We have already shared the most frequently asked java interview
questions for experience candidates. Also shared the tricky coding interview
questions in our previous posts. Today , we will learn about the top 50 java
collections interview questions and answers. We will divide this post into
three categories :
Beginner level (0-1 year experience (Freshers)) ,
Intermediate level (1-3 years experienced Java Developers)
Advanced level(3+ Experienced) java collections interview questions and
answers
Note : Please prepare all the below questions . Interviewer may choose to ask
any question.
Beginner
Level (0-1 yr): 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 Name the core Collection interfaces ?
The list of core
collection interfaces are : just mention the important ones
Important : Collection , Set , Queue , List , Map
Other interface also in the list : SortedSet, SortedMap , Deque,
ListIterator etc.
Q6 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 .
Q7 What is the difference
between Map and Set ?
Map object has unique keys each containing some value, while Set contain only
unique values.
Q8 What are the classes
implementing List and Set interface ?
Class implementing List interface : ArrayList , Vector
, LinkedList ,
Class implementing Set interface : HashSet , TreeSet
Q9 What is an iterator ?
Iterator is an interface . It is found in java.util package. It provides
methods to iterate over any Collection.
Q10 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 .
Q11 Which design pattern
followed by Iterator ?
It follows iterator design pattern. Iterator design pattern provides us to
navigate through the collection of objects by using a common interface without
letting us know about the underlying implementation.
Enumeration is an example of Iterator design pattern.
Q12 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 .
Q13 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.
Q14 How to reverse the List in
Collections ?
There is a built in reverse method in Collections class . reverse(List list)
accepts list as parameter.
Collections.reverse(listobject);
Q15 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);
Intermediate Level (1-3 yrs):
Java Collections Interview Questions and Answers
Q16 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%.
Q17 What is the difference
between HashMap and Hashtable ?
It is one of the most popular collections interview question for java developer
. Make sure you go through this once before appearing for the interview .
Main differences between HashMap and Hashtable are :
a. HashMap allows one null key and any number of null values while Hashtable
does not allow null keys and null values.
b. HashMap is not synchronized or thread-safe while Hashtable is synchronized
or thread-safe .
Q18 What is the difference
between peek(),poll() and remove() method of the Queue interface ?
Both poll() and remove() method is used to remove head object of the Queue. The
main difference lies when the Queue is empty().
If Queue is empty then poll() method will return null . While in similar case ,
remove() method will throw NoSuchElementException .
peek() method retrieves but does not remove the head of the Queue. If queue is
empty then peek() method also returns null.
Q19 What is the difference between Iterator and ListIterator.
Using Iterator we can traverse the list of objects in forward direction . But
ListIterator can traverse the collection in both directions that is forward as
well as backward.
Q20 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.
Q21 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.
Q22 Write java code showing
insertion,deletion and retrieval of HashMap object ?
Do it yourself (DIY) , if found any difficulty or doubts then please mention in
the comments.
Q23 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 .
Q24 Arrange the following in
the ascending order (performance):
HashMap , Hashtable ,
ConcurrentHashMap and Collections.SynchronizedMap
Hashtable < Collections.SynchronizedMap <
ConcurrentHashMap < HashMap
Q25
How HashMap works in Java ?
This is one of the most important question for java developers. HashMap
works on the principle of Hashing .
Q26 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 .
Q27 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,
Q28 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.
Explanation : Since Map requires key as well as value , for example , if we
want to add key-value pair then we will use put(Object key , Object value) . So
there are two parameters required to add element to the HashMap object .
In Collection interface add(Object o) has only one parameter.
The other reasons are Map supports valueSet , keySet as well as other
appropriate methods which have just different views from the Collection
interface.
Q29 When to use ArrayList and
when to use LinkedList in application?
ArrayList has constant time search operation O(1) .Hence, ArrayList is
preferred when there are more get() or search operation .
Insertion , Deletion operations take constant time O(1) for LinkedList. Hence,
LinkedList is preferred when there are more insertions or deletions involved in
the application.
Q30 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 .
Advance Level (3+ yrs): Java Collections Interview Questions
and Answers
Q31 How HashSet works
internally in java ?
This is one of the popular interview question . HashSet internally uses HashMap
to maintain the uniqueness of elements.
Q32 What is CopyOnWriteArrayList ? How it is
different from ArrayList in Java?
CopyOnWriteArrayList is a thread safe variant of
ArrayList in which all mutative operations like add , set are
implemented by creating a fresh copy of the underlying array.
It guaranteed not to throw ConcurrentModificationException.
It permits all elements including null. It is introduced in jdk 1.5 .
Q33 How HashMap works in Java ?
We are repeating this question , as it is one of the most important question
for java developer.HashMap works on the principle of Hashing .
Q35 What is BlockingQueue in
Java Collections Framework?
BlockingQueue implements the java.util.Queue interface .
BlockingQueue supports operations that wait for the queue to become
non-empty when retrieving an element , and wait for space to become
available in the queue when storing an element .
It does not accept null elements.
Blocking queues are primarily designed for the producer-consumer problems.
BlockingQueue implementations are thread-safe and can also be used in
inter-thread communications.
This concurrent Collection class was added in jdk 1.5
Q36 How TreeMap works in Java ?
TreeMap internally uses Red-Black tree to sort the elements in natural order.
Q37 All the questions related
to HashSet class can be found here ,
Q38 What is the difference
between Fail- fast iterator and Fail-safe iterator ?
This is one of the
most popular interview question for the higher experienced java developers .
Main differences between Fail-fast and Fail-safe iterators are :
a. Fail-fast throw ConcurrentModificationException while Fail-safe does not.
b. Fail-fast does not clone the original collection list of objects while
Fail-safe creates a copy of the original collection list of objects.
Q40 How do you use a custom object as key in Collection
classes like HashMap ?
If one is using the custom object as key then one needs to override equals()
and hashCode() method
and one also need to fulfill the contract.
If you want to store the custom object in the SortedCollections like SortedMap then
one needs to make sure that equals() method is consistent to the compareTo()
method. If inconsistent , then collection will not follow their contracts ,that
is , Sets may allow duplicate elements.
Q41 What is hash-collision in Hashtable ? How it was handled in
Java?
In Hashtable , if two different keys have the same hash value then it lead to
hash -collision. A bucket of type linkedlist used to hold the different keys of
same hash value.
Q42 Explain the importance of
hashCode() and equals() method ? Explain the contract also ?
HashMap object uses Key
object hashCode() method and equals() method to find out the index to put the
key-value pair. If we want to get value from the HashMap same both methods are
used . Somehow, if both methods are not implemented correctly , it will result
in two keys producing the same hashCode() and equals() output. The problem will
arise that HashMap will treat both output same instead of different and
overwrite the most recent key-value pair with the previous key-value pair.
Similarly all the collection classes that does not allow the duplicate values
use hashCode() and equals() method to find the duplicate elements.So it is very
important to implement them correctly.
Contract of hashCode() and equals() method
a. If object1.equals(object2)
, then object1.hashCode() ==
object2.hashCode() should always be true.
b. If object1.hashCode() ==
object2.hashCode() is true does not guarantee object1.equals(object2)
Q43 What is EnumSet in Java ?
EnumSet is a specialized Set implementation for use with
enum types. All of the elements in an enum set must come from a single enum
type that is specified explicitly or implicitly , when the set is
created.
The iterator never throws ConcurrentModificationException and is weakly
consistent.
Advantage over HashSet:
All basic operations of EnumSet execute in constant time . It is most likely to
be much faster than HashSet counterparts.
It is a part of Java Collections Framework since jdk 1.5.
Q44 What are
concurrentCollectionClasses?
In jdk1.5 , Java Api
developers had introduced new package called java.util.concurrent that have
thread-safe collection classes as they allow collections to be modified while
iterating . The iterator is fail-safe that is it will not throw
ConcurrentModificationException.
Some examples of concurrentCollectionClasses are :
a. CopyOnWriteArrayList
b. ConcurrentHashMap
Q45 How do you convert a given
Collection to SynchronizedCollection ?
One line code :
Collections.synchronizedCollection(Collection collectionObj) will convert
a given collection to synchronized collection.
Q46 What is
IdentityHashMap ?
IdentityHashMap
IdentityHashMap is a class present in java.util package.
It implements the Map interface with a hash table , using reference equality instead of object equality when
comparing keys and values.In other words , in IdentityHashMap two keys k1 and
k2 are considered equal if only if (k1==k2).
IdentityHashMap is not synchronized.
Iterators returned by the iterator() method are fail-fast , hence , will throw
ConcurrentModificationException.
Q47 What is WeakHashMap
?
WeakHashMap :
WeakHashMap is a class present in java.util package
similar to IdentityHashMap. It is a Hashtable based implementation of Map
interface with weak keys. An entry in WeakHashMap will automatically be removed
when its key is no longer in ordinary use. More precisely the presence of a
mapping for a given key will not prevent the key from being discarded by the
garbage collector.
It permits null keys and null values.
Like most collection classes this class is not synchronized.A synchronized
WeakHashMap may be constructed using the Collections.synchronizedMap() method.
Iterators returned by the iterator() method are fail-fast , hence , will throw
ConcurrentModificationException.
Q48 How will you make Collections readOnly ?
We can make the Collection readOnly by using the following lines
code:
General : Collections.unmodifiableCollection(Collection c)
Collections.unmodifiableMap(Map m)
Collections.unmodifiableList(List l)
Collections.unmodifiableSet(Set s)
Q49 What is UnsupportedOperationException?
This exception is thrown
to indicate that the requested operation is not supported.
Example of UnsupportedOperationException:
In other words, if you call add() or remove() method on the readOnly collection
. We know readOnly collection can not be modified . Hence ,
UnsupportedOperationException will be thrown.
Q50 Suppose there is an Employee class. We add Employee class objects to the
ArrayList. Mention the steps need to be taken , if I want to sort the objects
in ArrayList using the employeeId attribute present in Employee
class.
a. Implement the Comparable
interface for the Employee class and now to compare the objects by employeeId
we will override the emp1.compareTo(emp2)
b. We will now call
Collections class sort method and pass the list as argument , that is ,
Collections.sort(empList)
If you want to add more
java collections interview questions and answers or in case you have any
doubts related to the Java Collections framework , then please mention in the
comments.
