Interview Questions.

Question 1. How Does Get() Method Of Hashmap Works In Java?

Answer :

The get() approach of HashMap works within the principle of hashing. It is chargeable for storing an item into backend array. The hashcode() method is used along with a hash characteristic to find the precise area for the object into the bucket. If a collision happens then the entry object which includes both key and price is delivered to a linked listing and that connected listing is stored into the bucket place.

Question 2. What Is The Requirement For An Object To Be Used As Key Or Value In Hashmap?

Answer :

The key or value object ought to implement equals() and hashcode() approach. The hash code is used whilst you insert the key item into the map while equals are used while you try to retrieve a value from the map.

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Question three. What Will Happen If You Try To Store A Key Which Is Already Present In Hashmap?

Answer :

If you shop an existing key in the HashMap then it's going to override the antique price with the brand new cost and put() will go back the old cost. There will now not be any exception or error.

Question four. Can You Store A Null Key In Java Hashmap?

Answer :

Yes, HashMap lets in one null key that is stored at the primary place of bucket array e.G. Bucket[0] = fee. The HashMap does not name hashCode() on null key because it will throw NullPointerException, therefore when a consumer call get() approach with null then the fee of the first index is again.

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Question five. Can You Store A Null Value Inside Hashmap In Java?

Answer :

Yes, HashMap also lets in null price, you can store as many null values as you want as shown within the hashmap instance put up on this weblog.

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Question 6. How Does Hashmap Handle Collisions In Java?

Answer :

The java.Util.HashMap uses chaining to address collisions, which means new entries, an object which includes each key and values, are saved in a related listing along side current cost and then that connected list is saved within the bucket area. In the worst case, in which all key has the same hashcode, your hash table will be changed into a related listing and searching a price will take O(n) time instead of O(1) time.

Question 7. Which Data Structure Hashmap Represents?

Answer :

The HashMap is an implementation of hash table information structure that is idle for mapping one fee to other e.G. Identity to call as you could search for cost in O(1) time when you have the important thing.

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Question 8. Which Data Structure Is Used To Implement Hashmap In Java?

Answer :

Even though HashMap represents a hash table, it is internally implemented by way of the use of an array and connected listing information shape in JDK. The array is used as bucket even as a linked listing is used to keep all mappings which land inside the equal bucket. From Java 8 onwards, the linked list is dynamically changed via binary search tree, once some of elements in the linked list cross a sure threshold to improve overall performance.

Question 9. Can You Store A Duplicate Key In Hashmap?

Answer :

No, you cannot insert duplicate keys in HashMap, it does not permit reproduction keys. If you try to insert an present key with new or same fee then it'll override the vintage price however length of HashMap will not change i.E. It'll continue to be identical. This is one of the cause when you get all keys from the HashMap by using calling keySet() it returns a Set, no longer a Collection due to the fact Set does not allow duplicates.

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Question 10. Can You Store The Duplicate Value In Java Hashmap?

Answer :

Yes, you could placed reproduction values in HashMap of Java. It permits duplicate values, that's why when you retrieve all values from the Hashmap with the aid of calling values() technique it returns a Collection and not Set. Worth noting is that it would not return List due to the fact HashMap does not offer any ordering guarantee for key or cost.

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Question 11. Is Hashmap Thread-safe In Java?

Answer :

No, HashMap isn't thread-safe in Java. You should now not percentage an HashMap with multiple threads if one or more thread is editing the HashMap e.G. Placing or doing away with a map. Though, you may effortlessly share a read-handiest HashMap.

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Question 12. What Will Happen If You Use Hashmap In A Multithreaded Java Application?

Answer :

If you use HashMap in a multithreaded surroundings in the sort of way that a couple of threads structurally regulate the map e.G. Upload, get rid of or regulate mapping then the internal information shape of HashMap can also get corrupt i.E. Some links may go missing, a few might also point to wrong entries and the map itself may turn out to be absolutely vain. Hence, it's miles counseled now not to use HashMap in the concurrent utility, as a substitute, you should use a thread-safe map e.G. ConcurrentHashMap or Hashtable

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Question 13. What Are Different Ways To Iterate Over Hashmap In Java?

Answer :

Here are a number of the methods to iterate over HashMap in Java:

by means of the use of keySet and iterator
by using entrySet and iterator
via using entrySet and more advantageous for loop
through the use of keySet and get() technique
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Question 14. How Do You Remove A Mapping While Iterating Over Hashmap In Java?

Answer :

Even though HashMap gives cast off() approach to remove a key and a key/cost pair, you can not use them to do away with a mapping at the same time as traversing an HashMap, rather, you need to apply the Iterator's put off technique to remove a mapping as proven inside the following example:

Iterator itr = map.EntrySet().Iterator();

even as(itr.HasNext())

Map.Entry contemporary = itr.Subsequent();

if(modern.GetKey().Equals("matching")

itr.Cast off(); // this could get rid of the modern access.

You can see that we've got used Iterator.Dispose of() approach to do away with the contemporary access even as traversing the map.

Question 15. In Which Order Mappings Are Stored In Hashmap?

Answer :

Random order because HashMap would not offer any ordering guarantee for keys, values, or entries. When you iterate over an HashMap, you can get the special order each time you iterate over it.

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Question sixteen. Can You Sort Hashmap In Java?

Answer :

No, you cannot kind a HashMap because in contrast to List it isn't an ordered series. Albeit, you can sort contents of HashMap with the aid of keys, values or by using entries by way of sorting after which storing the end result into an ordered map e.G. LinkedHashMap or a looked after map e.G. TreeMap.

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Question 17. What Is Load Factor In Hashmap?

Answer :

A load component is a range of which controls the resizing of HashMap while a number of elements within the HashMap move the load component e.G. If the burden aspect is 0.Seventy five and when becoming greater than 75% full then resizing trigger which includes array copy.

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Question 18. How Does Resizing Happens In Hashmap?

Answer :

The resizing happens whilst map becomes full or while the dimensions of map crosses the weight thing. For instance, if the load factor is 0.75 and whilst end up greater than seventy five% full then resizing trigger which involves array replica. First, the scale of the bucket is doubled after which old entries are copied into a brand new bucket.

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Question 19. How Many Entries You Can Store In Hashmap? What Is The Maximum Limit?

Answer :

There is not any maximum limit for HashMap, you can save as many entries as you need because while you run out of the bucket, entries could be introduced to a connected listing which could guide an endless number of entries, of course till you exhaust all of the memory you have.

Btw, the scale() method of HashMap go back an int, which has a restrict, once a number of entries cross the restrict, size() will overflow and if your program is based on that then it's going to destroy. This issue has been addressed in JDK eight by introducing a new method referred to as mappingCount() which returns a protracted cost. So, you have to use mappingCount() for big maps. See Java SE eight for Really Impatient to research extra approximately new strategies delivered in existing interfaces in JDK eight.

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Question 20. What Is The Difference Between Capacity And Size Of Hashmap In Java?

Answer :

The capacity denotes how many entries HashMap can keep and length denotes what number of mappings or key/cost pair is currently present.

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Question 21. What Will Happen If Two Different Keys Of Hashmap Return Same Hashcode()?

Answer :

If two keys of HashMap return equal hash code then they will become inside the identical bucket, subsequently collision will arise. They will be saved in a related listing collectively.