Java MCQ's on Internal Working of HashMaps

This test contains 10 questions based on Internal Working of the HashMap.
Each question answered correctly awards you 1 point and each incorrect answer has a penalty of -0.25 points, no points are deducted for unattempted answers.

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How are key values stored in a hashmap ?

as Entry object	as a LinkedList
as an ArrayList	None of these

A

All the objects in the form of a key-value pair are stored as an Entry object.

What is the initial size of a HashMap when we create an object of a HashMap ?

0	8
16	None of these

C

When we create a HashMap like "HashMap map = new HashMap();" , it creates an empty hashmap of initial size of 16 elements.

What methods are used to find the correct value of a given key ?

hashCode()	equals()
both hashCode() and equals()	None of these

C

Both the hashCode() and equals() method are used to get the value of a given key. hasgCode() method is used to get the correct bucket location of the key. Then the equals() method is used to find the actual key.

How many times the key gets hashed while finding the correct location for saving the key value pair ?

2	1
4	3

A

While finding the location of the bucket for saving the key-value pair, the key is hashed twice, first by the hashCode() of the Object class and then by the internal hashing method of the HashMap class.

Output of a HashMap is ordered ?

True

False

B

A HashMap outputs in the order of the hashcode of the keys, so it is unordered but it will always produce same result for same set of keys.

What is the default load factor of a HashMap ?

1	0.5
0.75	0.25

C

Default load factor of a HashMap is 0.75

How are keys-values saved when the keys produce the same hashcode ?

as a linkedlist	as an array
as seperate objects	as vectors

A

All the keys that produce same hashcode are saved in the same bucket location as a linked list.

Which interface does a HashMap implement ?

NavigableMap	SortedMap
Map	HashTable

C

HashMap implements the Map interface.

Is a HashMap threadsafe ?

Yes	No
Depends	None of these

B

No, a hashmap is not thread safe, if we want to make a HashMap thread safe then we may have to use a ConcurrentHashMap.

Which of the following are correct for a HashMap and a HashTable ?

HashMap allows a null object but a HashTable does not.	Hashtable is threadsafe but a hashmap is not
HashTable uses enumeration to iterate while a HashMap uses iterator to iterate.	All of the above

D

All of the above are correct for hashmap and hashtable, these are the differences between the two.

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