Top Java Interview Questions to Nail Tech Interviews
Java is one of the primary contributors to the software development domain. It is not only essential to learn the skills but also to understand their applications in real life. We’re here to guide you in your preparation for Java interviews. We’ll discuss the types of questions asked and then go into some Java interview questions for each topic.
Being a software engineer, you might have some technical expertise in any one of the programming languages. This article discusses some basic and advanced level Java interview questions related to various Java concepts.
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Here’s what we’ll discuss in this article:
- Must-learn Topics for Java Interviews
- Benefits of Learning Java
- Top 10 Java Interview Questions
- Topic-wise Java Interview Questions
- Tricky Java Interview Questions
- How to Prepare for a Java Interview?
- Java Interview Questions FAQ
Must-learn Topics for Java Interviews
There is a wide range of topics you need to prepare to crack the interview for Java interviews. A few of them are:
- OOPs Concepts
- String Handling
- Loops and Data Types
- Collection Framework
- Serialisation & De-serialisation
- JDBC (Java Database Connectivity)
- Spring (MVC, Core, JDBC)
- Web Services (SOAP and REST)
- Hibernate ORM Framework
Benefits of Learning Java
1. You can do anything in Java
In this software development field, you can use Java to perform any task. Java can be used in developing Web applications, Windows applications, and games.
2. Widely used in tech companies
As Java can be used to perform any type of task, it is one of the most preferred programming languages by software companies and other companies in general.
3. Easy to learn
Learning a programming language is not easy. But, once you learn Java, you can become a good programmer by using the skills in developing many applications.
4. Collection of standard APIs
Java has an abundant collection of standard APIs. One can easily create graphical interfaces using Swing. They also have multiple features like multithreading, handling input and output operations, performing network operations, and accessing databases.
Top 10 Java Interview Questions
Let’s take a look at the top 10 java interview questions that are frequent in coding interviews at FAANG+ companies:
1. What is the difference between JDK, JRE, and JVM?
- JDK stands for Java Development Kit. It is the environment that is used to develop Java applications. It consists of JRE, JavaDoc, compiler, debuggers, etc.
JDK = JRE + Development Tools
- JRE stands for Java Runtime Environment. It is a package that provides the class libraries, JVM, and all the components required to run the Java application.
JRE = JVM + Libraries to execute the application
- JVM stands for Java Virtual Machine. It is an abstract machine providing a runtime environment in which the Java bytecode can be executed.
JVM = Runtime environment to execute Java bytecode
2. What is a JIT compiler?
Just-In-Time Compiler is used to improve the performance of an application during runtime. It compiles parts of the bytecode having similar functionality simultaneously, reducing the code’s compilation time.
3. Explain the difference between equals() method and equality operator (++) in Java.
It is a method defined in the Object class. This method checks the equality of contents between two objects.
Equality Operator (==):
It is a binary operator in Java. This operator checks if both the objects are pointing to the same memory location.
4. Why is the Java platform independent?
Java Compiler compiles the code and converts it to platform-independent bytecode, running on multiple systems. Hence, it does not depend on any hardware or software.
5. Why is Java not completely object-oriented?
Java is not purely object-oriented because it supports primitive data types like byte, boolean, char, short, int, float, long and double, which are not objects.
6. What are the differences between Stack and Heap Memory in Java?
7. What is Data Encapsulation?
Data Encapsulation is an Object-Oriented Programming concept of hiding the variables and methods in a single unit. It helps in keeping the code modular.
8. Why is a Character Array preferred over a string for storing confidential information?
Once a string is declared, it remains in the heap memory even after processing the string value. This can result in the theft of information. This risk can be eliminated when mutable objects like character arrays are used. Once the processing is done, the variable can be assigned a blank or null value. It saves the heap memory and also protects the information.
9. What happens when the static keyword is not included in the main method signature?
The code compiles successfully. But, as the JVM can’t map the main method signature, it throws a ‘NoSuchMethodError’ exception at runtime.
10. Does Finally block get executed if the code System.exit(0) is written at the end of Try block?
No. Once it encounters System.exit(0), the program’s control is immediately gone, and it gets terminated. This is why the finally block does not get executed after System.exit(0).
Topic-wise Java Interview Questions
We’ve compiled all the top java interview questions as per various Java concepts and categories such as Coding, Spring Boot, Multithreading, Data Structures, Collections, Selenium, and more. Take a look at these tech interview questions below:
Java Coding Interview Questions
Top Java 8 coding interview prep questions are listed below for reference.
- How to reverse a String in Java?
- Fibonacci series using recursion.
- How can you remove leading and trailing whitespaces from a String?
- How can one create a deadlock scenario by programming?
- How can you implement Binary Search?
- Create a pyramid of characters in Java
- Check whether two arrays contain the same elements.
- How can one print date in a specific format?
- How can you create a record in Java?
- How can one use forEach() method?
Java 8 Interview Questions
Java 8 or JDK 8.0 is the most significant release of the programming development platform. Let’s take a look at some Java 8 interview questions that could be asked in your tech interview:
- Describe the newly added features in Java 8.
- Explain some advantages of using Java 8.
- Differentiate MetaSpace from PermGen.
- What are Functional or SAM Interfaces?
- Explain some standard Java predefined functional interfaces.
- What is a lambda expression in Java, and how does it relate to a functional interface?
- What are the common ways to use lambda expressions?
- What is an Optional class?
- Explain Streams in Java 8.
- What are intermediate and terminal operations?
- How are collections different from streams?
- What are the important packages for the new Date and Time API in Java 8?
- Define Nashorn in Java 8.
- What is a Default method, and when do we use it?
- What is JJS?
- Explain the difference between Map and flatMap Stream Operations using examples.
- Given a list of employees, filter out the employees whose age is greater than 20 and print the employee names using Java 8 APIs.
- Given a list of employees, find the maximum age of employees.
- Given a list of employees, sort the employees based on age using Java APIs.
- Write a program to join all the employee names with “,” using Java 8.
Core Java Interview Questions
These are some of the top core java interview questions listed below:
- What is a JIT Compiler?
- What are the main differences between the Java platform and other platforms?
- What is a classloader?
- Is an empty .java file name a valid source file name?
- If no arguments are provided in the command line, what is the value stored in the String array passed into the main() method?
- What happens when we write static public void instead of public static void?
- What is the output of the below program?
- class Test
public static void main (String args)
for(int i=0; 0; i++)
- What is the difference between an object-oriented programming language and an object-based programming language?
- What do you understand about copy constructors in Java?
- Differentiate constructors from methods.
- What are the restrictions that are applied to Java static methods?
- What happens when the static modifier is removed from the signature of the main method?
- What are the main uses of this keyword?
- How can constructor chaining be done using this keyword?
- Explain aggregation using an example?
- What are the differences between this and super keywords?
- Can we use this() and super() both in a constructor?
- What is Object cloning?
- What is the covariant return type?
Java Collections Interview Questions
Here’s a list of Java Collections interview questions asked:
- What is the Collection framework in Java?
- What are the main differences between array and collection?
- Explain the various interfaces used in the Collections framework?
- Explain the differences between ArrayList and Vector.
- Explain the differences between ArrayList and LinkedList.
- Explain the differences between Iterator and Enumerator.
- Explain the differences between HashSet and TreeSet.a
- Explain the differences between HashSet and HashMap.
- Explain the differences between HashMap and TreeMap.
- What are the differences between Collection and Collections?
- What is a Blocking Queue?
- Explain the hashCode() method.
- How do you synchronize List, Set, and Map elements?
- What is hash-collision in Hashtable, and explain how it is handled in Java?
- Write a program to remove duplicates from ArrayList?
- Write a program to sort ArrayList in descending order.
- When do you use ArrayList and LinkedList?
- How can you make an ArrayList read-only in Java?
- Explain fail-fast and fail-safe iterators.
- Explain the purpose of RandomAccess Interface.
Java Multithreading Interview Questions
Some of the most widely asked Java multithreading interview questions for top tier tech companies are mentioned below:
- What is a thread in Java?
- What are the benefits of using Multithreading?
- What is the difference between a thread and a process?
- Differentiate between class lock and object lock.
- Explain the differences between a user thread and daemon thread.
- Write a program to create a daemon thread.
- What is the difference between notify() and notifyAll()?
- Why are wait(), notify(), and notifyAll() methods present in Object class?
- Explain the differences between Runnable and Callable interfaces.
- Explain thread pool.
- Explain the meaning of deadlock and instances when it can occur.
- Explain volatile variables in Java.
- What is Synchronization?
- What are synchronized methods and synchronized blocks? Which one is preferred?
- What is Livelock?
- What is Thread starvation?
- Can you start a thread twice?
- What are CyclicBarrier and CountDownLatch?
- Explain Thread Scheduler and Time Slicing.
- Explain Thread Group.
Selenium Java Interview Questions
Ten Selenium Java interview questions are given below for reference.
- What is automatic testing, and mention its advantages?
- Tell the disadvantages of using Selenium as a testing tool.
- What do you mean by a locator? Mention some of the different types of locators in Selenium.
- Can you use Selenium to launch a web browser?
- What does one mean by an exception test in Selenium?
- Explain the different navigation commands supported by Selenium.
- Elucidate the pause feature in Selenium IDE.
- Define same-origin policy and how Selenium handles it.
- Explain the various types of framework and connection of Selenium with Robot Framework.
- Mention the difference between driver.close() and driver.quit() command in Selenium?
Java Design Patterns Interview Questions
Take a look at these java interview questions on design patterns asked in tech interviews:
- Categorize the different Design Patterns in Java.
- What is Gang of Four (GOF)?
- What is an Abstract Factory Pattern?
- Explain Structural Patterns in Java.
- Describe the ways in which you can create a singleton pattern.
- What are Adapter Patterns? Illustrate its uses.
- Explain Decorator Pattern with an example.
- Explain Composite Design Pattern in Java.
- Explain the Design Patterns used in the JDK library.
- What is a proxy pattern?
- Explain the Chain of Responsibility Pattern.
- Differentiate Bridge Pattern from Adapter Pattern.
- Explain Dependency Injection and Service Locator patterns.
- Explain the Intercepting Filter Design Pattern and its benefits.
- Explain Data Access Object (DAO) pattern.
- Explain the differences between VO and JDO.
- What are Creational Patterns?
- Explain the Singleton pattern using examples.
- Explain the differences between Strategy and State Design patterns.
- Explain Builder Design Pattern.
Java Spring Interview Questions
Some questions you can come across during Java spring interviews are listed below.
- What is a Spring Framework?
- What do you mean by Dependency Injection?
- Explain Spring IOC Container.
- In Spring Framework, how many modules are there? Name them.
- Explain the different components of a Spring application.
- In how many ways can you do a dependency injection?
- Mention the differences between BeanFactory and ApplicationContext.
- How can you provide configuration data to the Spring container?
- What are inner beans in Spring?
- What is the meaning of Annotation-based container configuration?
Java Spring Boot Interview Questions
Take a look at the Java Spring Boot interview questions here:
- How does Spring Boot work?
- What is the function of @SpringBootApplication internally?
- Mention the purpose of using @ComponentScan in class files?
- Explain starter dependencies.
- What are some of the most common Spring Boot CLI comments?
- How can you disable a specific auto-configuration class?
- Tell me the difference between RequestMapping and GetMapping.
- How can you enable Actuator in Spring Boot Application?
- What is an IOC Container?
- What is thymeleaf?
- What are the basic annotations that Spring Boot offers?
Java Data Structures Interview Questions
These are some of the frequently asked java data structure interview questions:
- What is data structure?
- Explain multidimensional arrays.
- How are linear data structures different from non-linear data structures?
- How are linked lists more efficient than arrays?
- What are the applications of Doubly Linked Lists?
- Explain the process of storing a variable in memory.
- Write a program to implement stack using queues.
- Explain the data structures used for LRU cache.
- What is an AVL Tree?
- What is topological sorting in a graph?
Java Architect Interview Questions
Some of the most widely asked Java architect interview questions are:
- Mention the differences between JDBC and Hibernate.
- What is the meaning of sharding? Is it beneficial? If yes, how?
- Define CAP theorem.
- What is scalability?
- Mention the advantages of NoSQL over traditional RDBMS.
- Explain Domain-Driven Design.
- Why is it necessary to use Clustering?
- Do you think there is any difference between a Binary Semaphore and a Mutex? If yes, mention them.
- Explain ACID Property of a System.
- Explain DRY and DIE principles.
Java Concurrency Interview Questions
The ten common Java concurrency interview questions are as follows:
- Mention the main components of concurrency API.
- How can you implement the producer-consumer problem using BlockingQueue?
- In Concurrency, what is the Atomic action?
- Explain the ExecutorService Interface.
- What does one mean by Callable and Future in Java?
- What is the FutureTask class in Java?
- Define race-condition.
- What do you mean by lock interface in Concurrency API in Java?
- Mention the differences between Synchronous programming and Asynchronous programming in Java.
- Mention the differences between ScheduledExecutorService and ExecutorService interface?
Java Algorithms Interview Questions
The most common questions for Java algorithm interviews at top tech organizations are listed below.
- What is the complexity of Algorithms?
- What are the Asymptotic Notations?
- Explain the Bubble sort algorithm.
- What do you mean by Divide and Conquer algorithms?
- Define the BFS algorithm.
- Define Greedy algorithms and mention some examples of the same.
- What do you mean by a linear search?
- How can you count the leaf nodes of the binary tree?
- What does one mean by the Criteria of Algorithm Analysis?
- Mention the differences between the Singly Linked List and Doubly Linked List data structure?
Java Full Stack Developer Interview Questions
Below listed are ten commonly asked questions in Java full stack developer interviews.
- What do you mean by MVC and MVP? Also, mention the differences.
- Define pair programming.
- How can you enhance a website’s efficiency and scalability?
- What is a RESTful API?
- What do you mean by referential transparency in functional programming?
- Which architectural designs are commonly used to design applications?
- Define long polling.
- Mention the main differences between GraphQL and REST.
- What is a Request Dispatcher?
Java Garbage Collection Interview Questions
Some of the most widely asked Java Garbage Collection interview questions are listed below.
- Mention the key differences between ParNew and DefNew Young Generation Garbage collector.
- Mention the differences between -XX:ParallelGC and -XX:ParallelOldGC?
- How can you monitor garbage collection activities?
- In JVM, does garbage collection occur in permanent generation space?
- Can one force garbage collection in Java?
- Mention the differences between serial and parallel garbage collectors?
- How can you store garbage collection logs in a specified file?
- How can one find total and used memory in Java?
- Define G1 garbage collector.
- What do you mean by ConcurrentMarkSweep Garbage Collector?
Java Scenario Based Interview Questions
For reference, we have listed some commonly asked scenario-based Java interview questions.
- Suppose you have a requirement to generate online reports or feed files by pulling out thousands of historical records from a database. What are the questions that you will ask, and how will you design them?
- You require finding and changing a text from ‘Client’ to ‘Customer’ in 300+ HTML files.
- You must maintain a history of insertion, modification, and deletion to the ‘Customer’ table. What will be your approach to achieving this?
- You need to design an application, which validates data with 100+ rules to comply with the government compliance protocols and tax laws. The given compliance requisites can change, and the application should quickly and easily adapt to changing requirements.
- If one is working with an online trading application, they may want to change the functionality to queue trades placed after hours and duly process them when the trading hours resume. The person may also need to asynchronously handle order statuses sent from the stock exchange like partially filled, rejected, or fully filled, and update the online order information accordingly.
How will you solve this?
- You need to change the logic of a module that several other modules depend on. How will you make the necessary changes without impacting the dependent modules?
Abstract And Interface In Java Interview Questions
Common abstract and interface interview questions in Tier-1 tech companies are as follows:
- In Java, can abstract classes have constructors?
- Create an instance of an abstract class.
- When should you prefer abstract class over interface?
- Can an abstract class contain the main method in Java?
- What is the meaning of a cloneable interface, and how many methods does it consist of?
- Mention the differences between Abstract class and interface.
- How would you define an interface?
- Is it possible for an abstract class to be final in Java?
Exception Handling In Java Interview Questions
Some exception handling java interview questions are given below for your reference.
- Mention the differences between error and exception in Java.
- Is it possible to keep other statements in between try, catch, and finally blocks?
- What do you mean by unreachable catch block error?
- Define run time exceptions in Java along with an example.
- Explain checked and unchecked examples in Java.
- Is it possible to throw an exception manually? If yes, how can you do it?
- Explain exceptions hierarchy in Java.
- Can you write only try block without a catch and finally blocks?
- How does exception handling work in Java?
Tricky Java Interview Questions
Here are some tricky interview questions on Java to boost the inner coding interview ninja are listed below.
- How would a large number of unused import statements affect the performance?
- Mention a scenario where a hotspot will be able to optimize your code?
- Tell us how a decorator design pattern works in I/O classes?
- How can one call one constructor to another constructor in a class?
- Mention the purpose of the intern() method in the String class?
- What happens when an exception is thrown from a finally block?
- Why is it more preferable to use hibernate than JDBC for database interaction?
- How can you make your web application use HTTPS protocol?
How to Prepare for a Java Interview?
Java is an ocean when it comes to its concepts. There are various concepts on which an interviewer can ask questions. Below are some preparation tips that can help:
- Brush up on your fundamentals and learn some basics of Java.
- Practice as many coding questions as possible before the interview. It helps you build confidence.
- Go through some previously asked questions of the company.
- Take mock interviews with a friend or a colleague before appearing for the actual interview. This helps in building confidence and getting constructive feedback.
FAQs on Java Interview Questions
Q. What is the average duration of Java Interviews?
A. Java Interviews mostly range from an average of 45 mins to 1 hour.
Q. What should I prepare for the Java Interview?
A. You can prepare on some topics like Data Structures and Algorithms, Object-Oriented Programming Concepts, Collections, Arrays, etc.
Q. What are the screening questions asked in Java Interviews?
A. Few Questions would be:
- Explain the differences between JDK, JRE, and JVM.
- Does Java use Pointers?
- Explain the process of connecting to a database in Java.
- What is Multithreading?
Q. What are Objects?
A. A Java Object is an instance of Java Class. They are created at runtime and have their own identity, behavior, and state.
Q. What is a constructor?
A. A constructor is a block of code similar to a method called automatically when an object is created.
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