77 Java Collections Interview Questions 2023 (Freshers/Experienced)

The Collection Framework is one of the most important supporting pillars for the fundamental concepts of Java. And you would know that if you are an aspiring Java candidate. Java, being one of the most popular and most used programming languages in the world has been an important topic of discussion when it comes to hiring a candidate.
This is why you would find several Java Collection questions in your interview. This gives you a reason to worry and you should be if you are going to take an interview for a job that requires Java expertise as well as some knowledge of Java Collection Framework.
One thing is clear that you need to consider the Java Collection questions before you step up in that interview room so that you are fully prepared and know what you can expect and what kind of questions there would be asked in the interview. The question is, where can you find the most asked Java Collection interview questions? Well here! We are not going to disappoint because we have to happen to have a list of the most asked Java Collection interview questions that you need to know about.
So move ahead and take a look at the following list of most asked Java Collection Interview questions and be ready and confident for your upcoming interview.

Java Collections Interview Questions

Q1.What are Collection related features in Java 8?
Q2.What is Java Collections Framework? List out some benefits of Collections framework?
Q3.What is the benefit of Generics in Collections Framework?
Q4.What are the basic interfaces of Java Collections Framework?
Q5.Why Collection doesn’t extend Cloneable and Serializable interfaces?
Q6.Why Map interface doesn’t extend Collection interface?
Q7.What is an Iterator?
Q8.What is the difference between Enumeration and Iterator interface?
Q9.Why there is not a method like Iterator.add() to add elements to the collection?
Q10.Why Iterator don’t have a method to get next element directly without moving the cursor?
Q11.What is different between Iterator and ListIterator?
Q12.What are different ways to iterate over a list?
Q13.What do you understand by iterator fail-fast property?
Q14.What is difference between fail-fast and fail-safe?
Q15.How to avoid ConcurrentModificationException while iterating a collection?
Q16.Why there are no concrete implementations of Iterator interface?
Q17.What is UnsupportedOperationException?
Q18.How HashMap works in Java?
Q19.What is the importance of hashCode() and equals() methods?
Q20.Can we use any class as Map key?
Q21.What are different Collection views provided by Map interface?
Q22.What is difference between HashMap and Hashtable?
Q23.How to decide between HashMap and TreeMap?
Q24.What are similarities and difference between ArrayList and Vector?
Q25.What is difference between Array and ArrayList? When will you use Array over ArrayList?

Tricky java collections interview questions

Q26.What is difference between ArrayList and LinkedList?
Q27.Which collection classes provide random access of its elements?
Q28.What is EnumSet?
Q29.Which collection classes are thread-safe?
Q30.What are concurrent Collection Classes?
Q31.What is BlockingQueue?
Q32.What is Queue and Stack, list their differences?
Q33.What is Collections Class?
Q34.What is Comparable and Comparator interface?
Q35.What is difference between Comparable and Comparator interface?
Q36.How can we sort a list of Objects?
Q37.While passing a Collection as argument to a function, how can we make sure the function will not be able to modify it?
Q38.How can we create a synchronized collection from given collection?
Q39.What are common algorithms implemented in Collections Framework?
Q40.What is Big-O notation? Give some examples?
Q41.What are best practices related to Java Collections Framework?
Q42.What is Java Priority Queue?

Java Collections Interview Questions And Answers

Q1.What is the Collection framework in Java?
Collection Framework is a combination of classes and interface, which is used to store and manipulate the data in the form of objects. It provides various classes such as ArrayList, Vector, Stack, and HashSet, etc. and interfaces such as List, Queue, Set, etc. for this purpose.
Q2.What are the main differences between array and collection?
Array and Collection are somewhat similar regarding storing the references of objects and manipulating the data, but they differ in many ways. The main differences between the array and Collection are defined below:
Arrays are always of fixed size, i.e., a user can not increase or decrease the length of the array according to their requirement or at runtime, but In Collection, size can be changed dynamically as per need.
Arrays can only store homogeneous or similar type objects, but in Collection, heterogeneous objects can be stored.
Arrays cannot provide the ?ready-made? methods for user requirements as sorting, searching, etc. but Collection includes readymade methods to use.
Q3.How to synchronize ArrayList?
We can synchronize ArrayList in two ways.
Using Collections.synchronizedList() method
Using CopyOnWriteArrayList<T>
Q4.When to use ArrayList and LinkedList?
LinkedLists are better to use for the update operations whereas ArrayLists are better to use for the search operations.
Q5.How to make Java ArrayList Read-Only?
We can obtain java ArrayList Read-only by calling the Collections.unmodifiableCollection() method. When we define an ArrayList as Read-only then we cannot perform any modification in the collection through  add(), remove() or set() method.
Q6.How to remove duplicates from ArrayList?
There are two ways to remove duplicates from the ArrayList.
Using HashSet: By using HashSet we can remove the duplicate element from the ArrayList, but it will not then preserve the insertion order.
Using LinkedHashSet: We can also maintain the insertion order by using LinkedHashSet instead of HashSet.
The Process to remove duplicate elements from ArrayList using the LinkedHashSet:
Copy all the elements of ArrayList to LinkedHashSet.
Empty the ArrayList using clear() method, which will remove all the elements from the list.
Now copy all the elements of LinkedHashset to ArrayList.
Q7.What is the advantage of the generic collection?
There are three main advantages of using the generic collection.
If we use the generic class, we don’t need typecasting.
It is type-safe and checked at compile time.
Generic confirms the stability of the code by making it bug detectable at compile time.
Q8.What is hash-collision in Hashtable and how it is handled in Java?
Two different keys with the same hash value are known as hash-collision. Two separate entries will be kept in a single hash bucket to avoid the collision. There are two ways to avoid hash-collision.
Separate Chaining
Open Addressing
Q9.What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
Q10.What is the default size of load factor in hashing based collection?
The default size of load factor is 0.75. The default capacity is computed as initial capacity * load factor. For example, 16 * 0.75 = 12. So, 12 is the default capacity of Map.
Q11.What do you understand by fail-fast?
The Iterator in java which immediately throws ConcurrentmodificationException, if any structural modification occurs in, is called as a Fail-fast iterator. Fail-fats iterator does not require any extra space in memory.
Q12.What does the hashCode() method?
The hashCode() method returns a hash code value (an integer number).
The hashCode() method returns the same integer number if two keys (by calling equals() method) are identical.
However, it is possible that two hash code numbers can have different or the same keys.
If two objects do not produce an equal result by using the equals() method, then the hashcode() method will provide the different integer result for both the objects.
Q13.Why we override equals() method?
The equals method is used to check whether two objects are the same or not. It needs to be overridden if we want to check the objects based on the property.
For example, Employee is a class that has 3 data members: id, name, and salary. However, we want to check the equality of employee object by the salary. Then, we need to override the equals() method.
Q14.What is the difference between HashSet and TreeSet?
The HashSet and TreeSet, both classes, implement Set interface. The differences between the both are listed below.
HashSet maintains no order whereas TreeSet maintains ascending order.
HashSet impended by hash table whereas TreeSet implemented by a Tree structure.
HashSet performs faster than TreeSet.
HashSet is backed by HashMap whereas TreeSet is backed by TreeMap.
Q15.What is the difference between Set and Map?
The differences between the Set and Map are given below.
Set contains values only whereas Map contains key and values both.
Set contains unique values whereas Map can contain unique Keys with duplicate values.
Set holds a single number of null value whereas Map can include a single null key with n number of null values.
Q16.What is the difference between HashSet and HashMap?
The differences between the HashSet and HashMap are listed below.
HashSet contains only values whereas HashMap includes the entry (key, value). HashSet can be iterated, but HashMap needs to convert into Set to be iterated.
HashSet implements Set interface whereas HashMap implements the Map interface
HashSet cannot have any duplicate value whereas HashMap can contain duplicate values with unique keys.
HashSet contains the only single number of null value whereas HashMap can hold a single null key with n number of null values.
Q17.What is the difference between HashMap and TreeMap?
The differences between the HashMap and TreeMap are given below.
HashMap maintains no order, but TreeMap maintains ascending order.
HashMap is implemented by hash table whereas TreeMap is implemented by a Tree structure.
HashMap can be sorted by Key or value whereas TreeMap can be sorted by Key.
HashMap may contain a null key with multiple null values whereas TreeMap cannot hold a null key but can have multiple null values.

Java Collections Interview Questions For Selenium Testers

Q1.What is Set in Java Collections framework and list down its various implementations?
A Set refers to a collection that cannot contain duplicate elements. It is mainly used to model the mathematical set abstraction. The Java platform provides three general-purpose Set implementations which are:
Q2.What is the HashSet class in Java and how does it store elements?
java.util.HashSet class is a member of the Java collections framework which inherits the AbstractSet class and implements the Set interface. It implicitly implements a hashtable for creating and storing a collection of unique elements. Hashtable is an instance of the HashMap class that uses a hashing mechanism for storing the information within a HashSet. Hashing is the process of converting the informational content into a unique value that is more popularly known as hash code. This hashcode is then used for indexing the data associated with the key. The entire process of transforming the informational key into the hashcode is performed internally.
Q3.Can you add a null element into a TreeSet or HashSet?
In HashSet, only one null element can be added but in TreeSet it can’t be added as it makes use of NavigableMap for storing the elements. This is because the NavigableMap is a subtype of SortedMap that doesn’t allow null keys. So, in case you try to add null elements to a TreeSet, it will throw a NullPointerException.
Explain the emptySet() method in the Collections framework?
The Collections.emptySet() is used to return the empty immutable Set while removing the null elements. The set returned by this method is serializable. Below is the method declaration of emptySet().
public static final <T> Set<T> emptySet()
Q4.What is LinkedHashSet in Java Collections Framework?
A java.util.LinkedHashSet is a subclass of the HashSet class and implements the Set interface. It is an ordered version of HashSet which maintains a doubly-linked List across all elements contained within. It preserves the insertion order and contains only unique elements like its parent class.
LinkedHashSet<String> hs = new LinkedHashSet<String>();
Q5.What is Map interface in Java?
The java.util.Map interface in Java stores the elements in the form of keys-values pairs which is designed for faster lookups. Here every key is unique and maps to a single value. These key-value pairs are known as the map entries. This interface includes method signatures for insertion, removal, and retrieval of elements based on a key. With such methods, it’s a perfect tool to use for key-value association mapping such as dictionaries.
Q6.Why Map doesn’t extend the Collection Interface?
The Map interface in Java follows a key/value pair structure whereas the Collection interface is a collection of objects which are stored in a structured manner with a specified access mechanism. The main reason Map doesn’t extend the Collection interface is that the add(E e) method of the Collection interface doesn’t support the key-value pair like Map interface’s put(K, V) method. It might not extend the Collection interface but still is an integral part of the Java Collections framework.
Q7.List down the different Collection views provided by the Map interface in the Java Collection framework?
The Map interface provides 3 views of key-value pairs which are:
key set view
value set view
entry set view
All these views can be easily navigated through using the iterators.
Q8.What is the ConcurrentHashMap in Java and do you implement it?
ConcurrentHashMap is a Java class that implements ConcurrentMap as well as Serializable interfaces. This class is the enhanced version of HashMap as it doesn’t perform well in the multithreaded environment. It has a higher performance rate compared to the HashMap.
Q9.Differences between comparable and comparator?
Comparable Interface is actually from java.lang package.
It will have a method compareTo(Object obj)to sort objects
Comparator Interface is actually from java.util package.
It will have a method compare(Object obj1, Object obj2)to sort objects
Q10.How can we sort a list of Objects?
To sort the array of objects we will use  Arrays.sort() method.
If we need to sort collection of object we will use Collections.sort().
Q11.What is difference between fail-fast and fail-safe?
Fail fast is nothing but immediately report any failure. whenever a problem occurs fail fast system fails.
in java Fail fast iterator while iterating through collection of objects sometimes concurrent
modification exception will come there are two reasons for this.
If one thread is iterating a collection and another thread trying to modify the collection.
And after remove() method call if we try to modify collection object
Q12.What is difference between Iterator ,ListIterator and Enumeration?
Enumeration interface implemented in java 1.2 version.So Enumeration is legacy interface.
Enumeration uses elements() method.
Iterator is implemented on all Java collection classes.
Iterator uses iterator() method.
Iterator can traverse in forward direction only.
ListIterator is implemented only for List type classes
ListIterator uses listIterator() method.
Read more :
What is difference between Iterator ,ListIterator and Enumeration?
Q13.What is difference between Set and List in Java?
A set is a collection that allows unique elements.
Set does not allow duplicate elements
Set allows only one null value.
Set having classes like :
List having index. and ordered  collection
List allows n number of null values.
List will display Insertion order with index.
List having classes like :


So these are some of the best and some of the most asked Java Collection interview questions that you need to know about. We have picked these questions with the assistance of some experts and professionals who knows a thing or two about Java Collection interviews and they want to help candidates who would be taking the interview.
You should keep in mind that these won’t be the only questions that would be asked in the interview, but you would certainly feel confident if you know the answer to some mostly asked Java Collection interview questions.
That would be all for now and we hope that you found everything you have been looking for. We hope we were helpful and best of luck with your upcoming interview.