Top 40 Java Interview Questions and Answers 2023

Java is one of the most widely used programming languages and is frequently used in job interviews for software development roles. During a Java interview, you can expect a range of questions that assess your knowledge and understanding of the language’s concepts, features, and best practices.

Java Collections Interview Questions

Q1: What is the difference between ArrayList and LinkedList in Java?

Answer: The main difference between ArrayList and LinkedList is their underlying data structure. ArrayList uses a dynamic array to store elements, while LinkedList uses a doubly-linked list. ArrayList provides faster access to elements using indexes, but LinkedList performs better when adding or removing elements from the middle of the collection.

Q2: What is the purpose of the Map interface in Java Collections?

Answer: The Map interface in Java Collections is used to store key-value pairs. It provides methods to add, remove, and retrieve values based on their associated keys. Some commonly used implementations of Map interface are HashMap, TreeMap, and Linked HashMap.

Q3:What is the difference between HashMap and Hash Table in Java?

Answer: HashMap and Hash Table are both implementations of the Map interface, but there are a few differences between them. HashMap is not synchronized and allows null keys and values, whereas Hash Table is synchronized and doesn’t allow null keys or values. Additionally, HashMap is generally considered more efficient in terms of performance.

Q4. What is the difference between HashSet and TreeSet in Java?

Answer: HashSet and TreeSet are both implementations of the Set interface, but they have some differences. HashSet doesn’t maintain any order of elements and allows null values, while TreeSet stores elements in sorted order (according to their natural ordering or a custom Comparator) and doesn’t allow null values.

Q5. What is the difference between ArrayList and Vector in Java?

Answer: ArrayList and Vector are both dynamic array implementations, but Vector is synchronized, whereas ArrayList is not. This means that Vector is thread-safe, but it may have a performance impact in single-threaded scenarios. In most cases, ArrayList is preferred over Vector.

Q6. What is the purpose of the Comparable interface in Java?

Answer: The Comparable interface is used to define the natural ordering of objects. It provides a method called compareTo that is used to compare two objects and determine their relative order. Classes that implement Comparable can be sorted using methods like Collections. sort().

Q7. What is the difference between fail-fast and fail-safe iterators?

Answer: Fail-fast and fail-safe iterators are two different approaches to handling concurrent modifications during iteration.

Fail-fast iterators, such as those provided by Array List and HashMap, throw a ConcurrentModificationException if a collection is modified while an iterator is iterating over it. They detect any structural modifications and ensure that the iterator operates on the original collection snapshot. They provide fast-fail behavior to prevent unpredictable behavior.

Fail-safe iterators, such as those provided by Concurrent HashMap and CopyOnWriteArrayList, work on a cloned copy of the collection instead of the original. They allow modifications to the collection while iterating without throwing any exceptions.

Q8. What is the purpose of the Comparator interface in Java?

Answer: The Comparator interface is used to define custom comparison logic for objects. It provides a method called “compare” that takes two objects and returns an integer value indicating their relative order. Comparator allows sorting objects based on different criteria without modifying their original implementation.

Q9. What is the difference between HashSet and LinkedHashSet in Java?

Answer: HashSet and LinkedHashSet are both implementations of the Set interface, but they differ in how they maintain the order of elements:

HashSet does not maintain any order of elements. It uses the hash code of the objects to determine their storage location, providing fast retrieval of elements but without any specific order.

LinkedHashSet, on the other hand, maintains the insertion order of elements. It uses a doubly-linked list in addition to the hash table, which allows it to preserve the order in which elements were added.

Q10. What is the purpose of the Collections class in Java?

Answer: The Collections class in Java provides various utility methods for working with collections. It contains static methods that operate on or return collections, including sorting, searching, and modifying collections.

Java OOPs Interview Questions

Q11. What is the difference between a class and an object in Java?

Answer: A class is a blueprint or template that defines the properties and behaviors of an object. An object is an instance of a class that represents a real-world entity. In simpler terms, a class is like a blueprint, and an object is the actual building created from that blueprint.

Q12. What are the four principles of object-oriented programming?

Answer: The four principles of object-oriented programming are:

Encapsulation: The bundling of data and methods together within a class.
Inheritance: The ability to create new classes based on existing classes.
Polymorphism: The ability to use a single interface to represent different types of objects.
Abstraction: The process of hiding unnecessary details and exposing only essential features.

Q13. What is the difference between method overloading and method overriding?

Answer: Method overloading occurs when multiple methods in the same class have the same name but different parameters. Method overriding, on the other hand, happens when a subclass provides its implementation of a method that is already defined in its superclass.

Q14. What is a constructor in Java?

Answer: A constructor is a special method that is used to initialize objects of a class. It has the same name as the class and is called automatically when an object is created. Constructors can be used to set initial values for object attributes or perform any other necessary setup tasks.

Q15. What is the difference between static and non-static variables/methods?

Answer: Static variables/methods belong to the class itself and are shared among all instances of the class. Non-static variables/methods are specific to each instance of the class. Static variables/methods can be accessed without creating an instance of the class, whereas non-static variables/methods require an object to be created first.

Q16. What is the purpose of the “final” keyword in Java?

Answer: The “final” keyword can be used in three different contexts:

To create constant variables that cannot be modified.
To prevent a class from being inherited.
To prevent a method from being overridden in a subclass.

Q17. What is the difference between composition and inheritance?

Composition and inheritance are two ways to achieve code reuse. Inheritance allows a class to inherit the properties and methods of another class, promoting an “is-a” relationship. Composition, on the other hand, involves creating an instance of another class within a class, promoting a “has-a” relationship.

Q18. What is the significance of the “this” keyword in Java?

The “this” keyword is a reference to the current object. It is often used to differentiate between instance variables and method parameters that have the same name. It can also be used to invoke other constructors in the same class.

Q19. What is method overloading in Java?

Method overloading is the ability to define multiple methods with the same name but different parameters within a class. Java determines which method to call based on the number, type, and order of the arguments passed during method invocation.

Q20. What is an abstract class in Java?

An abstract class is a class that cannot be instantiated but can be extended by other classes. It serves as a blueprint for its subclasses, defining common properties and methods. Abstract classes may contain abstract methods, which are declared without implementation and must be implemented by the concrete subclasses.

Java String Interview Questions

Q21. What is a string in Java?

A string in Java is an object that represents a sequence of characters. It is used to store and manipulate textual data.

Q22. How do you create a string object in Java?

You can create a string object in Java by using the new keyword and the String class constructor, or by directly assigning a value to a string variable using double quotes.

Example: String str1=new String(“Hello”);

String str2= “World”;

Q23. What is the difference between String, StringBuilder, and StringBuffer?

String objects are immutable, meaning their values cannot be changed after they are created. Each modification to a string results in creation of a new string object.
StringBuilder and StringBuffer are mutable classes that can be used to modify strings. They provide methods for appending, inserting, and deleting characters in a string. The main difference between them is that StringBuffer is thread-safe (synchronized) while StringBuilder is not.

Q24. How do you concatenate strings in Java?

In Java, you can concatenate strings using the + operator or the concat() method.

Example using the + operator:

String str1=”Hello”;

String str2= “World”;

String result= str1+ ” “+str2; // Concatenation using +

Example using the concat() method:

String str1=”Hello”;

String str2= “World”;

String result = str1.concat(” “) concat(str2);// Concatenation using concat( )

Q25. Can you modify a string in Java?

No, you cannot directly modify a string in Java because strings are immutable. However, you can perform operations on strings to create a new modified string.

Q26. What is the difference between the equals() method and the == operator when comparing strings?

The equals() method is used to compare the contents of two string objects for equality. It checks if the characters in the strings are the same. The == operator, on the other hand, checks if two string references point to the same object in memory.

String str1 = “Hello”;

String str2 = “Hello”;

String str3 = new String(“Hello”);

System.out.println(str1.equals(str2)); // true

System.out.println(str1 == str2); // true

System.out.println(str1.equals(str3)); // true

System.out.println(str1 == str3); // false

Q27. How do you convert a string to uppercase or lowercase in Java?

You can convert a string to uppercase or lowercase in Java using the toUpperCase() and toLowerCase() methods, respectively.

Example: String str = “Hello World”; String uppercase = str.toUpperCase(); // Converts to uppercase String lowercase = str.toLowerCase(); // Converts to lowercase

Q28. What is the difference between the length() method and the size() method for strings?

In Java, the length() method is used to get the number of characters in a string, while the size() method is not a valid method for strings. The size() method is commonly used with collections such as lists or sets to get the number of elements.

Question 29: How do you check if a string contains a specific substring in Java?

You can check if a string contains a specific substring in Java using the contains() method or by using the indexOf() method to find the position of the substring.

Example using contains() method:

String str = “Hello World”;
boolean containsSubstring = str.contains(“World”); // true
Example using indexOf() method:

String str = “Hello World”;
int index = str.indexOf(“World”);
boolean containsSubstring = index != -1; // true if the substring is found

Q30. How do you reverse a string in Java?

You can reverse a string in Java by converting it to a StringBuilder or StringBuffer object and then using the reverse() method.

String str = “Hello”; StringBuilder reversed = new StringBuilder(str).reverse(); String result = reversed.toString();

Java Multithreading Interview Questions 

Q31. What is multithreading in Java?

Multithreading in Java refers to the concurrent execution of two or more parts of a program, where each part is known as a thread. Threads allow programs to perform multiple tasks simultaneously.

Q32. How can you create a thread in Java?

There are two ways to create a thread in Java:

By extending the Thread class.
By implementing the Runnable interface and passing it to the Thread constructor.

Q33. What is the difference between Thread and Runnable in Java?

The Thread class represents an actual thread, while the Runnable interface defines the task that a thread will execute. By implementing Runnable, you can achieve better separation of concerns and improve code reusability.

Q34. How can you start a thread in Java?

You can start a thread by calling its start() method. This method internally calls the run() method of the thread, which contains the code to be executed concurrently.

Q35. What is the difference between start() and run() methods in Java?

The start() method is used to start a new thread and internally calls the run() method. The run() method contains the code to be executed by the thread and should not be called directly if you want to achieve concurrent execution.

Q36. What are the ways to synchronize threads in Java?

Thread synchronization can be achieved using:

The synchronized keyword, which can be applied to methods or code blocks.
The Lock interface and its implementations like ReentrantLock.

Q37. What is the purpose of the wait(), notify(), and notifyAll() methods in Java?

These methods are used for inter-thread communication in Java.

The wait() method causes a thread to wait until it is notified by another thread.
The notify() method wakes up a single waiting thread.
The notifyAll() method wakes up all the waiting threads.

Q38. What is a deadlock in multithreading, and how can it be avoided?

Deadlock occurs when two or more threads are blocked forever, waiting for each other to release resources. Deadlock can be avoided by using proper synchronization, avoiding nested locks, and implementing timeouts.

Q39. What is the volatile keyword in Java?

The volatile keyword is used to indicate that a variable’s value may be modified by multiple threads. It ensures that changes made to the variable by one thread are immediately visible to other threads.

Q40. What are the benefits of multithreading in Java?

Multithreading in Java offers several benefits, including:

Improved performance and utilization of system resources.
Enhanced responsiveness and user experience in GUI applications.
Simplified handling of multiple concurrent tasks.
Efficient utilization of multi-core processors.

Tips to prepare for Java Interview

1. Review the Basics: Ensure you have a strong understanding of core Java concepts like object-oriented programming, data types, control structures, exception handling, and collections. Familiarize yourself with key Java APIs and frameworks.
2. Data Structures and Algorithms: Brush up on fundamental data structures (arrays, linked lists, stacks, queues, trees, graphs) and algorithms (sorting, searching, recursion). Understand their implementation, time complexity, and best use cases.
3. Java Platform: Learn about Java platforms (Java SE, Java EE, Java ME) and their respective APIs. Understand the differences between JDK, JRE, and JVM. Stay updated with the latest Java version features and enhancements.
4. Multithreading and Concurrency: Be comfortable with multithreading concepts, synchronization, locks, and thread safety. Understand the Java concurrency API (Thread, Executor framework, synchronizers) and be familiar with common concurrency issues and their resolutions.
5. Design Patterns: Study common design patterns like Singleton, Factory, Observer, Builder, and MVC. Understand when and how to apply them in different scenarios. Be prepared to explain their purpose and benefits.
6. Databases and SQL: Have a good understanding of working with databases, SQL queries, and JDBC (Java Database Connectivity). Be familiar with concepts like ACID properties, normalization, and database indexing.
7. Java Frameworks: Depending on the role you’re applying for, familiarize yourself with popular Java frameworks such as Spring, Hibernate, JUnit, and Maven. Understand their core functionalities and how they are used in real-world projects.
8. Practice Coding: Solve coding problems and practice coding exercises to improve your problem-solving skills. Platforms like LeetCode and HackerRank offer coding challenges specific to Java. Review your solutions, analyze time complexity, and optimize where possible.
9. Mock Interviews: Conduct mock interviews with a friend or mentor to simulate real interview scenarios. Practice answering common interview questions and discussing your thought process while solving coding problems.
10. Stay Updated: Follow Java-related blogs, forums, and websites to stay updated with the latest trends, news, and advancements in the Java ecosystem. Keep an eye on new Java features, libraries, and frameworks.
11. Prepare Questions: Prepare a list of thoughtful questions to ask the interviewer about the company, team, projects, or any technical aspects that you would like to know more about. This demonstrates your interest and curiosity.