Java Collections Interview Questions and Answers

    11 Votes

7) What are the advantages of using Interface type in collection ?

  • We can switch the implementation without changing the code.
  • It is easy to test our code with testing and mocking frameworks.
  • We can use the most generalized interface for data exchange (enumerator instead of a list, if you only want to iterate over the values).


8) What is the difference between Array and ArrayList ?

Fixed length Size of the Array can be grown based on the need.
Can't use Generics. Can use Generics
Provides length variable which denotes length Provides size() method to calculate size
Can store primitives We can't store primitives

9) Can you explain the difference between fail fast iterator and fail safe iterator ?

Fail fast iterator
: This iterator throws "ConcurrentModificationException" when a thread iterating a collection realizes that the same collection is modified by another thread while under operation. Iterator identifies change in collection is by looking at it's modification count. Ex: HashMap

Fail safe iterator : If we are using fails safe iterator, the above scenario is handled and java doesn't throw the ConcurrentModificationException. To handle this scenario, iterator makes a clone of the collection and works on it. Ex: ConcurrentHashMap

10) Can you explain about concurrent modification exception ?

When two or more threads try to modify an object where concurrent modification is not permitted, java throws concurrent modification exception which is a runtime exception. This exception is thrown by fail fast iterators.

11) what is the difference between iterator and list iterator ?

IteratorList Iterator
Only forward movement is allowed. It can move backwards as well as forward while reading the elements.
While traversing we can't obtain the index at any point. We can obtain the index at any point while traversing
We can only check the availability of the next element We can check availability of previous and next element.
We can't modify element while traversing. Elements can be modified while traversing.

12) Explain the get() and put() logic in hashmap ?

Put (Key,value) 

  • First, check for null in the key. If null, then key value pair will be store in 0'th position. Reason for that is, Hashcode for null is 0.
  • Else, calculate hash code of key by calling hashcode() method
  • Based on the returned hashcode, hashmap implementation goes to the bucket(Stores Key Value pair) who's identifier is matching the returned hashcode. Once bucket is identified, hashmap search the basket for a matching key using equals method. If it finds one, value will be updated
  • If a matching key is not found in the basket, java creates a new Key-Value pair in the basket.

Get (Key)

  • First, use the key to get the hashcode.
  • Go to the basket with matching identifier as hash code.
  • Iterate using equals method to find the matching key in the basket and once key is identified, return value which is stored along with key.
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