Top 20 Java Collections Interview Questions and Answers

Q1  What is Collection ? What is a Collections Framework ? What are the benefits of Java Collections Framework ?

Collection : A collection (also called as container) is an object  that groups multiple elements into a single unit.

Collections Framework : Collections framework provides unified architecture for manipulating and representing collections.

Benefits of Collections Framework :

1. Improves program quality and speed
2. Increases the chances of reusability of software
3. Decreases programming effort.




Q2 What is the root interface in collection hierarchy ? 

Root interface in collection hierarchy is Collection interface . Few interviewer may argue that
Collection interface extends Iterable interface. So iterable should be the root interface. But you should reply iterable interface present in java.lang package not in java.util package .It is clearly mentioned in Oracle Collection  docs , that Collection interface is a member of the Java Collections framework.  For Iterable interface Oracle doc , iterable interface is not mentioned as a part of the Java Collections framework .So if the question includes  collection hierarchy , then you should answer the question as Collection interface (which is found in java.util package).

Q3 What is the difference between Collection and Collections ?

Collection is  an interface while Collections is a java class , both are present in java.util package and  part of java collections framework.

Q4 Which collection classes are synchronized or thread-safe ?

Stack, Properties , Vector and Hashtable can be used in multi threaded environment because they are synchronized classes (or thread-safe). 

Q5 What is the difference between List and Set ?

Set contain only unique elements while List can contain duplicate elements.
Set is unordered while List is ordered . List maintains the order in which the objects are added .

Q6 What is the difference between Iterator and Enumeration ?

The main difference between Iterator and Enumeration is that Iterator has remove() method while Enumeration doesn't.
Hence , using Iterator we can manipulate objects by adding and removing the objects from the collections. Enumeration behaves like a read only interface as it can only traverse the objects and fetch it .

Q7 Which methods you need to override to use any object as key in HashMap ?

To use any object as key in HashMap , it needs to implement equals() and hashCode() method .

Q8  What is the difference between Queue and Stack ?

Queue is a data structure which is based on FIFO ( first in first out ) property . An example of Queue in real world is buying movie tickets in the multiplex or cinema theaters.

Stack is a data structure which is based on LIFO (last in first out) property . An example of Stack in real world is  insertion or removal of CD  from the CD case.

Q9 How to convert the array of strings into the list ?

Arrays class of java.util package contains the method asList() which accepts the array as parameter.
So,

String[]  wordArray =  {"Love Yourself"  , "Alive is Awesome" , "Be in present"};
List wordList =  Arrays.asList(wordArray);

Q10 How to reverse the List in Collections ?

There is a built in reverse method in Collections class . reverse(List list) accepts list as parameter.

Q11 What is the difference between ArrayList and Vector ?

It is one of the frequently asked collection interview question , the main differences are
Vector is synchronized while ArrayList is not . Vector is slow while ArrayList is fast . Every time when needed, Vector increases the capacity twice of its initial size while ArrayList increases its ArraySize by 50%. find detailed explanation   ArrayList vs Vector  .

Q12 What is the difference between Array and ArrayList in Java ?

This question checks whether student understand the concept of static and dynamic array. Some main differences between Array and ArrayList are :
a. Array is static in size while ArrayList is dynamic in size.
b. Array can contain primitive data types while ArrayList can not contain primitive data types.

find detailed explanation ArrayList vs Array in Java

Q13 What is the difference between HashSet and TreeSet ?

Main differences between HashSet and TreeSet are :
a.  HashSet maintains the inserted elements in random order while TreeSet maintains elements in the sorted order
b. HashSet can store null object while TreeSet can not store null object.
find detailed explanation here TreeSet vs HashSet in Java

Q14 Write the code for iterating the list in different ways in java ? 

There are two ways to iterate over the list in java :
a. using Iterator
b. using for-each loop

Coding part : Do it  yourself (DIY) , in case of any doubts or difficulty please mention in the comments .

Q15 What is the difference between HashMap and ConcurrentHashMap ?

This is also one of the most popular java collections interview question . Make sure this question is in your to do list before appearing for the interview .
Main differences between HashMap and ConcurrentHashMap are :
a. HashMap is not synchronized while ConcurrentHashMap is synchronized.
b. HashMap can have one null key and any number of null values while ConcurrentHashMap does not allow null keys and null values .
find detailed explanation here ConcurrentHashMap vs HashMap in Java

Q16 Arrange the following in the ascending order (performance):
HashMap , Hashtable , ConcurrentHashMap and Collections.SynchronizedMap 

Hashtable  <  Collections.SynchronizedMap  <  ConcurrentHashMap  <  HashMap

Q17 How HashMap works in Java ?

This is one of the most important question for java developers. HashMap  works on the principle of Hashing . Find detailed information here to understand what is hashing and how hashmap works in java .

Q18 What is the difference between LinkedList and ArrayList in Java ?

Main differences between LinkedList and ArrayList are :
a. LinkedList is the doubly linked list implementation of list interface , while , ArrayList is the resizable array implementation of list interface.
b. LinkedList can be traversed in the reverse direction using descendingIterator() method  provided by the Java Api developers , while , we need to implement our own method to traverse ArrayList in the reverse direction . find the detailed explanation here ArrayList vs LinkedList in java.



Q19 What are Comparable and Comparator interfaces ? List the difference between them ?

We already explained what is comparable and comparator interface in detail along with examples here,  Comparable vs Comparator in Java

Q20 Why Map interface does not extend the Collection interface in Java Collections Framework ?

One liner answer : Map interface is not compatible with the Collection interface.

Categories: Java interview Questions

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