Top 50 Java Collections Interview Questions and Answers in 2024

Java Collections is a fundamental concept in the Java programming language, and it’s essential for aspiring Java developers to have a strong understanding of it before appearing for interviews. In this comprehensive article, we’ll explore the top 50 Java Collections interview questions and their respective answers.

Generic Java Collections Interview Questions

1. What are the advantages of the Collection Framework in Java?

The Java Collection Framework offers several advantages, including:

• Performance: It provides highly efficient and effective data structures, enhancing program speed and accuracy.
• Maintainability: Code developed with the collection framework is easy to maintain, supporting data consistency and interoperability.
• Reusability: Classes in the Collection Framework can easily integrate with other types, increasing code reusability.
• Extensibility: The framework allows developers to customize primitive collection types according to their requirements.

1. What do you understand by the Collection Framework in Java?

The Java Collection Framework provides an architecture to store and manage a group of objects. It allows developers to access pre-packaged data structures and algorithms for manipulating data. The collection framework includes interfaces, classes, and algorithms that support various operations such as searching, sorting, insertion, manipulation, and deletion, making data manipulation easy and efficient.

1. Describe the Collection hierarchy in Java.

The Java Collection hierarchy consists of the following:

• Iterable interface: The root interface, containing the iterator() method for iterating over a collection.
• Collection interface: A subinterface of Iterable, representing a group of objects.
• List interface: Extends Collection and represents an ordered collection of elements, allowing duplicates.
• Set interface: Extends Collection and represents a unique collection of elements, not allowing duplicates.
• Queue interface: Extends Collection and follows the First-In-First-Out (FIFO) principle.
• Map interface: A separate data structure that stores key-value pairs.

1. List down the primary interfaces provided by the Java Collections Framework.

The primary interfaces provided by the Java Collections Framework are:

• Collection interface: The root interface of the Java Collection Framework.
• List interface: Represents an ordered collection of elements, allowing duplicates.
• Set interface: Represents a unique collection of elements, not allowing duplicates.
• Queue interface: Follows the First-In-First-Out (FIFO) principle.
• Map interface: Stores data in the form of key-value pairs.

1. Why doesn’t the Collection interface extend the Cloneable and Serializable interfaces?

The Collection interface in Java specifies a group of objects called elements. The maintainability and ordering of elements depend entirely on the concrete implementations provided by each Collection. Thus, there is no need to extend the Cloneable and Serializable interfaces.

1. List down the major advantages of using a Generic Collection.

The main advantages of using a Generic Collection in Java are:

• Provides stronger type checks at compile-time.
• Eliminates the need for typecasting.
• Enables the implementation of generic algorithms, making the code customizable, type-safe, and easier to read.

1. What is the main benefit of using the Properties file?

The main advantage of using the Properties file in Java is that if the values in the Properties file are changed, it will automatically be reflected without having to recompile the Java class. It is mainly used to store information that is liable to change, such as usernames and passwords, making application management easy and efficient.

1. What do you understand by Iterator in the Java Collection Framework?

Iterator in Java is an interface of the Collection Framework present in the java.util package. It is a cursor in Java used to iterate over a collection of objects. The Iterator interface provides the following major functionalities:

• Traverse a collection object’s elements one by one.
• Known as a Universal Java Cursor, as it is applicable for all classes of the Collection Framework.
• Supports READ and REMOVE operations.
• Iterator method names are easy to implement.

1. What is the need for overriding the equals() method in Java?

The initial implementation of the equals() method helps in checking whether two objects are the same or not. However, if you want to compare objects based on their properties, you will have to override this method.

1. How are Collection objects sorted in Java?

Sorting in Java Collections is implemented via the Comparable and Comparator interfaces. When the Collections.sort() method is used, the elements get sorted based on the natural order specified in the compareTo() method. On the other hand, when the Collections.sort(Comparator) method is used, it sorts the objects based on the compare() method of the Comparator interface.

List Java Collections Interview Questions

1. What is the use of the List interface?

The List interface in Java is an ordered collection of elements. It maintains the insertion order and allows duplicate values to be stored. This interface contains various methods that enable smooth manipulation of elements based on their index. The main classes implementing the List interface are ArrayList, LinkedList, Stack, and Vector.

1. What is ArrayList in Java?

ArrayList is the implementation of the List interface, where elements can be dynamically added or removed from the list. ArrayList in the Collection Framework provides positional access and insertion of elements. It is an ordered collection that permits duplicate values. The size of an ArrayList can be increased dynamically if the number of elements is more than the initial size.

ArrayList<String> list = new ArrayList<>();

1. How would you convert an ArrayList to an Array and an Array to an ArrayList?

An Array can be converted into an ArrayList by making use of the asList() method provided by the Arrays class. It is a static method that accepts List objects as a parameter.

String[] array = {"apple", "banana", "orange"};List<String> list = Arrays.asList(array);

Whereas an ArrayList can be converted into an Array using the toArray() method of the ArrayList class.

List<String> list = new ArrayList<>(Arrays.asList("apple", "banana", "orange"));String[] array = list.toArray(new String[list.size()]);

1. How will you reverse a List?

An ArrayList can be reversed using the reverse() method of the Collections class.

List<String> myList = new ArrayList<>(Arrays.asList("AWS", "Java", "Python", "Blockchain"));Collections.reverse(myList);System.out.println(myList); // Output: [Blockchain, Python, Java, AWS]

1. What do you understand by LinkedList in Java? How many types of LinkedList does Java support?

LinkedList in Java is a data structure that contains a sequence of links. Here, each link contains a connection to the next link.

LinkedList<String> list = new LinkedList<>();

Java LinkedList class uses two types of LinkedList to store the elements:

• Singly Linked List: In a singly LinkedList, each node stores the data and a pointer or reference to the next node in the list.
• Doubly Linked List: In a doubly LinkedList, each node has two references, one to the next node and another to the previous node.

1. What is a Vector in Java?

Vector is similar to arrays, where the elements of the Vector object can be accessed via an index. Vector implements a dynamic array and is not limited to a specific size; it can shrink or grow automatically whenever required. It is similar to ArrayList, but with two differences:

• Vector is synchronized.
• Vector contains many legacy methods that are not part of the collections framework.

Vector<String> vector = new Vector<>();

Queue Java Collections Interview Questions

1. What are the various methods provided by the Queue interface?

Below are some of the methods of the Java Queue interface:

• boolean add(Object): Inserts the specified element into the queue and returns true if successful.
• boolean offer(Object): Inserts the specified element into the queue.
• Object remove(): Retrieves and removes the head of the queue.
• Object poll(): Retrieves and removes the head of the queue, or returns null if the queue is empty.
• Object element(): Retrieves, but does not remove, the head of the queue.
• Object peek(): Retrieves, but does not remove, the head of the queue, or returns null if the queue is empty.

1. What do you understand by BlockingQueue?

The BlockingQueue interface belongs to the java.util.concurrent package. This interface enhances flow control by activating blocking if a thread is trying to dequeue an empty queue or enqueue an already full queue. While working with the BlockingQueue interface in Java, you must remember that it does not accept a null value. If you try to do so, it will instantly throw a NullPointerException.

1. What is a priority queue in Java?

A priority queue in Java is an abstract data type similar to a regular queue or stack data structure, but has a special feature called priority associated with each element. In this queue, a high-priority element is served before a low-priority element, irrespective of their insertion order. The PriorityQueue is based on the priority heap. The elements of the priority queue are ordered according to their natural ordering or by a Comparator provided at queue construction time, depending on which constructor is used.

1. What is the Stack class in Java, and what are the various methods provided by it?

The Java Stack class is an important part of the Java Collection Framework and is based on the Last-In-First-Out (LIFO) principle. In other words, elements are added and removed from the rear end. The action of adding an element to a stack is called push, while removing an element is referred to as pop. Below are the various methods provided by this class:

• empty(): Checks if the stack is empty.
• push(Object): Pushes an item to the top of the stack.
• pop(): Removes the object from the top of the stack.
• peek(): Looks at the object at the top of the stack without removing it.
• search(Object): Searches for an item in the stack to get its index.

Set Java Collections Interview Questions

1. What is Set in the 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:

• HashSet
• TreeSet
• LinkedHashSet

1. What is the HashSet class in Java, and how does it store elements?

The java.util.HashSet class is a member of the Java Collections Framework that inherits the AbstractSet class and implements the Set interface. It implicitly implements a hash table for creating and storing a collection of unique elements. The hash table is an instance of the HashMap class that uses a hashing mechanism for storing information within a HashSet. Hashing is the process of converting informational content into a unique value, known as a hash code. This hash code is then used for indexing the data associated with the key. The entire process of transforming the informational key into the hash code is performed internally.

1. 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 NavigableMap is a subtype of SortedMap that doesn’t allow null keys. So, if you try to add null elements to a TreeSet, it will throw a NullPointerException.

1. Explain the emptySet() method in the Collections Framework.

The Collections.emptySet() is used to return an empty immutable Set while removing null elements. The set returned by this method is serializable.

public static final <T> Set<T> emptySet()

1. What is LinkedHashSet in the Java Collections Framework?

The java.util.LinkedHashSet is a subclass of the HashSet class and implements the Set interface. It is an ordered version of HashSet that 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> set = new LinkedHashSet<>();

Map Java Collections Interview Questions

1. What is the Map interface in Java?

The java.util.Map interface in Java stores elements in the form of key-value pairs, designed for faster lookups. Every key is unique and maps to a single value. These key-value pairs are known as 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.

1. Why doesn’t the Map interface extend the Collection interface?

The Map interface in Java follows a key/value pair structure, whereas the Collection interface is a collection of objects 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 the Map interface’s put(K, V) method. It might not extend the Collection interface, but it is still an integral part of the Java Collections Framework.

1. List down the different Collection views provided by the Map interface in the Java Collection Framework.

The Map interface provides three views of key-value pairs:

• Key set view
• Value set view
• Entry set view

All these views can be easily navigated through using iterators.

1. What is ConcurrentHashMap in Java, and how do you implement it?

ConcurrentHashMap is a Java class that implements the ConcurrentMap and Serializable interfaces. This class is an enhanced version of HashMap as it doesn’t perform well in a multi-threaded environment. It has a higher performance rate compared to HashMap.

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();map.put("1", "Welcome");map.put("2", "to");map.put("3", "Edureka");map.put("4", "Demo");

1. Can you use any class as a Map key?

Yes, any class can be used as a Map key as long as the following points are considered:

• The class overriding the equals() method must also override the hashCode() method.
• The class should adhere to the rules associated with equals() and hashCode() for all instances.
• The class field which is not used in the equals() method should not be used in the hashCode() method as well.
• The best way to use a user-defined key class is by making it immutable. It helps in caching the hashCode() value for better performance and ensures that the hashCode() and equals() methods are not changing in the future.

Differences Java Collections Interview Questions

1. Differentiate between Collection and Collections.

1. Differentiate between an Array and an ArrayList.

1. Differentiate between Iterable and Iterator.

1. Differentiate between ArrayList and LinkedList.