Java HashMap is a critical component of the JDK. There are few classes in Java’s collection system that are more popular than ArrayList. When a person says, “If you could have only one data structure, make it a hash table,” it’s no surprise that HashMap is so valuable. Searching for a value takes O(1) time if you know the key to a hash table. Many different hash tables exist in Java, such as ConcurrentHashMap and LinkedHashMap, but the HashMap is your go-to implementation.
There are additional hash table implementations in the JDK if you have a specific need. LinkedHashMap, for example, can be used if you want to maintain the order of the mapping. TreeMap is a sorted map implementation that you can organize your mappings.
It is also possible to use a ConcurrentHashMap from the JDK 5 to construct a thread-safe hash table that can be used in a concurrent application without affecting Scalability.
Also, if you’re just getting started with JDK API development, I urge you to look at my list of the top free core Java courses. In addition to helping you perform better in interviews, this will help you better grasp the foundations.

Interview Questions for HashMap in Java

From the Java Interviews, here is a set of HashMap questions. This collection includes questions about the internal implementation of HashMap, the Map API, how you use HashMap, and common best practices when utilizing HashMap in a Java application.

1. Explain The put() method of a Java HashMap

HashMap’s put() method follows the hashing idea. To put an object into a backend array, this is what it does. An item can be retrieved from the bucket using a hash function and the hashcode() method. Whenever a key/value collision happens, a linked list of the entry object’s key/value pairs is created and saved in the bucket location.

2. What is the definition of a key or value in a hash table?

The key or value object must implement the equals() and hashcode() ways. Key objects are entered into the map using their hash code, and values are retrieved using their equivalents.

3. Is it possible to store an already existing key in a HashMap?

You can override an existing key in the HashMap with a new value, and put() will return the previous value if you save an existing key. There will be no snafus or blunder.

4. In Java HashMap, can you store a null key?

Yes, a null key can be stored in a HashMap’s first bucket, e.g., bucket[0] = value. Using a null key in the HashMap’s get() method results in the return of the first index, which is why the hashCode() method isn’t called on a null key.

5. In Java, is it possible to store a null value in a HashMap, or is it not?

The hashmap example in this blog shows that you can store as many null values as you like in the Hashmap.

6. HashMap handles collisions in Java uniquely and efficiently. Explain how?

This is the Java utility class. HashMap uses chaining to store new items in a linked list and the old ones for collisions. Then, the linked list is placed into its bucket location. ‘
Your hash table will be transformed into a linked list if every key has the same hashcode, hence looking for a value will require O(n) time instead of O(1).
It’s good to consult suitable data structure and algorithm courses, which cover basic data structures like an array, linked list, and binary tree and advanced concepts like O(n) sorting algorithms, Radix sort, Counting sort, and others.

7. What kind of data structure is HashMap?

If you know the key, you can search for the key-value pair in O(1) time in the HashMap implementation of a hash table data structure like id to name.

8. Java’s HashMap implementation uses this data structure?

Even though HashMap represents a hash table, it is implemented in the JDK utilizing an array and linked list data structure. If you have a bucket, an array data structure is used to store all the mappings that fall into it. When the number of linked list members reaches a predetermined threshold, a binary search tree is dynamically substituted for the linked list.

9. In a HashMap, can you store a duplicate key

No, HashMap does not enable duplicate keys to be inserted. The last value will be overwritten if you try to insert an existing key with the same or a new value, but the size of the HashMap will remain unchanged, i.e., the same. When you call keySet to obtain all the keys from a HashMap, this is one of the reasons (). Because a Set is not a Collection, it returns a Set.

10. Is it possible to store the same value in a Java hash table?

Java’s HashMap allows you to store duplicate values. When you call the values() function to get all the values from the Hashmap, it returns a Collection rather than a Set because it allows duplicate values. Noting that HashMap does not guarantee the order of keys or values, it doesn’t yield a list.
Learn about the essential features of Java collections, such as List, Set, and Map, by checking out these free online courses on Java Collections.

11. HashMap in Java is thread-safe, or is it not?

The HashMap class in Java is not a thread-safe method. If one or more threads modify the HashMap (e.g., inserting or removing a map), you should not share a HashMap with several threads. However, you can share a read-only HashMap.

12. In a multithreaded Java program, how will HashMap behave?

The internal data structure of HashMap may become corrupted if you use it in a multithreaded scenario where many threads structurally affect the map, such as adding, removing, or modifying mappings. The map itself may become utterly worthless. ConcurrentHashMap or Hashtable are better options for a thread-safe map than HashMap, which is not recommended for concurrent applications.

13. Is it possible to iterate over a HashMap in Java in various ways?

The following are some examples of how Java developers can iterate over a HashMap:
• keySet an iterator can be used for this purpose.
• entrySet and Iterator are two tools that help with this.
• Loops can be improved by using entrySet.
• keySet and the get() method can be used for this purpose
Each of these methods can be demonstrated in this article, which provides an example for them.

14. In Java, how do you remove the mapping from a HashMap?

Although HashMap has a remove() method, you cannot use it while traversing a HashMap to remove a mapping. A mapping can be removed by removing it using the Iterator’s delete method like the following example:
terator itr = map.entrySet().iterator(); while(itr.hasNext()){ Map.Entry current = itr.next(); if(current.getKey().equals(“matching”){ itr.remove(); // this will remove the current entry. }
}
While traversing the map, you can see that we used the Iterator. Remove () method to remove the current entry. You may read more about it here.

15. How are HashMaps mappings arranged?

Keys, values, and entries in HashMap are not guaranteed to be in any particular order. It is possible to achieve a different order when iterating over a HashMap.

16. Can HashMap be sorted in Java?

You cannot sort a HashMap since it is not an ordered collection, as a list. As a side note, it is entirely possible to sort the data in a HashMap using sorting and then save it to an ordered map like LinkedHashMap or a sortable map like TreeMap by sorting and then storing the result.

17. What is the HashMap load factor

Resizing of the HashMap is triggered by the load factor, which is a percentage that determines how many elements must be removed from the HashMap before resizing can be triggered.

18. How does resizing a HashMap occur?

When the map fills up, or the load factor exceeds, the map is resized. It will be necessary to do array copy resizing if the load factor is 0.75 and the array grows more than 75% filled. The bucket’s capacity is doubled before copying existing entries into a new one.
How many HashMap entries can you store? What is the highest amount of time that can be saved? When the bucket fills up, items are added to a linked list, which can hold an unlimited number of entries; hence there is no upper limit for the number of entries you can store in a HashMap.
Also, the HashMap size() function has a limit; once the number of entries exceeds the limit, the size() method will overflow, and if your program relies on the overflow, it will break.
In JDK 8, a new method named mappingCount() has been introduced, which returns a long value. So, if you have a large map, you should use mappingCount(). New methods introduced in JDK 8 are covered in Java SE 8 for Really Impatient.

19. What is the distinction between a HashMap’s capacity and its size in Java?

HashMap’s capacity is the number of entries it can hold, and its size is the number of mappings or key/value pairs currently in the database.
What will happen if HashMap returns the same hashcode() for two separate keys of the same type? There will be a collision if two keys in a HashMap return the same hash code; hence the buckets will be emptied. They will be linked to each other in a database.
Some of the most common Java HashMap interview questions have been covered here. I’ve tried my best to respond to them, but please share them in the comments section below if you have any more thoughts. You should study the HashMap class and implementation thoroughly because it is a key Java class and a crucial part of the Java interview process.
We hope that these questions will not only improve your understanding of HashMap but that they’ll spur you on to learn more about it and the hash table data structure in general. If you have any other Java-related HashMap queries that you’d want to share with us, please do so.

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