Last Updated on November 20th, 2024

Definitely, Collections are the most important for a Java developer. You can’t imagine even a minor project in Java without the use of Collections. Moreover, in order to work in an industry level project, you must have a good grasp on Collections in Java. Apart from going through theoretical concepts multiple times, it is highly advisable that developers need much practice & revision of concepts. One of the ways to achieve this is by attempting FAQs/MCQs on collections.

In this article, we will be doing theoretical practice on ‘Java Collections’ in the form of MCQs that are frequently asked in the interviews as well. Definitely, you must find this article ‘Collection MCQ in Java with Answers Explained’ 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) Beginner to intermediate 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.

Please note that the questions in this article should be beneficial for beginner to intermediate level developers. If you are looking for questions to experienced level, kindly visit MCQs on Collection In Java.

Collection MCQ in Java with Answers Explained

Q#1. Which among the following has elements in insertion order?

A) HashMap
B) TreeMap
C) SortedMap
D) LinkedHashMap

Answer: D

Explanation: LinkedHashMap maintains the order of elements based on their insertion order. When iterating through a LinkedHashMap, the elements are returned in the order they were inserted. HashMap does not maintain any order, TreeMap maintains elements sorted according to their natural ordering or a specified comparator, and SortedMap is an interface that TreeMap implements to provide a sorted map, not necessarily in insertion order.

Q#2. A List in Java accesses any element from its ___?

A) key
B) index
C) value
D) position

Answer: B

Explanation: A List in Java accesses any element from its index. Lists are ordered collections that allow for precise control over where each element is inserted. They can be accessed by their integer index, which starts from 0 for the first element and increases consecutively.

Q#3. Which among the following allow duplicate elements?

A) ArrayList
B) HashSet
C) LinkedHashSet
D) TreeSet

Answer: A

Explanation: ArrayList allows duplicate elements. It is an ordered collection (also known as a sequence) and permits all elements, including duplicates. In contrast, Since HashSet, LinkedHashSet, and TreeSet are implementations of the Set interface, they do not allow duplicate elements.

Q#4. Which class in the collection framework has its implementation based on a balanced tree data structure?

A) HashMap
B) LinkedList
C) TreeMap
D) ArrayList

Answer: C

Explanation: TreeMap has its implementation based on a balanced tree data structure. Specifically, it uses a Red-Black tree to store its key-value pairs, which ensures that the map is sorted according to the natural ordering of its keys or by a specified comparator. This provides log(n) time complexity for basic operations such as get, put, and remove.

Q#5. Which one of these classes provides the features of maintaining insertion order and allowing null elements?

A) HashSet
B) LinkedHashSet
C) TreeSet
D) PriorityQueue

Answer: B

Explanation: LinkedHashSet provides the features of maintaining insertion order and allowing null elements. It combines the unique element property of a HashSet with the insertion order property of a LinkedList. HashSet does not maintain insertion order, TreeSet does not allow null elements and sorts elements according to their natural order, and PriorityQueue orders elements based on their priority and does not guarantee insertion order.

Q#6. Which one of the following statements is NOT true about the Collection interface?

A) The Collection interface is the root interface of the Java Collections Framework.
B) The Collection interface extends the Iterable interface.
C) The Collection interface includes methods for accessing elements by index.
D) The Collection interface can be used to represent groups of objects.

Answer: C

Explanation: The Collection interface does not include methods for accessing elements by index. Methods for accessing elements by index are provided by the List interface, which is a subinterface of Collection. The Collection interface is the root interface of the Java Collections Framework, it extends the Iterable interface, and it can be used to represent groups of objects.

Q#7. Which of these interfaces must contain unique elements?

A) List
B) Queue
C) Set
D) Deque

Answer: C

Explanation: The Set interface must contain unique elements. This means no duplicates are allowed in a Set. In contrast, the List interface allows duplicates, the Queue interface does not enforce uniqueness, and the Deque (double-ended queue) also does not require elements to be unique.

Q#8. You need to store elements in a collection that guarantees no duplicates. Which collection should you use?

A) ArrayList
B) HashSet
C) LinkedList
D) PriorityQueue

Answer: B

Explanation: HashSet guarantees that no duplicates will be stored in the collection. It is an implementation of the Set interface, which enforces uniqueness of its elements. ArrayList and LinkedList allow duplicates, and PriorityQueue does not enforce uniqueness of its elements.

Q#9. Which collection type preserves the insertion order of key-value pairs?

A) HashMap
B) TreeMap
C) LinkedHashMap
D) SortedMap

Answer: C

Explanation: LinkedHashMap preserves the insertion order of key-value pairs. This means that when you iterate over the entries in a LinkedHashMap, they will appear in the order they were inserted. HashMap does not guarantee any specific order, TreeMap sorts the entries based on their keys, and SortedMap is an interface for maps that maintain a sorted order, typically implemented by TreeMap.

Q#10. Which of these is not an interface in the Collections Framework?

A) List
B) Set
C) ArrayList
D) None of the above

Answer: C

Explanation: ArrayList is not an interface in the Collections Framework; it is a class. List, and Setare interfaces in the Collections Framework. List is used for ordered collections, Set is used for collections that do not allow duplicates.

Q#11. Which of these interfaces declares core methods that all collections will have?

A) List
B) Set
C) Collection
D) Map

Answer: C

Explanation: The Collection interface declares core methods that all collections will have. It is the root interface in the Java Collections Framework hierarchy. Interfaces like List, Set, and Queue extend the Collection interface and add specific behaviors. Map, on the other hand, is not a subtype of Collection and defines a different type of data structure for storing key-value pairs.

Q#12. Which one of the methods below is not defined in the Collection interface?

A) add
B) remove
C) get
D) size

Answer: C

Explanation: The method get is not defined in the Collection interface. The Collection interface provides methods like add, remove, and size for adding elements, removing elements, and getting the size of the collection, respectively. However, accessing elements by index (like get(int index)) is specific to the List interface, which extends Collection.

Q#13. Which of the below does not implement the Map interface?

A) HashMap
B) TreeMap
C) LinkedHashMap
D) ArrayList

Answer: D

Explanation: ArrayList does not implement the Map interface. It is a class in Java that implements the List interface, which is used for ordered collections and allows duplicate elements. HashMap, TreeMap, and LinkedHashMap are all classes that implement the Map interface, which is used for storing key-value pairs and provides methods for accessing, inserting, and removing elements based on keys.

Q#14. Which of the following statements are true about ArrayList and Vector in Java?

A) Both ArrayList and Vector are thread-safe and implement the RandomAccess interface.
B) Both ArrayList and Vector have the same initial capacity and resize strategy.
C) ArrayList is non-synchronized and Vector is synchronized, making Vector thread-safe.
D) There is no significant difference between ArrayList and Vector; both offer the same functionalities.

Answer: C

Explanation: Both ArrayList and Vector are dynamic arrays that can store and access elements by index. However, they differ in thread-safety and resizing behavior:

• Thread-safety:

  • ArrayList: Not synchronized, meaning it’s not thread-safe for concurrent access from multiple threads.
  • Vector: Synchronized, making it thread-safe but potentially slower for single-threaded operations due to synchronization overhead.

• Resizing:

  • ArrayList: When capacity is reached, it increases its size by 50% of the current size.
  • Vector: When capacity is reached, it doubles its size.

Q#15. Which of the following methods helps insert elements at a specific position in a collection within the Java Collection Framework?

A) add(element)
B) put(key, value)
C) addAll(elements)
D) get(index)

Answer: A

Explanation:

• add(element) is a general method available in most collection classes (like ArrayList, LinkedList, etc.) that allows inserting an element at the end of the collection. However, for targeted insertion at a specific position:
  • ArrayList and LinkedList provide the add(int index, element) method, where index specifies the desired insertion position.
• put(key, value) is used with Map implementations (like HashMap, TreeMap) to insert key-value pairs.
• addAll(elements) adds all elements from another collection to the current collection, not at a specific position.
• get(index) retrieves an element at a specific position but doesn’t modify the collection by inserting elements.

Q#16. Which of the following methods is a new addition for Sets introduced in Java 9?

A) add(element)
B) contains(element)
C) isEmpty()
D) of(elements...)

Answer: D

Explanation:

• add(element) has existed in Sets since earlier Java versions and allows adding elements.
• contains(element) has also been available previously to check if an element exists in the Set.
• isEmpty() is another existing method to determine if the Set is empty.
• of(elements…) is a static factory method introduced in Java 9 for creating immutable Set objects. It allows concise initialization with a variable number of elements.

Q#17. Which of the following is an interface in the Java Collection Framework?

A) ArrayList
B) Collections
C) HashMap
D) Collection

Answer: D

Explanation:

• ArrayList, and HashMap are all concrete classes that implement specific interfaces within the Collection Framework.
• The Collections is a helper class that provides utility methods for working with collections (such as lists, sets, and maps).
• Collection is a fundamental interface in the Java Collection Framework that defines core operations for collections, such as adding, removing, and checking element existence. Many collection classes (like ArrayList, HashSet, etc.) inherit functionalities from the Collection interface.

Q#18. What is the relationship between HashSet and HashMap in the Java Collection Framework?

A) HashSet is a subclass of HashMap, and both store unique elements.
B) HashMap is a subclass of HashSet, and both store key-value pairs.
C) HashSet and HashMap are unrelated; HashSet stores unique elements, while HashMap stores key-value pairs.
D) Both HashSet and HashMap use the same internal mechanism for storing elements but differ in access methods.

Answer: C

Explanation:

• HashSet and HashMap are distinct classes that utilize hashing for efficient element storage and retrieval.
• HashSet implements the Set interface and stores a collection of unique elements. It doesn’t use key-value pairs.
• HashMap implements the Map interface and stores key-value pairs. Each key must be unique, and it allows efficient retrieval of values based on the key.
• While both leverage hashing, they cater to different data structures: HashSet for sets of unique elements and HashMap for key-value associations.

Q#19. Which of these classes provide an implementation of the Map interface?

A) ArrayList
B) TreeSet
C) HashMap
D) LinkedList

Answer: C

Explanation: HashMap provides an implementation of the Map interface. The Map interface is used for storing key-value pairs, and HashMap is one of its primary implementations. ArrayList and LinkedList implement the List interface, and TreeSet implements the Set interface.

Q#20. What is the difference between List and ArrayList in the Java Collection Framework?

A) List is a concrete class, while ArrayList is an interface.
B) ArrayList offers additional methods beyond the basic functionalities provided by the List interface.
C) List allows duplicate elements, while ArrayList enforces uniqueness.
D) There's no significant difference; both List and ArrayList serve the same purpose.

Answer: B

Explanation:

• List is an interface that defines core operations for ordered collections of elements, including adding, removing, accessing elements by index, and checking size. It allows for different implementations that can manage elements in various ways.
• ArrayList is a concrete class that implements the List interface. It provides a dynamic array-based implementation, offering functionalities outlined in the List interface and additional methods specific to ArrayList. These might include functionalities for managing the underlying array size, ensuring efficient resizing when capacity is reached.
• List defines the “what” (core functionalities for ordered collections).
• ArrayList defines the “how” (a specific implementation using dynamic arrays).

Q#21. Which of the following is true about LinkedHashSet in Java?

A) It does not allow null elements.
B) It maintains the insertion order of elements.
C) It allows duplicate elements.
D) It is synchronized by default.

Answer: B

Explanation: LinkedHashSet maintains the insertion order of elements. This means that when iterating through the LinkedHashSet, the elements will be returned in the order in which they were inserted. LinkedHashSet allows null elements and does not allow duplicate elements, similar to HashSet. It is also not synchronized by default, meaning it is not thread-safe without external synchronization.

Q#22. What are the difference between HashSet and TreeSet in the Java Collection Framework?

A) Both HashSet and TreeSet allow duplicate elements and maintain insertion order.
B) Both HashSet and TreeSet use hashing for element storage, offering constant time average retrieval.
C) HashSet stores unique elements without a specific order, while TreeSet maintains elements in a sorted order.
D) HashSet is synchronized for thread-safe access, while TreeSet is not.

Answer: C

Explanation:

• HashSet:
  • Implements the Set interface.
  • Stores a collection of unique elements, ensuring no duplicates exist.
  • Employs hashing for efficient storage and retrieval (average constant time).
  • Doesn’t maintain any specific order for elements (insertion order is not preserved).
• TreeSet:
  • Implements the SortedSet interface (a sub-interface of Set).
  • Stores a collection of unique elements, similar to HashSet.
  • Maintains elements in a naturally sorted order (ascending order by default).
  • Uses a tree-based data structure for efficient retrieval and ordered iteration. Sorting overhead might impact performance compared to HashSet for some operations.

In addition,

• HashSet: Focuses on efficient storage and retrieval of unique elements without a specific order.
• TreeSet: Prioritizes maintaining elements in a sorted order while ensuring uniqueness.

Q#23. What is the difference between ArrayList and LinkedList classes in the Collection Framework?

A) ArrayList uses a doubly linked list to store elements, while LinkedList uses a dynamic array.
B) ArrayList is synchronized, while LinkedList is not synchronized.
C) ArrayList provides constant-time positional access, while LinkedList provides linear-time positional access.
D) ArrayList allows duplicate elements, while LinkedList does not allow duplicate elements.

Answer: C

Explanation: The main difference between ArrayList and LinkedList is in their internal implementations and performance characteristics.

• ArrayList uses a dynamic array to store elements, which provides constant-time positional access (i.e., O(1) time complexity) because it allows direct access to any element using its index. This makes ArrayList more suitable for scenarios where frequent access to elements by index is required.
• LinkedList uses a doubly linked list to store elements, which provides linear-time positional access (i.e., O(n) time complexity) because it requires traversing the list from the beginning to reach a specific element by index. However, LinkedList is more efficient than ArrayList when it comes to insertions and deletions in the middle of the list, as these operations only require updating references rather than shifting elements.
• ArrayList and LinkedList both allow duplicate elements and are not synchronized by default.

Q#24. Which of the following concepts make extensive use of arrays?

A) Recursion
B) Linked Lists
C) Binary Trees
D) Sorting Algorithms

Answer: D

Explanation: Sorting algorithms make extensive use of arrays. Many sorting algorithms, such as QuickSort, MergeSort, and Bubble Sort, are designed to sort elements stored in arrays efficiently. Arrays provide a convenient structure for accessing and manipulating elements in a contiguous block of memory, which is essential for the operations performed by sorting algorithms. While recursion, linked lists, and binary trees can also use arrays, they do not rely on arrays as extensively as sorting algorithms do.

Q#25. Entries in a stack are ‘ordered’. What is the meaning of this statement?

A) A collection of stacks is sorted.
B) Stack entries are stored in a FIFO (First In, First Out) manner.
C) Stack entries are stored in a LIFO (Last In, First Out) manner.
D) A stack keeps track of the number of entries it contains.

Answer: C

Explanation: The statement “entries in a stack are ‘ordered'” means that stack entries are stored in a LIFO (Last In, First Out) manner. This means that the most recently added element is the first one to be removed. This ordering mechanism is fundamental to the stack data structure, where the push operation adds an element to the top of the stack and the pop operation removes the element from the top of the stack. Options A, B, and D do not correctly describe the ordering mechanism specific to stacks.

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