Last Updated on March 20th, 2024

Being a Java developer, you must be familiar with the importance of Collections in Java. Undoubtedly, Collections are the most important for you. You can’t imagine even a small project in Java without the use of Collections. Moreover, in order to work in an industry level project, you must have a good knowledge on Collections in Java. Apart from going through theoretical concepts multiple times, it is highly advisable that developers need much coding practice on collections.

In this article, we will be doing theoretical & code practice on ‘Collections in Java’ in the form of MCQs that are frequently asked in the interviews as well. Definitely, you must find this article beneficial whether it is an interview or a written test from these MCQ on Collections in Java.

You may also go through the article Collection In Java to refresh your concepts.

Who can get benefitted from these questions?

1) Job seekers who are going to appear in a Java interview or written/online test.

2) Students and learners who want to practice their concepts and test the skills.

3) Experienced developers who want to revise their concepts before appearing in any interview.

4) All who want to appear in any online test or in an interview.

In summary, MCQ on Collections In Java can benefit anyone who wants to improve their understanding of Java Collection concepts, whether they are job seekers, students, experienced developers, or any others.

MCQ on Collections In Java

Q#1. Which of the following statement(s) is/are correct about the List collection?

(a) Just like an array, List indexes start from 0.
(b) List interface has got many default methods in Java 8.
(c) List allows you to have 'null' elements.
(d) List doesn't support Generics.

Answer: (a), (b), (c)

Explanation: The List interface indexes start from 0, allows null elements & also supports Generics. It allows duplicate elements and maintains an ordered collection. Furthermore, List interface has got many default methods in Java 8.

Q#2. What is the correct way to create an ArrayList of Strings?

(a) ArrayList<String> list = new ArrayList[];
(b) ArrayList list = new List<String>();
(c) ArrayList list = new ArrayList<String>();
(d) ArrayList<String> list = new ArrayList<>();

Answer: (d) ArrayList<String> list = new ArrayList< >();

Explanation: While creating an ArrayList of a specific type, the variable declaration should include the type.

Q#3. Which collection classes allow you to retrieve elements based on the insertion order?

(a) ArrayList
(b) LinkedList
(c) LinkedHashMap
(d) LinkedHashSet

Answer: (a), (b), (c), (d)

Explanation: The LinkedList & ArrayList classes implement the List interface and allows us to retrieve elements based on the insertion order. LinkedHashMap is a map implementation that maintains the insertion order of key-value pairs. Similarly, the LinkedHashSet is a set implementation that maintains the insertion order of elements.

Q#4. What is the output of the following code?

ArrayList<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
System.out.println(list.remove(0));

(a) Java
(b) Python
(c) true
(d) Compilation error

Answer: a) Java

Explanation: The remove() method is used to remove an element at the specified index in an ArrayList. In this case, it removes “Java” at index 0 and returns it.

Q#5. Which of the following is incorrect statement about the Hastable class?

(a) Hashtable class implements Map interface.
(b) Hashtable class extends the Dictionary class.
(c) Hashtable class implements Cloneable interface.
(d) Hashtable class extends Properties class.

Answer: d) Hashtable class extends Properties class.

Explanation: Hashtable class doesn’t extend Properties class. Alternatively, the Properties class extends Hashtable class.

Q#6. Which of the following collection class/(s) is/are synchronized and thread-safe?

(a) Stack
(b) Hashtable
(c) Properties
(d) Vector

Answer: (a), (b), (c), (d)

Explanation: All of the above classes are synchronized and thread safe in java.

Q#7. Which one is the correct way to iterate over elements in an ArrayList?

(a) for (int i = 0; i < list.size(); i++)
(b) for (int i : list)
(c) for (Object obj : list)
(d) for (Iterator it = list.iterator(); it.hasNext();)

Answer: c) for (Object obj : list)

Explanation: The correct way to iterate over elements in an ArrayList is by using a for-each loop and declaring the loop variable as the type of the elements in the list.

Q#8. Which of the following statement is incorrect about Deque interface in Java?

(a) It is typically pronounced as "deck".
(b) It stands for doubly-ended queue.
(c) It is mostly used to implement some algorithms and data structures.
(d) It can either be used as a queue(FIFO) or as a stack(LIFO).

Answer: b) It stands for doubly-ended queue.

Explanation: The Deque interface extends the Collection interface and adds support for inserting elements at both the ends. It stands for “double-ended queue”, not the doubly-ended queue. All other statements are correct about Deque. You may also read When to use Deque in some more detail.

Q#9. Suppose you are working in a Java project. You have a requirement where you need a dynamic or resizable array-like data structure that can permit you to add, remove, and access elements at a specific index efficiently. It should also automatically adjust its size when the elements are added or removed. Which of the following class will you use to satisfy your requirement?

(a) Vector
(b) LinkedList
(c) ArrayList
(d) HashMap

Answer: c) ArrayList

Explanation: The ArrayList class provides a resizable array implementation of the List interface. It dynamically grows and shrinks as elements are added or removed.

Q#10. Which collection class provides a way to store elements in key-value pairs with no duplicate keys?

(a) ArrayList
(b) Hashtable 
(c) TreeMap
(d) LinkedHashMap

Answer: c) TreeMap

Explanation: The TreeMap class provides a way to store elements in key-value pairs with no duplicate keys. It orders the keys in ascending order by default.

Q#11. What is the output of the following code?

LinkedList<String> list = new LinkedList<>();
list.add("Java");
list.add("Python");
System.out.println(list.poll());

(a) Java
(b) Python
(c) true
(d) Compilation error

Answer: a) Java
Explanation: The poll() method of LinkedList retrieves and removes the first element in a LinkedList. In this case, it retrieves and removes ‘Java’.

Q#12. Which one of the following collection classes can be used to store unique & sorted objects?

(a) HashSet
(b) TreeSet
(c) LinkedHashMap
(d) ArrayList

Answer: b) TreeSet

Explanation: The TreeSet class provides a way to store unique elements in a sorted order. It can order the elements using ascending order. TreeSet does not preserve the insertion order of elements but elements are sorted by keys.

Q#13. Apart from providing methods for adding, removing, and examining elements in a specific order, which of the following interfaces provides methods like peek() and poll()?

(a) Set
(b) List
(c) Queue
(d) Map

Answer: c) Queue

Explanation: The Queue interface provides methods for adding, removing, and examining elements in a specific order, following the FIFO (First-In-First-Out) principle. It also includes additional queue-specific operations like peek() and poll().

Q#14. Which method is used to add an element to the end of an ArrayList?

(a) addLast()
(b) addEnd()
(c) add()
(d) append()

Answer: c) add()

Explanation: The add() method itself is used to add an element to the end of an ArrayList.

Q#15. Which collection class allows duplicate elements and maintains their insertion order?

(a) HashSet
(b) TreeSet
(c) LinkedHashSet
(d) HashMap

Answer: c) LinkedHashSet

Explanation: The LinkedHashSet class allows duplicate elements and maintains their insertion order. It combines the features of a HashSet and a LinkedList.

Q#16. Which method is used to check if a specific element is present in a Set?

(a) containsElement()
(b) containsKey()
(c) containsValue()
(d) contains() 

Answer: d) contains()

Explanation: The contains() method is used to check if a specific element is present in a Set. If the element is found, It returns true, otherwise false.

Q#17. Which collection class stores objects in the form of key-value pairs and maintains the order in which they were inserted?

(a) Hashtable
(b) LinkedHashMap
(c) TreeMap
(d) HashSet

Answer: b) LinkedHashMap

Explanation: The LinkedHashMap class provides a way to store key-value pairs. It maintains the insertion order of the elements.

Q#18. Which collection class is synchronized and thread-safe?

(a) ArrayList
(b) HashSet
(c) ConcurrentHashMap
(d) LinkedList

Answer: c) ConcurrentHashMap

Explanation: The ConcurrentHashMap class is synchronized and thread-safe, allowing multiple threads to access it simultaneously without data discrepancies.

Q#19. Which collection class in Java provides a fixed-size collection that does not allow adding or removing elements?

(a) ArrayList
(b) LinkedList 
(c) HashSet
(d) Arrays

Answer: d) Arrays Explanation: The Arrays class in Java provides utility methods for working with fixed-size arrays, which do not allow adding or removing elements once initialized.

Q#20. Which method is used to retrieve the first element from a Queue without removing it?

(a) peek()
(b) getFirst()
(c) element()
(d) front()

Answer: a) peek()

Explanation: The peek() method is used to retrieve the first element from a Queue without removing it. If the Queue is empty, it returns null.

Q#21. Which interface should be implemented to create a custom Comparator for sorting objects in a TreeSet?

(a) Comparable
(b) Sortable
(c) Comparator
(d) ComparatorSet

Answer: c) Comparator

Explanation: To create a custom Comparator for sorting objects in a TreeSet, the Comparator interface should be implemented.

Q#22. What is the output of the following code?

LinkedHashSet<Integer> set = new LinkedHashSet<>();
set.add(10);
set.add(20);
set.add(30);
System.out.println(set);

(a) 10 20 30 
(b) [30, 20, 10]
(c) [10, 20, 30]
(d) Compilation error

Answer: c) [10, 20, 30]

Explanation: The LinkedHashSet maintains the insertion order of elements. It will output the elements in the order they were added.

Q#23. What is the output of the following code?

ArrayList<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
System.out.println(list.contains("Python"));

(a) true
(b) false
(c) Python
(d) Compilation error

Answer: a) true

Explanation: The contains() method is used to check if a specific element exists in the ArrayList. In this case, it returns true as “Python” is present.

Q#24. Which method is used to obtain a synchronized (thread-safe) version of a List collection?

(a) synchronized()
(b) synchronizedList()
(c) synchronizedCollection()
(d) synchronize()

Answer: b) synchronizedList()

Explanation: The synchronizedList() method is used to obtain a synchronized (thread-safe) version of a List collection, such as ArrayList.

Q#25. What is the output of the following code?

TreeSet<Integer> set = new TreeSet<>();
set.add(24);
set.add(5);
set.add(50);
System.out.println(set.first());

(a) 10
(b) 5
(c) 20
(d) Compilation error

Answer: b) 5

Explanation: The first() method of the TreeSet is used to retrieve the first (lowest) element. In this case, it retrieves the smallest element, which is 5.

Q#26. Which method is used to retrieve the last element without removing it from a LinkedList?

(a) getLast()
(b) lastElement()
(c) getLastElement()
(d) peekLast()

Answer: a) getLast()

Explanation: The getLast() method is used to retrieve the last element from a LinkedList. It returns the last element without removing it from the list.

Q#27. What is the output of the following code snippet?

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("Scala");
System.out.println(list.subList(1, 3));

(a) [Java, Python]
(b) [Python, Scala]
(c) [Python]
(d) [Scala]

Answer: b) [Python, Scala]

Explanation: The subList(1, 3) method returns a fraction of the list from index 1 (inclusive) to index 3 (exclusive), so the output is [Python, Scala].

Q#28. What is the output of the following code?

LinkedHashSet<String> set = new LinkedHashSet<>();
set.add("Java");
set.add("Python");
System.out.println(set.remove("Scala"));

(a) true
(b) false
(c) Java
(d) Compilation error

Answer: b) false

Explanation: The remove() method is used to remove a specific element from a LinkedHashSet. In this case, “Scala” is not present, so it returns false.

Q#29. Which method is used to retrieve the key associated with the maximum value in a TreeMap?

(a) getMaxKey()
(b) firstKey()
(c) lastKey()
(d) getMaxValue()

Answer: c) lastKey()

Explanation: The lastKey() method is used to retrieve the key associated with the largest value in a TreeMap. Since the TreeMap is ordered by keys, the largest key is the last key.

Q#30. What is the output of the following code?

HashSet<Integer> set1 = new HashSet<>();
set1.add(1);
set1.add(2);
set1.add(3);

HashSet<Integer> set2 = new HashSet<>();
set2.add(2);
set2.add(3);
set1.add(4);

set1.retainAll(set2);
System.out.println(set1);

(a) [1, 4]
(b) [2, 3]
(c) [1, 2, 3]
(d) [1, 2, 3, 4]

Answer: b) [2,3]

Explanation: The retainAll() method is used to retain only the elements that exist in both sets. In this case, the intersection of set1 and set2 is [2,3].

Q#31. What is the output of the following code?

HashMap<String, Integer> map = new HashMap<>();
map.put("Java", 10);
map.put("Python", 20);
System.out.println(map.keySet());

(a) ["Java", "Python"]
(b) [10, 20]
(c) "Java", "Python"
(d) [Java, Python]

Answer: d) [Java, Python]

Explanation: The keySet() method is used to retrieve a set of all the keys in a HashMap. In this case, it returns [Java, Python].

Q#32. Which method is used to check if a LinkedList is empty?

(a) isEmpty()
(b) checkEmpty()
(c) hasElements()
(d) hasData()

Answer: a) isEmpty()

Explanation: The isEmpty() method is used to check if a LinkedList is empty. It returns true if the LinkedList contains no elements.

Q#33. What is the output of the following code?

LinkedList<String> list = new LinkedList<>();
list.add("Java");
list.add("Python");
list.add("Scala");
System.out.println(list.removeLast());

(a) Java
(b) Python
(c) Scala
(d) Compilation error

Answer: c) Scala
Explanation: The removeLast() method is used to retrieve and remove the last element in a LinkedList. In this case, it retrieves and removes “Scala”.

Q#34. What happens when an element is added to a CopyOnWriteArrayList while multiple threads are iterating over it?

(a) The newly added element is immediately visible to all iterators.
(b) Iterators throw a ConcurrentModificationException.
(c) Iterators continue without considering the newly added element.
(d) The iteration blocks until the addition is complete.

Answer: c) Iterators continue without considering the newly added element.

Explanation: CopyOnWriteArrayList iterators operate on a copy of the underlying array, so they do not see modifications made after the iterator was created.

Q#35. What is the main difference between HashSet and TreeSet in Java?

(a) HashSet allows duplicate elements, while TreeSet does not.
(b) HashSet maintains insertion order, while TreeSet does not.
(c) HashSet is unsorted, while TreeSet is sorted.
(d) TreeSet allows null values, while HashSet does not.

Answer: c) HashSet is unsorted, while TreeSet is sorted.

Explanation: HashSet does not maintain any specific order, while TreeSet sorts elements in a specific order.

Q#36. Which collection class in Java does not allow null values?

(a) HashSet
(b) ArrayList
(c) HashMap
(d) TreeSet

Answer: d) TreeSet

Explanation: TreeSet does not allow null values because it uses the natural ordering of elements or a custom comparator, which cannot compare null.

Q#37. What is the difference between ArrayList and LinkedList in terms of inserting an element in the middle of the collection?

(a) ArrayList is faster than LinkedList.
(b) LinkedList is faster than ArrayList.
(c) Both ArrayList and LinkedList have the same performance.
(d) It depends on the size of the collection.

Answer: b) LinkedList is faster than ArrayList.

Explanation: LinkedList performs better than ArrayList when inserting elements in the middle because it does not require shifting elements.

Q#38. Which collection class in Java is best suited for frequent search operations on a large collection?

(a) HashSet
(b) ArrayList
(c) LinkedList
(d) TreeMap

Answer: d) TreeMap

Explanation: TreeMap uses a balanced tree structure, providing efficient search operations (logarithmic time complexity) on large collections.

Q#39. What is the output of the following code snippet?

List<Integer> list = new ArrayList<>();
list.add(4);
list.add(2);
list.add(3);
list.add(2);
System.out.println(list.lastIndexOf(2));

(a) 1
(b) 2
(c) 3
(d) 4

Answer: c) 3

Explanation: The lastIndexOf(2) method returns the index of the last occurrence of the specified element in the list. In this case, the last occurrence of 2 is at index 3, so the output is 3.

Q#40. Which statement(s) is/are true about CopyOnWriteArrayList in Java?

(a) CopyOnWriteArrayList class implements List and RandomAccess interfaces.
(b) CopyOnWriteArrayList is a concurrent replacement for a synchronized List.
(c) It allows duplicate elements and heterogeneous Objects.
(d) It throws ConcurrentModificationException.

Answer: (a), (b), (c)

Explanation: CopyOnWriteArrayList is one of the solution for ConcurrentModificationException. Hence it can’t throw ConcurrentModificationException. All other statements are true for CopyOnWriteArrayList.

---

♥ You may also go through other articles related to Collection In Java such as:

Top 20 Java Collection Interview Questions

Enhancement Of Java Collection Classes

♥ You may also attempt MCQs on other topics in Java by visiting our Quizzes section.

---