High 40 Java HashMap Interview Questions and Solutions for 3 to five Years Skilled

40 HashMap Interview Questions in Java and Solutions

Right here is my assortment of 40 HashMap based mostly Java interview questions. You
ought to undergo this record earlier than showing for any Java interviews e.g.
telephonic spherical or face-to-face spherical. I’ve purposefully stored the reply
quick and to-the-point so that you just need not spend an excessive amount of time. 

I’ve
additionally included hyperlinks for extra curious builders as a result of I’ve already
mentioned many of those questions in detailed right here. You may verify them if
you could have extra time otherwise you wish to study extra about these ideas. 

An HashMap is a Java implementation of common knowledge construction hash desk,
which lets you retailer key worth pairs e.g. books and their costs. It
additionally implement the java.util.Map interface which is Java’s interface for a
class which lets you retailer key worth pairs. This implies you possibly can cross
HashMap to a way, which is anticipating java.util.Map interface. 

Each key and worth object should implement equals() and hashcode() technique if
you intent to make use of them as key and worth i HashMap. The hashcode() technique of
secret is used to search out the bucket location, the place entry object is saved.
Equally, equals() technique of key and worth is used if you retrieve an
object from HashMap. See Core Java for the Impatient for extra particulars on how
HashMap internally works. 

The important thing distinction between them is that HashMap just isn’t thread-safe whereas
Hashtable is thread-safe. This tread-safety is achieved utilizing
synchronization, which additionally makes Hashtable slower than HashMap. One other
value noting distinction is that HashMap was first added in Java 1.4 whereas
Hashtable is current from the beginning of JDK. 

The ConcurrentHashMap is an scalable and concurrent model of HashMap. The
key distinction once more is similar because it was in earlier query, HashMap just isn’t
thread-safe however ConcurrentHashMap is thread-safe, although it’s extra scalable
than Hashtable or synchronized HashMap due to its implementation e.g. it
divides the map into a number of segments and it by no means lock complete map, as an alternative
solely segments are locked if you add or take away entries from
ConcurrentHashMap. You may study extra about its implementation on my put up
How ConcurrentHashMap works in Java.

A HashMap works within the precept of hashing. The important thing object is linked with
worth objects utilizing hash operate. Additionally they saved collectively as entry. It
means that you can seek for an object in O(1) time when you have key object. When
you retailer an object into HashMap, the hash operate is utilized on its key to
discover the bucket location, which is nothing however the applicable index in
underlying array. It is nothing however hash desk knowledge construction, in case you are not
conversant in hash desk, I recommend you studying a superb e book on knowledge
constructions e.g. Introduction to Algorithms by Thomas H. Cormen. 

HashMap is a Map, whereas HashSet is a Set. HashMap is used to retailer key worth
pairs, whereas HashSet can be utilized to retailer only one object e.g. keys. 

6. Which knowledge construction is used to implement HashMap? (reply)

HashMap is implementation of hash desk knowledge construction however it makes use of array
internally to retailer components. The array is also called bucket in context
of HashMap. 

When hash operate return similar bucket location for 2 completely different objects
then its referred to as collision as a result of there may be already an object saved at that
location. In case of collision completely different collision decision technique is
used to resolve it e.g. double hashing means one other attempt to discover a new
location or chaining e.g. retailer each values in the identical bucket location as
linked record, which is what java.util.HashMap does to resolve
collisions. 

The important thing distinction between HashMap and TreeMap is the truth that HashMap is
not ordered or sorted however TreeMap is a sorted Map, the place key worth pairs are
saved in a sorted order imposed by Comparator (pure order) or Comparator
(customized order). One other notable distinction is that, other than equals() and
hashCode(). the important thing must also be Comparable to one another. 

Right here can also be a pleasant desk which highlights key distinction between HashMap,
TreeMap, LinkedHashMap, and Hashtable in Java:

You may put as many as you need, till you run out of reminiscence however when you
cross the threasold of Integer.MAX_VALUE you’ll begin dealing with issues
as a result of the scale technique additionally return an int, which implies when you retailer extra
than Intger.MAX_VALUE, the scale technique will return incorrect worth. 

12. How does HashMap handles null keys in Java? (reply)

13. Does HashMap is thread-safe? What is going to occur when you expose HashMap to
a number of threads? (reply)

15. What’s distinction between HashMap and LinkedHashMap in Java?
(reply)

19. Are you able to identify some new strategies added into HashMap in Java 8?
(reply)

20. What’s distinction between measurement() and mappingCount() technique of HashMap?
(reply)

21. Does the iterator returned by HashMap is fail-safe or fail-fast?
(reply)

22. How do you get all keys from HashMap in Java? (reply)

23. How do you get all values from HashMap in Java? (reply)

25. What’s distinction between HashMap and ArrayList in Java? (reply)

As I stated, you can’t type HashMap as a result of it’s an un-ordered assortment,
however you possibly can seize all its values and kind them identical to another
assortment utilizing Comparator after which retailer all entries within the order of
values right into a LinkedHashMap, which retains mapping within the order they have been
added. 

You should utilize this neat trick to initialize HashMap in Java. This is named
Double brace initialization sample, it saves time however it’s not inspired
as a result of it creates an Nameless class everytime you employ it. 

This is named reverse lookup as a result of you are attempting to get a key from
worth and you need to keep in mind that HashMap permits duplicate values
means, multiple key can map to similar worth e.g. this technique ought to
return a set of keys. 

The primary distinction between HashMap and WeakHashMap is the truth that keys in
WeakHashMap are wrapped inside WeakReference object, which implies if an
object just isn’t referred wherever else aside from the Map itself, then it’s
topic to rubbish assortment.

35. How does get() technique of HashMap works in Java? (reply)

36. How does put() technique of HashMap works in Java? (reply)

37. What’s time complexity of get() and put() in HashMap? (reply)

38. What’s the order of entries in HashMap? (reply)

39. Are you able to retailer duplicate keys in HashMap? What occurs when you attempt to
put a key which already exists in HashMap? (reply)

No, you can’t retailer a replica worth within the HashMap. For those who attempt to retailer a
key which already exists within the Map then its worth is up to date with the brand new
worth and put() technique returns the outdated worth. 

40. Does HashMap permits duplicate values? (reply)

Sure, HashMap permits duplicate values and that is why the values() technique,
which return all values, return a Assortment object and never a Set. 

That is all about among the essential HashMap interview questions from Java developer job
interviews. These questions won’t solely aid you to do effectively in your job interview
but in addition aid you to study and perceive HashMap higher, which could be very
essential for a Java developer. It additionally encourage you to study extra about
HashMap e.g. how they’re applied internally, what are the necessities
for key and worth objects, efficiency of HashMap, thread-safety,
immutability and different essential properties of HashMap. 

Different Java Interview Questions it’s possible you’ll wish to probe for follow

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