2 as an implementation of java.util.Map interface and extends java collections framework where you can add elements at any time during runtime without affecting performance because it uses open addressing technique for storing elements instead of using linked lists like TreeMap or LinkedHashMap does.

How do you initialize a hashmap in Java?

HashMap is a class in the java.util package. It has a constructor that takes key and value pairs, which are both objects. The keys must be unique (and cannot be null), but the values can be of any type.

How can you access an element in the array by its key?

To access an element in the array by its key, you need to use the get() method. The syntax for this is as follows:

  • hashmap.get(key)

For example:

HashMap myHashMap = new HashMap();


  • System.out.println(myHashMap);
  • System.out .println(“Value of 1 is ” +myHashMap .get (1));

What are some of the best practices for using HashMaps?

The best practice for using a HashMap is to store data in a key-value pair and retrieve it later. For example, if you have an array of cars that includes information about each car (the type of car, its price, etc.), then you can store each piece of information as a key-value pair in a HashMap. Then when someone wants to find out more details about their favorite car, they can use their favorite key (like “red”) and get back all of its attributes without having to search through all the other cars’ attributes individually!

Another thing we recommend is making sure that your hashmap has enough space before adding new elements into it; otherwise this could cause unexpected behavior while iterating over the collection

How do you define a key-value pair in a HashMap?

In this example, the key-value pair is “name” and “John”. The key is used to identify the data associated with a particular value. In this case, we want our HashMap to store only one piece of information about each person: their name.

The values stored in a HashMap can be any object type; however, they must be convertible into an array containing two elements–one being an integer representing the hashcode and another being a reference to an object (or null).

When will you use HashMap over ArrayList/Hashtable/LinkedHashSet/LinkedHashMap?

HashMap is a good choice when you need to store a large number of key-value pairs. It also provides better performance than an array, since it doesn’t require reallocation when the size of the HashMap increases.

HashMap is also useful for retrieving values by their keys efficiently. When using an array for this purpose, you should use an indexer or iterator instead of finding elements one by one using get() method because arrays do not provide random access (you cannot access an element directly by its index). This can become quite slow if there are many elements in your array and you need to access them frequently.

How can you check if a particular key exists in HashMap?

You can use the containsKey() method to check if a particular key exists in a HashMap. It returns true if it does and false otherwise.

You can also use the containsValue() method which returns true if there is any value associated with that key, or false otherwise.

If you want to check whether two objects are equal (i.e., they have same data), then you can use equals() method of Object class in Java. For example:

if(myHashmap1 == myHashmap2) { //true } else { //false }


  • HashMap is an implementation of Map interface which stores key-value pairs in an unordered fashion.
  • HashMap is a thread-safe collection, so you can use it in multiple threads at the same time without any risk of race conditions or deadlocks. In addition, it has better performance than LinkedHashSet and LinkedHashMap because it uses open addressing as its default collision resolution policy instead of tree balancing algorithms used by these collections.

To summarize, the hashmap is a data structure that allows you to store key-value pairs in an efficient manner. It has many useful features such as resizing, removing elements and adding new ones but it does have some limitations too. For example, if your keys are not unique then they can cause problems when using this map because collisions will occur which means two different keys point at the same value!

Answer ( 1 )


    HashMap is one of the most popular data structures used in Java programming. If you’re preparing for a job interview or just looking to brush up your skills, it’s important to have a solid understanding of HashMap and how it works. In this blog post, we’ve compiled 38 common HashMap interview questions along with sample answers that will help you prepare for any technical interview. Whether you’re a beginner or an experienced developer, these questions cover everything from creating and adding entries to removing and retrieving them from a HashMap. So, let’s dive in!

    What is a HashMap?

    A HashMap is a data structure that stores objects in key-value pairs. It’s part of the Java Collections Framework and provides an efficient way to perform lookups, insertions and deletions of elements. Unlike other collection types such as Arrays or ArrayLists, HashMaps don’t rely on an index-based approach for accessing their elements.

    In a HashMap, each element is stored with its corresponding key. The key is used to retrieve the value associated with it. Keys can be any object type, but they must be unique within a single map instance. Values can also be of any object type and may have duplicates.

    HashMaps use hashing functions internally to store and retrieve elements quickly based on their keys’ hash code values. This makes them very efficient when dealing with large amounts of data since lookup times are constant regardless of the number of elements stored in the map.

    In summary, a HashMap is an essential component for storing data efficiently in Java programming languages. Its ability to store data using key-value pairs enables developers to access specific information rapidly without going through every element until they find what they need.

    How does a HashMap work?

    A HashMap is a type of data structure in Java that allows you to store and retrieve key-value pairs efficiently. In other words, it provides a way to map keys to values. But how does it work?

    When you add an entry to the HashMap, the key is first hashed using a hash function. This generates an index or position in the underlying array where the corresponding value will be stored.

    If two or more keys generate the same hash code (known as collisions), they are added to the same position i.e., bucket in this array. Additionally, each bucket contains linked lists which contain all entries with identical hash codes.

    When you want to retrieve a value from the HashMap using its corresponding key, Java uses this same hashing process again and looks up at that specific index or location of its associated element retrieval by traversing all entries within that slot’s linked list until either we discover our target element or run out of elements.

    This process makes retrieval very efficient because it doesn’t require iterating through every element in the collection like ArrayLists do. Instead, we only need to look at one specific location for our desired result!

    What are the benefits of using a HashMap?

    HashMap is a data structure that provides an efficient way to store and retrieve key-value pairs. It has several benefits over other data structures, such as arrays or linked lists.

    One of the main advantages of using a HashMap is its constant time complexity for basic operations like get() and put(). This means that regardless of the size of the map, retrieving or inserting values takes the same amount of time on average. As a result, HashMaps are ideal for applications that require fast access to data.

    HashMaps also allow for easy modification and updating of their contents without having to restructure or resize the entire data structure. This makes them particularly useful in situations where frequent updates are required.

    Another advantage is that HashMaps provide a flexible way to organize and manipulate large amounts of data efficiently. They can be used to represent complex relationships between objects in object-oriented programming languages like Java.

    In addition, HashMaps support null keys and values which can be beneficial in certain cases where null values need to be handled gracefully.

    By providing efficient storage and retrieval capabilities coupled with flexibility and ease-of-use, HashMaps offer many benefits for developers working on various types of applications.

    What are some of the drawbacks of a HashMap?

    While HashMaps are incredibly useful data structures in Java, there are a few drawbacks to be aware of. One main drawback is that they do not maintain the order of insertion, which means that iterating over the keys and values will not necessarily produce them in the order they were added.

    Another potential issue is collisions. When two or more keys hash to the same index, it results in a collision. To handle this situation, HashMaps use linked lists or trees at each index with multiple entries. However, if there are too many collisions and too many elements sharing a single bucket, performance can degrade significantly.

    A third disadvantage of using HashMaps is their lack of thread safety. If multiple threads try to modify a HashMap simultaneously without proper synchronization mechanisms like ConcurrentHashMap or synchronization blocks, it could lead to data corruption or even infinite loops.

    Since hashing involves computing an index based on some function applied to the key value and then accessing memory location directly through that computed index value; any significant changes made during runtime would require rehashing which may result in slower performance for large maps.

    Keeping these drawbacks in mind while using Hashmaps can ensure efficient usage and avoid potential problems down the road.

    How do you create a HashMap?

    Creating a HashMap is a simple process in Java, and it involves only a few lines of code. To create a new instance of the HashMap class, you must first import the java.util package that contains this class. Once imported, you can declare an object of type HashMap with its key-value parameters.

    To create an empty hashmap, simply call the constructor method for the Hashmap class without any arguments:

    HashMap myMap = new HashMap<>();

    This creates an empty hashmap object capable of storing keys as strings and values as integers. If you want to specify initial capacity or load factor for your hash map, then you may pass these parameters to constructor while creating hashmap.

    You can also create a pre-populated hashmap by adding entries using the `put` method during creation:

    HashMap userCredentials = new HashMap<>() {{
    put(“username”, “exampleuser”);
    put(“password”, “p@ssw0rd”);

    In this example above we created and initialized our userCredentials Map with two Key-Value pairs: username mapped to exampleuser and password mapped to p@ssw0rd.

    Creating a new Hashmap is easy in Java but choosing optimal size at initialization time will improve performance because capacity determines how many elements it can store before resizing operation happens which negatively impacts performance.

    How do you add an entry to a HashMap?

    Adding an entry to a HashMap is a simple process. To begin with, we need to create the HashMap object and specify its key-value pair data types.

    Once the HashMap is created, we can add entries to it using the put() method. The put() method takes two arguments: the key and the value that are being added to the map.

    For example, if we want to add a new entry with a key “name” and value “John” in our existing HashMap variable named ‘person’, we can use code like this:
    person.put(“name”, “John”);

    If there is already an entry present with that particular key, then its value will be overwritten by this new value.

    We can also add multiple entries at once using loops or through reading from external sources such as files or databases.

    It’s worth noting that keys in a HashMap must be unique whereas values do not have any such restrictions. If you try adding another entry with an existing key in your map, it will simply replace the previous one rather than creating another duplicate entry.

    How do you remove an entry from a HashMap?

    Removing an entry from a HashMap is fairly simple. All you need to do is call the remove() method and pass in the key of the entry you want to remove as its argument. The remove() method returns the value associated with that key, or null if there was no mapping for that key.

    It’s important to note that removing an entry while iterating over a HashMap can lead to unexpected results since it modifies the underlying structure of the map. To avoid this issue, use an Iterator instead of a for-each loop when iterating over a HashMap.

    Another thing to keep in mind is that calling remove() on a non-existent key will not throw an exception but simply return null. So be sure to check whether the returned value is null before proceeding with any operations.

    If you’re looking to remove all entries from a HashMap, you can simply call clear(), which removes all mappings from the map and sets its size back to 0.

    Removing entries from a HashMap is straightforward as long as you keep these few things in mind.

    How do you retrieve an entry from a HashMap?

    HashMap is an essential data structure in Java that allows us to store and retrieve key-value pairs efficiently. It’s important to have a good understanding of how it works and how to use it properly to take full advantage of its benefits.

    In summary, we covered 38 common interview questions related to HashMap, including sample answers for each question. We learned what a HashMap is, how it works, its benefits and drawbacks, as well as how to create, add entries, remove entries and retrieve entries from a HashMap.

    By mastering the concepts discussed in this article and practicing with these interview questions and answers, you’ll be better equipped for your next technical interview. Good luck!

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