Top 50 Java Collections Interview Questions and Answers in 2022

Collection Framework is one of the most authoritative pillars that support the fundamental concepts of the Java scheduling speech. If you are an aspiring Java Developer, it is very crucial for you to have a strong cognition of these core concepts before you appear for an interview. Through the metier of this article, I will plowshare the Top 50 Java Collections Interview Questions and Answers that will decidedly help you in clearing your consultation with flying colors. The questions in this article have been divided into the play along sections :

Generic –  Java Collections Interview Questions

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

Below postpone contains the major advantages of the Java Collection Framework :

Feature Description
Performance The collection framework provides highly effective and efficient data structures that result in enhancing the speed and accuracy of a program.
Maintainability The code developed with the collection framework is easy to maintain as it supports data consistency and interoperability within the implementation.
Reusability The classes in Collection Framework can effortlessly mix with other types which results in increasing the code reusability.
Extensibility The Collection Framework in Java allows the developers to customize the primitive collection types as per their requirements.

2. What do you understand by Collection Framework in Java?

The Java Collection framework provides an architecture to store and manage a group of objects. It permits the developers to access prepackaged data structures american samoa well as algorithm to manipulate data. The solicitation model includes the postdate :

  • Interfaces
  • Classes
  • algorithm

All these classes and interfaces support diverse operations such as Searching, Sorting, Insertion, Manipulation, and Deletion which makes the data handling in truth slowly and quick. 3. Describe the Collection hierarchy in Java. Java Collection Heirarchy - Java Collection Interview Questions - Edureka

4. List down the primary interfaces provided by Java Collections Framework?

Below are the major interfaces provided by the Collection Framework :

  • Collection Interface: java.util.Collection is the root of the Java Collection framework and most of the collections in Java are inherited from this interface.
public interface Collectionextends Iterable
  • List Interface: java.util.List is an extended form of an array that contains ordered elements and may include duplicates. It supports the index-based search, but elements can be easily inserted irrespective of the position. The List interface is implemented by diverse classes such as ArrayList, LinkedList, Vector, etc .
public interface List extends Collection
  • Set Interface: java.util.Set refers to a collection classify that can not contain duplicate elements. Since it doesn ’ deoxythymidine monophosphate define an order for the elements, the index-based search is not supported. It is majorly used as a mathematical set abstraction model. The Set interface is implemented by versatile classes such as HashSet, TreeSetand LinkedHashSet .
public interface Set extends Collection
  • Queue Interface: java.util.Queue in Java follows a FIFO approach i.e. it orders the elements in First In First Out manner. Elements in Queue will be added from the rise end while removed from the front .
public interface Queue extends Collection
  • Map Interface: java.util.Map is a two-dimensional data structure in Java that is used to store the data in the form of a Key-Value pair. The key here is the unique hashcode and value represent the element. Map in Java is another form of the Java Set but can’t contain duplicate elements.

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

The Collection interface in Java specifies a group of objects called elements. The maintainability and rate of elements is completely dependent on the concrete implementations provided by each of the Collection. thus, there is no consumption of extending the Cloneable and Serializable interfaces .

    6. List down the major advantages of the Generic Collection.

    Below are the independent advantages of using the generic collection in Java :

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

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

    The independent advantage of using the properties file in Java is that in case the values in the properties file is changed it will be automatically reflected without having to recompile the java class. Thus it is chiefly used to store information which is liable to change such as username and passwords. This makes the management of the application easy and effective. Below is an model of the same :

    import java.util.*;
    public class PropertiesDemo{
    public static void main(String[] args)throws Exception{ 
    FileReader fr=new FileReader(""); 
    Properties pr=new Properties();

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

    Iterator in Java is an interface of the Collection framework portray in java.util box. It is a Cursor in Java which is used to iterate a collection of objects. Below are a few other major functionalities provided by the Iterator interface :

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

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

    The initial execution of the equals method helps in checking whether two objects are the lapp or not. But in case you want to compare the objects based on the property you will have to override this method acting .

    10. How the Collection objects are sorted in Java?

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

    List – Java Collections Interview Questions

    11. What is the use of the List interface?

    The List interface in Java is an ordered collection of elements. It maintains the insertion holy order and allows duplicate values to be stored within. This interface contains assorted methods which enables smooth handling of elements based on the chemical element index. The independent classes implementing the List interface of the Collection framework are ArrayList, LinkedList, Stack, and Vector .

    12. What is ArrayList in Java?

    ArrayList is the execution of List Interface where the elements can be dynamically added or removed from the number. ArrayList in the Collection model provides positional access and insertion of elements. It is an regulate collection that permits double values. The size of an ArrayList can be increased dynamically if the act of elements is more than the initial size. Arraylist - Java Collections Interview Questions - EdurekaSyntax :

    ArrayList object = new ArrayList ();

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

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


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

    List_object.toArray(new String[List_object.size()])

    14. How will you reverse an List? ArrayList can be reversed using the reverse ( ) method acting of the Collections class. Syntax:

    public static void reverse(Collection c)

    For Example:

    public class ReversingArrayList { 
    public static void main(String[] args) { 
    List myList = new ArrayList(); 
    myList .add("Blockchain"); 
    System.out.println("Before Reversing"); 
    System.out.println("After Reversing"); 

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

    LinkedList in Java is a datum social organization that contains a sequence of links. here each connection contains a association to the adjacent radio link. Syntax:

    Linkedlist object = new Linkedlist();

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

    • Singly Linked List: In a singly LinkedList, each node in this list stores the data of the node and a pointer or reference to the next node in the list.

    SinglyLinkedList - Java Collections - Edureka

    • Doubly Linked List: In a doubly LinkedList, it has two references, one to the next node and another to the previous node.

    DoublyLinkedList - Java Collections - Edureka 16. What is a Vector in Java?

    Vectors are alike to arrays, where the elements of the vector object can be accessed via an index into the vector. Vector implements a moral force array. besides, the vector is not limited to a specific size, it can shrink or grow automatically whenever required. It is alike to ArrayList, but with two differences :

    • Vector is synchronized .
    • Vector contains many bequest methods that are not partially of the collections model .

    Vector - Java Collections - Edureka Syntax :

    Vector object = new Vector(size,increment);

    Queue – Java Collections Interview Questions

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

    Below are some of the methods of Java Queue interface :

    method acting description
     boolean add(object) Inserts the specify element into the line up and returns truthful if it is a success .
     boolean offer(object) Inserts the pin down component into this line up .
     Object remove() Retrieves and removes the read/write head of the line up .
     Object poll() Retrieves and removes the drumhead of the queue, or returns nothing if the queue is empty .
     Object element() Retrieves, but does not remove the headway of the line up .
     Object peek() Retrieves, but does not remove the head of this queue, or returns nothing if the queue is empty .

    18. What do you understand by BlockingQueue?

    BlockingQueue interface belongs to the java.util.concurrent  package. This interface enhances flow control by activating block, in case a weave is trying to dequeue an evacuate queue or enqueue an already full line up. While working with the BlockingQueue interface in Java, you must remember that it does not accept a null value. In case you try to do that it will instantaneously throw a NullPointerException. The under visualize represents the work of the BlockingQueue interface in Java. BlockingQueue - Java Collections Interview Questions - Edureka

    19. What is a priority queue in Java?

    A precedence queue in Java is an abstract datum type similar to a regular queue or stack data structure but has a special feature called precedence associated with each element. In this line up, a high priority component is served before a humble priority chemical element regardless of their interpolation regulate. The PriorityQueue is based on the precedence heap. The elements of the precedence queue are ordered according to the natural order, or by a Comparator provided at queue structure time, depending on which builder is used .

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

    Java Stack classify is an authoritative part of the Java Collection framework and is based on the basic principle of last-in-first-out. In other words, the elements are added american samoa well as removed from the rise end. The action of adding an chemical element to a stack is called push while removing an element is referred to as popular. Below are the respective methods provided by this class :

    Methods Description
    empty() Checks if the stack is empty
    push() Pust an item to the top of the stack
    pop() Remove the object from the stack
    peek() Looks at the object of a stack without removing it
    search() Searches item in the push-down list to get its index

    Set  – Java Collections Interview Questions

    21. What is Set in Java Collections framework and list down its various implementations?

    A Set refers to a collection that can not contain duplicate elements. It is chiefly used to model the mathematical set abstraction. The Java platform provides three general-purpose Set implementations which are :

    1. HashSet
    2. TreeSet
    3. LinkedHashSet

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

    java.util.HashSet class is a extremity of the Java collections model 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 case of the HashMap class that uses a hash mechanism for storing the data within a HashSet. Hashing is the process of converting the informational contentedness into a singular value that is more popularly known as hash code. This hashcode is then used for indexing the data associated with the cardinal. The integral action of transforming the informational samara into the hashcode is performed internally .

    23. 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 ’ thyroxine 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 nothing elements to a TreeSet, it will throw a NullPointerException .

    24. Explain the emptySet() method in the Collections framework?

    The Collections.emptySet ( ) is used to return the empty immutable Set while removing the nothing elements. The set returned by this method acting is serializable. Below is the method acting announcement of emptySet ( ). Syntax:

    public static final  Set emptySet()

    25. 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 arranged version of HashSet which maintains a doubly-linked list across all elements contained within. It preserves the insertion decree and contains only alone elements like its parent classify. Syntax:

    LinkedHashSet hs = new LinkedHashSet();

    Map  – Java Collections Interview Questions

    26. 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 search. hera every cardinal is unique and maps to a one 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 keystone. With such methods, it ’ s a perfect cock to use for key-value association map such as dictionaries. 27. Why Map doesn’t extend the Collection Interface? The Map interface in Java follows a key/value pair social organization whereas the Collection interface is a collection of objects which are stored in a integrated manner with a intend access mechanism. The chief reason Map doesn ’ thymine extend the Collection interface is that the add ( E vitamin e ) method of the Collection interface doesn ’ deoxythymidine monophosphate hold the key-value match like Map interface ’ randomness put ( K, V ) method acting. It might not extend the Collection interface but even is an integral part of the Java Collections framework .

    28. 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 .

    29. What is the ConcurrentHashMap in Java and do you implement it?

    ConcurrentHashMap is a Java class that implements ConcurrentMap adenine well as Serializable interfaces. This class is the enhance version of HashMap as it doesn ’ metric ton perform well in the multithreaded environment. It has a higher performance rate compared to the HashMap. Below is a small model demonstrating the execution of ConcurrentHashMap :

    package edureka;
    import java.util.concurrent.*;
    public class ConcurrentHashMapDemo {
        public static void main(String[] args) 
            ConcurrentHashMap m = new ConcurrentHashMap(); 
            m.put(1, "Welcome"); 
            m.put(2, "to"); 
            m.put(3, "Edureka's");
            m.put(4, "Demo");
            // Here we cant add Hello because 101 key 
            // is already present in ConcurrentHashMap object 
            m.putIfAbsent(3, "Online"); 
            System.out.println("Checking if key 3 is already present in the ConcurrentHashMap object: "+ m);
            // We can remove entry because 101 key 
            // is associated with For value 
            m.remove(1, "Welcome");
            System.out.println("Removing the value of key 1: "+m);
            // Now we can add Hello 
            m.putIfAbsent(1, "Hello");
            System.out.println("Adding new value to the key 1: "+m);
            // We cant replace Hello with For 
            m.replace(1, "Hello", "Welcome"); 
            System.out.println("Replacing value of key 1 with Welcome: "+ m); 

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

    Yes, any class can be used as Map Key deoxyadenosine monophosphate 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 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. Also if the class is made immutable it will ensure that the hashCode() and equals() are not changing in the future.

    Differences – Java Collections Interview Questions

    31. Differentiate between Collection and Collections.

    Collection Collections
    java.util.Collection is an interface java.util.Collections is a class
    Is used to represent a group of objects as a single entity It is used to define various utility method for collection objects
    It is the root interface of the Collection framework It is a utility class
    It is used to derive the data structures of the Collection framework It contains various static methods which help in data structure manipulation

    32. Differentiate between an Array and an ArrayList.

    Array ArrayList
    java.util.Array is a class java.util.ArrayList is a class
    It is strongly typed It is loosely types
    Cannot be dynamically resized Can be dynamically resized
    No need to box and unbox the elements Needs to box and unbox the elements

    33. Differentiate between Iterable and Iterator.

    Iterable Iterator
    Iterable is an interface Iterator is an interface
    Belongs to java.lang package Belongs to java.util package
    Provides one single abstract method called iterator() Provides two abstract methods called hasNext() and next()
    It is a representation of a series of elements that can be traversed It represents the object with iteration state

    34. Differentiate between ArrayList and LinkedList.

    ArrayList LinkedList
    Implements dynamic array internally to store elements Implements doubly linked list internally to store elements
    Manipulation of elements is slower Manipulation of elements is faster
    Can act only as a List Can act as a List and a Queue
    Effective for data storage and access Effective for data manipulation

    35. Differentiate between Comparable and Comparator.

    Comparable Comparator
    Present in java.lang package Present in java.util package
    Elements are sorted based on natural ordering Elements are sorted based on user-customized ordering
    Provides a single method called compareTo() Provides to methods equals() and compare()
    Modifies the actual class Doesn’t modifies the actual class

    36. Differentiate between List and Set.

    List Set
    An ordered collection of elements An unordered collection of elements
    Preserves the insertion order Doesn’t preserves the insertion order
    Duplicate values are allowed Duplicate values are not allowed
    Any number of null values can be stored Only one null values can be stored
    ListIterator can be used to traverse the List in any direction ListIterator cannot be used to traverse a Set
    Contains a legacy class called vector Doesn’t contains any legacy class

    37. Differentiate between Set and Map.

    Set Map
    Belongs to java.util package Belongs to java.util package
    Extends the Collection interface Doesn’t extend the Collection interface
    Duplicate values are not allowed Duplicate keys are not allowed but duplicate values are
    Only one null values can be stored Only one null key can be stored but multiple null values are allowed
    Doesn’t maintain any insertion order Doesn’t maintain any insertion order

    38. Differentiate between List and Map.

    List Map
    Belongs to java.util package Belongs to java.util package
    Extends the Collection interface Doesn’t extend the Collection interface
    Duplicate elements are allowed Duplicate keys are not allowed but duplicate values are
    Multiple null values can be stored Only one null key can be stored but multiple null values are allowed
    Preserves the insertion order Doesn’t maintain any insertion order
    Stores elements based on Array Data Structure Stores data in key-value pairs using various hashing techniques

    39. Differentiate between Queue and Stack.

    Queue Stack
    Based on FIFO (First-In-First-Out) principle Based on LIFO (Last-In-First-Out) principle
    Insertion and deletion takes place from two opposite ends Insertion and deletion takes place the same end
    Element insertion is called enqueue Element insertion is called push
    Element deletion is called dequeue Element deletion is called pop
    Two pointers are maintained one point to the first element and the other one points the last element on the list Only one pointer is maintained which points to the top element on the stack

    40. Differentiate between PriorityQueue and TreeSet.

    PriorityQueue TreeSet
    It is a type of Queue It is based on a Set data structure
    Allows duplicate elements Doesn’t allows duplicate elements
    Stores the elements based on an additional factor called priority Stores the elements in a sorted order

    41. Differentiate between the Singly Linked List and Doubly Linked List.

    Singly Linked List(SLL) Doubly Linked List(DLL)
    Contains nodes with a data field and a next node-link field Contains nodes with a data field, a previous link field, and a next link field
    Can be traversed using the next node-link field only Can be traversed using the previous node-link or the next node-link
    Occupies less memory space Occupies more memory space
    Less efficient in providing access to the elements More efficient in providing access to the elements

    42. Differentiate between Iterator and Enumeration.

    Iterator Enumeration
    Collection element can be removed while traversing it Can only traverse through the Collection
    Used to traverse most of the classes of the Java Collection framework Used to traverse the legacy classes such as Vector, HashTable, etc
    Is fail-fast in nature Is fail-safe in nature
    Is safe and secure Is not safe and secure
    Provides methods like hasNext(), next() and remove() Provides methods like hasMoreElements() and nextElement()

    43. Differentiate between HashMap and HashTable.

    HashMap HashTable
    It is non-synchronized in nature It is synchronized in nature
    Allows only one null key but multiple null values Doesn’t allow any null key or value
    Has faster processing has slower processing
    Can be traversed by Iterator Can be traversed by Iterator and Enumeration
    Inherits AbstractMap class Inherits Dictionary class

    44. Differentiate between HashSet and HashMap.

    HashSet HasMap
    Based on Set implementation Based on Map implementation
    Doesn’t allow any duplicate elements Doesn’t allow any duplicate keys but duplicate values are allowed
    Allows only a single null value Allows only one null key but any number of null values
    Has slower processing time Has faster processing time
    Uses HashMap as an underlying data structure Uses various hashing techniques for data manipulation

    45. Differentiate between Iterator and ListIterator.

    Iterator ListIterator
    Can only perform remove operations on the Collection elements Can perform add, remove and replace operations the Collection elements
    Can traverse List, Sets and maps Can traverse only Lists
    Can traverse the Collection in forward direction Can traverse the collection in any direction
    Provides no method to retrieve the index of the element Provides methods to retrieve the index of the elements
    iterator() method is available for the entire Collection Framework listIterator() is only available for the collections implementing the List interface

    46. Differentiate between HashSet and TreeSet.

    HashSet TreeSet
    Uses HasMap to store elements Uses Treemap to store elements
    It is unordered in nature By default, it stores elements in their natural ordering
    Has faster processing time Has slower processing time
    Uses hasCode() and equals() for comparing Uses compare() and compareTo() for comparing
    Allows only one null element Doesn’t allow any null element
    Takes up less memory space Takes up more memory space

    47. Differentiate between Queue and Deque.

    Queue Deque
    Refers to single-ended queue Refers to double-ended queue
    Elements can be added or removed from only one end Elements can be added and removed from either end
    Less versatile More versatile

    48. Differentiate between HashMap and TreeMap.

    HashMap TreeMap
    Doesn’t preserves any ordering Preserves the natural ordering
    Implicitly implements the hashing principle Implicitly implements the Red-Black Tree Implementation
    Can store only one null key Cannot store any null key
    More memory usage Less memory usage
    Not synchronized Not synchronized

    49. Differentiate between ArrayList and Vector.

    ArrayList Vector
    Non-synchronized in nature Synchronized in nature
    It is not a legacy class Is a legacy class
    Increases size by 1/2 of the ArrayList Increases size by double of the ArrayList
    It is not thread-safe It is thread-safe

    50. Differentiate between failfast and failsafe.

    failfast failsafe
    Doesn’t allow modifications of a collection while iterating Allows modifications of a solicitation while iterating

    Throws ConcurrentModificationException Don’t throw any exceptions
    Uses the original collection to traverse over the elements Uses a copy of the original collection to traverse over the elements
    Don’t require extra memory Require extra memory

    then this brings us to the end of the Java Collections interview questions. The topics that you learned in this Java Collections Interview Questions are the most sought skill sets that recruiters look for in a Java Professional. These sets of Java Collection Interview Questions will decidedly help you ace your occupation consultation. Good luck with your interview! Check out the Java Certification Training by Edureka, a entrust on-line learning caller with a net of more than 250,000 satisfy learners spread across the globe. We are here to help you with every footprint on your travel, for becoming a besides this java interview questions, we come up with a course of study which is designed for students and professionals who want to be a Java Developer. The course is designed to give you a principal begin into Java program and educate you for both core and advanced Java concepts along with respective Java frameworks like Hibernate & Spring. Got a motion for us ? Please mention it in the comments section of this “ Java Collections Interview Questions ” and we will get back to you vitamin a soon as potential or you can besides join Java Training in Ernakulam .

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