The class of java.util.HashMap is by far one of the most important libraries of Java. Rarely is there any project which is written in java, and it does not use the library of HashMap in their implementation. It is the way that this data structure is implemented, which makes this almost indispensable for any Java-related program. HashMap, in its definition, is very simple. HashMap is a data structure that is implemented using a very tabular approach.

It is rightly said that if there would be only one data structure, it should be HashMap. Not only is this very useful but also very time efficient. A simple lookup in HashMap only takes about O(1) time (meaning all the lookup in a HashMap is done in constant time). You will find many implementations of HashMap in the programming language of Java, such as HashTable, concurrenthashmaps, etc. However, if you are looking for a general implementation you should not stray far from basic HashMap.

Each of these implementations of HashMap comes with its own sets of features and use cases. If you want the order in which you have written the mapping to be preserved, you should be looking to use the Linked HashMap. If you wish to the mapping of yours to be sorted, then you should use the TreeMap implementation of HashMap, which does the sorting for you. 

In a similar fashion, if you are looking for a Hash table that happens to be thread-safe, which can also be used in a concurrent application while keeping the scalability, you would be looking at a concurrent HashMap. 

As you can see, there are various ways in which you can use HashMap. To really know the importance of HashMap, you should see the interview questions for any software engineering job profile. You will not be able to optimize time properly if you do not know how to implement a HashMap correctly. So, if you are a programmer, you cannot go away from learning the ins and outs of HashMap. So we have listed some outstanding hashmap interview questions below, which should help you prepare for your next interview.

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Hashmap Interview Questions & Answers

Q1. How will you use the put() method of HashMap in Java?

Ans. The method of put() simply works by making use of the principle known as hashing. You would make use of this function to store the object in question into the backend (which is an array of memory). You would have to use another function together with this put() function to be able to ascertain the exact position of this object in the backend. The hashcode() function is used as the helper function to put() in the way which we have earlier described.

So, the way these two functions work together is that they go and search for the available key and the memory location in the backend. If they happen to collide, then the object which has both values (meaning the values of both key and value) is then added into a dynamic implementation of a list, also known as a linked list. The liked list here is stored in the memory location, which was earlier found.

Q2. What are the basic needs that an object needs to have in order for it to be used as either the key or the value of a HashMap?

Ans. Both key and the value which we use in any HashMap and its implementation should also have these two functions written in them. The name of these two functions is equals() and hashcode(). The function that has the name of hashcode() is used when we insert the value of the key in any HashMap. At the same time, the function of equals() is called only when we are trying to get back the value that was already stored in the HashMap.

Q3. Let us say that you have lost track of the keys which you have already used in your HashMap. The next key which you want to add is already present in the HashMap. What do you think will happen in this case?

Ans. If you feel that there should be an error, you will not be wrong if we were not talking about HashMap. However, if you are using HashMap and you happen to insert a key whose value was already present in the hashmap, the java compiler will not throw any error. Instead, what will happen is that the compiler would just go and find the value associated with the key that you have entered, and it will only update the value there.

If you use the function put() to add the value into the hashmap, this function’s return value would be the old value associated with that key. So, let us assume that for key A, you have a stored value of 1. If you now put the value of A to be, let us say 50, then the value which will be returned by the put() function will be 1, and the final value stored in the key A would be 50.

Q4. In case the need arises of storing a null value. Would you be able to use HashMap in that case?

Ans. Yes, we would be able to use hashmap in that case without any issues because hashmaps do not reduce the value which you can store. You can store as many null values as your heart desires in any hashmap.

Q5. How is collision handled with the hashmap implementation in Java?

Ans. The code library of java.util.HashMap uses the method of chaining to handle the collisions. The way that this chaining works is any new entry to the hashmap, which contains both the key and value pair would be stored in a linked list, and this list would then be stored where the existing data of the hashmaps reside (the bucket location).

In the case when all the keys which you have in your hashmap turn out to have the same hashcode, then your hashmap will no longer be hashmap. It will be turned into a linked list. The lookup time will increase for the order of one (o(1)), i.e., constant time, to order of N (O(N)) that is linear time.

Q6. Will you be able to store a key that has a null value in a java’s implementation of hashmap?

Ans. Yes, in the java’s implementation of the hashmap, you will be able to store keys with a null value. But you will only be able to store just one such key, which will always be stored at the start of the bucket list. The hashmap will not call the function hashcode() here because if that function is used on a null key, it will throw a Null Pointer Exception error. You will have to use the get function instead of using the null value as the key if you want the first index of your hashmap to be returned to you.

Read: Sorting in Data Structure: Categories & Types 

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Q7. Which data structure do you think does hashmap is designed to mimic?

Ans. The hashmap is supposed to mimic the look and functionality of the hash table data structure. Hash table data structure is your ideal choice if you want to store the values in the form of a key and value pair. The main reason why this hash table is preferred is because of the lookup time. So, if you have the key you want to search for, then in just order of one or constant time, you will be able to get back the value you desire.

Q8. Since hashmap is not one of the core data structures like structure or arrays. What do you think goes on under the hood whenever you create a hashmap? Which data structure’s combination do you think gave us the hashmaps?

Ans. Even though the hashmap actually represents a hashtable, but under the hood, it is not directly implemented. We use two well-known data structures for breathing life into a hashmap. We use a linked list and arrays. A linked list is used to store the key and value pair dynamically, and the array is used to store all these linked lists into them; together, they form a hashmap.

From Java 8, if the linked list becomes sufficiently large, then instead of linked list Java implements a binary search tree instead. This change is done to save time and improve the overall performance of hashmap.

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Q9. In case we want to store multiple values under the same key, do you think it will be possible in the hashmap implementation present in Java?

Ans. No, you will not be able to store duplicate keys in the hashmap. If you try to store a new value in a key already present in the hashmap, then the hashmap would simply remove the value that was earlier stored in that key and replace it with the new one.

The size of the hashmap, in this case, would not change, meaning there will be no addition of keys into the hashmap. This feature is one of the reasons we use the function keyset() to get back all the keys of a hashmap and that this function returns a set and not a collection (because in a set all the values have to be unique).

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Q10. Will you be able to store duplicate values in HashMap?

Ans. Yes, you will be able to store duplicate values in the hashmap. That is the reason when you retrieve all the values from the hashmap. Then instead of getting a set, we get a collection. It does not give the value in the form of a list because ordering is not guaranteed.

Q11. In Java’s implementation of the hashmap, is it thread-safe?

Ans. No, the vanilla Hashmap is not at all thread-safe in Java. Ideally, you should avoid sharing the hashmap to a thread that can change the value and edit the hashmap in any form or shape. You can, however, use a read-only hashmap and then feed it into the thread. This way, the values contained in them will not change.

Q12.  What do you think is going to happen if we use the hashmap in an application which uses multi threads?

Ans. If you happen to pass a hashmap to an application that uses more than one thread, and each of these threads can modify, add or remove the values which are stored in the hashmap, then the internal data through which the hashmap was constructed would be corrupted. The links which were used to form the hashmap would go missing, and then the hashmap would lose its structure, and its purpose would also be lost. Hence it is always recommended that you always use thread safe hashmap if you are to use them in an application that is threaded. 

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Q13. If we want to iterate over the entire HashMap in Java. What are the different ways in which we would be able to do that?

Ans. There are many ways in which you would be able to iterate over a hashmap in java; some of them are listed below:

1. You can use the function keySet coupled with an iterating variable.
2. You can use the function enterySet coupled with an iterating variable.
3. You can use the function entrySet and use it with an enhanced loop.
4. You can use the keySet and get a method.

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