Top 51 Core Java Interview Questions and Answers for Freshers & Experienced Developers

Introduction

Java is a programming language and also a platform for computing. Java was first released by Sun Microsystems in 1995. There are lots of applications and websites that will not work unless Java is installed in them. Each day there are more and more applications created using Java. It is a very simple, powerful and robust programming language which provides a fast, secure, and reliable platform to work on. Java is being used everywhere, from laptops to data centers, game consoles to scientific supercomputers, and cell phones to the Internet.

Due to its ease of use and simplicity, Java is used almost everywhere in the IT sector. This means that the professionals who are skilled in Java are always required. If you are someone who is likely to attend a job interview that would test you on your Java skills, then please go through the list of Java interview questions and answers provided herein. You may also refer to them just to add on to your knowledge.

Core Java Interview Questions and Answers cover:

1. Core Java Interview Questions for Freshers
2. Core Java interview Questions for Experienced
3. Core Java Interview Questions for Senior Developers

Core Java Interview Questions and Answers

1. Explain the difference between JDK, JRE, and JVM.
2. What makes Java a platform-independent programming language?
3. Would it be correct to say that Java is not 100% Object-oriented?
4. What is a constructor in Java?
5. Differentiate between a constructor and a method? Can we mark constructors final?
6. What is a class in Java?
7. State the significance of public, private, protected class.
8. What is the final class?
9. What is a wrapper class?
10. What is an abstract class?
11. Describe synchronization with respect to multithreading.
12. What is the purpose of garbage collection in Java, and when is it used?
13. What are the different ways of implementing thread? Which one is more advantageous?
14. What if the main() method is declared as private? What happens when the static modifier is removed from the signature of the main() method?
15. What is the first argument of the String array in the main() method?
16. What is a Java servlet?
17. Describe the Life-cycle of a Java Servlet.
18. Explain the Request Dispatcher?
19. List the steps to connect to a database in java?
20. What do you mean by a JDBC Driver?
21. What is a Java applet?
22. What is numeric promotion?
23. What is false sharing in the context of multi-threading?
24. What are the methods used to implement the key Object in HashMap?
25. What is an immutable object?
26. Differentiate between StringBuffer and StringBuilder in Java?
27. What is the difference between a factory and abstract factory patterns?
28. Differentiate JAR and WAR files
29. What is a JIT compiler?
30. What is the multi-catch block in Java?
31. What is a package in Java?
32.  What is the final keyword in Java?
33.  What are the various types of inheritance in Java?​​​​
34.  What is the most important feature of Java8?
35. What is the difference between path and classpath variables?
36. Can you explain serialization and deserialization?
37. What is a ternary operator?
38. What is platform independence?
39. Which is the latest Java release?
40. What are the differences between C++ and Java?
41. List the features of the Java Programming language?
42. What makes a HashSet different from a TreeSet?
43. What is a package in Java? List down its major advantages.
44. What is the importance of reflection in Java?
45. What is Java String Pool?
46. What is a Map in Java?
47. Why is Inheritance used in Java?
48. Why is Java dynamic?
49. What is a JSP page?
50. What is the difference between Systems?out, System. err, and System. in?
51. Explain the different authentications in Java Servlets.

Java Interview Questions & Answers for Developers

For a Java enthusiast who is looking to attend a Java interview anytime soon, here are the most popular interview questions and answers that might come your way. We have tried to include the top frequently asked Java interview questions as well as their answers to help freshers and experienced professionals with their job interviews. Here are some of the Java interview questions for experienced and freshers.

Q1). Java Interview Questions- Explain the difference between JDK, JRE, and JVM.

Ans:- 

JDK	JRE	JVM
JDK stands for Java Development Kit and is the tool used to compile, package and document Java programs.	JRE stands for Java Runtime Environment and is a runtime environment that is a prerequisite to run java byte-code.	JVM stands for Java Virtual Machine and is a specification that facilitates a run-time environment in which Java’s bytecode can be implemented.

Q2).Java Interview Questions- What makes Java a platform-independent programming language?

Ans:- Java is platform-independent because it provides the feasibility to run and compile the program in one platform and execute the program on any other platform of your choice.

Q3). Java Interview Questions- Would it be correct to say that Java is not 100% Object-oriented?

Ans:- Yes, it would be correct to say that Java is not 100% Object-oriented because it utilizes eight kinds of primitive data types and they are - Boolean, byte, char, int, float, double, long, short. These data types are not objects.

Q4). Java interview question for experience -What is a constructor in Java?

Ans:- A constructor is a block of code that is used in a Java environment to initialize an object. It carries the same name that you had given to the class. A Java constructor is automatically called when an object is created.

Q5). Core Java interview questions for experienced- Differentiate between a constructor and a method? Can we mark constructors final?

Ans:- A constructor constructs the value, by providing data for the object. It is a special type of method that is used to initialize the object. The constructor has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. A method can be invoked using the dot operator and has its own name, and a return type. No, declaring the constructor as final is not possible.

Q6). Core Java interview questions - What is a class in Java?

Ans:- A class, in the environment of Java, are some templates that are used for object creation, and to define certain object data types and their methods.

Q7). Java coding interview questions- State the significance of public, private, protected class.

Ans:- 

• Public: When a class is public, the public class is visible in other packages, the field is visible everywhere.
• Private: Private variables or methods can be used by an instance of the same class only which declares the variable or method. A private feature can be accessed by the class that owns the feature.
• Protected: Protected variable is available to all classes in the same package. It is also available to all subclasses of the class that owns the protected feature. Subclasses that reside in a different package also are provided with access from the class that owns the protected feature.

Q8).Java interview programming - What is the final class?

Ans:- A final class is a constant value of a final variable. Extending A final class is not possible ie., final class may not be subclassed. A final method cannot be overridden when its class is inherited.

Read: Java Developer Role & Responsibilities - Job Description

Q9). Core Java tough interview questions -What is a wrapper class?

Ans:- The classes that “wrap” the primitive data type into an object of that class are called Java wrapper classes.

Core Java interview Questions for Experienced

Here are the Java interview questions and answers for experienced professionals who have gained some experience and are looking to upgrade their career.

Q10). Java questions and answers - What is an abstract class?

Ans:- Abstract serves as a template. In order to use an abstract class, it must be extended or subclassed. An abstract class may not be instantiated. Any class with an abstract method is abstract itself, hence is declared as such.

Q11).Java interview questions and answers for experienced - Describe synchronization with respect to multithreading.

Ans:- Synchronization is the method to control the access of multiple threads to shared resources, with respect to multithreading. One thread can modify a shared variable when not in synchronization even when another thread is in the process of using or updating the same shared variable. This can lead to significant errors.

Q12). Core Java interview questions -What is the purpose of garbage collection in Java, and when is it used?

Ans:- The main purpose of garbage collection is to analyze a program, identify and discard the objects that are no longer needed. It is helpful as the resources can be reclaimed and reused. A Java object is subjected to garbage collection when the object is not reachable to the program in which it is used.

Q13). Core Java interview questions and answers for experienced professionals - What are the different ways of implementing thread? Which one is more advantageous?

Ans:- The thread can be implemented by using runnable interface Inheriting from the Thread class. The use of the Runnable interface is more advantageous because when going for multiple inheritances, the only interface can help.

Q14). Core Java interview questions -What if the main() method is declared as private? What happens when the static modifier is removed from the signature of the main() method?

Ans:- When the main() method is declared as private, the program compiles but during runtime it shows “main() method not public.” Message. When the static modifier is removed from the signature of the main() method, the Program compiles but at runtime throws an error “NoSuchMethodError”.

Q15). Java interview questions and answers -What is the first argument of the String array in the main() method?

Ans:- Unlike in C/C++ where the first element by default is the program name, the string array in main() has no element, the String array is empty.

Q16).Java interview questions - What is a Java servlet?

Ans:- A Servlet in Java refers to the server-side technologies which are used to extend the competence of web servers by giving it support for a dynamic response as well as data persistence.

Read: Java String Functions & Methods with Examples

Q17). Java interview questions - the Life-cycle of a Java Servlet.

Ans:- Here is the life-cycle of a Java Servlet-

• Servlet is loaded
• Servlet is instantiated
• Servlet is destroyed
• Servlet is initialized
• Service the request

Q18). Java interview questions -Explain the Request Dispatcher?

Ans:- Request Dispatcher is a Java interface that is used to forward a request to some other resource which can be HTML, JSP or any other Java servlet within the same application.

Q19). Java interview questions - List the steps to connect to a database in java?

Ans:- Follow the given steps to connect to a database in java-

• Close the Connection
• Register the Drive Class
• Create Connection
• Create Statement
• Execute Queries

Q20). Java interview questions -What do you mean by a JDBC Driver?

Ans:- JDBC Driver in Java refers to a software component that allows Java applications to relate with the database.

There are 4 types of JDBC drivers in Java-

• JDBC-ODBC bridge driver
• Native-API driver (partially java driver)
• Network Protocol driver (fully java driver)
• Thin driver (fully java driver)

Core Java Interview Questions For Senior Developers & Java Interview Questions For 5 Years Experience

Here are the top Java interview questions and answers for experienced senior developers you should study.

Q21). Java Interview Questions -What is a Java applet?

Ans:- The applet is a Java program that is designed for transmitting the Java code over the internet. Ttion works automatically by Java-enabled Web Browser. The applet can respond to the user input immediately as it is dynamically programmed.

Q22). Java Interview Questions - What is numeric promotion?

Ans:- Numeric promotions of a numeric operator are used for the conversion of the operands into a common type. In order to perform calculations easily, numeric promotion, conversion is performed. It is the conversion of a smaller numeric type to a larger numeric type so that integer and floating-point operations can be performed over it. Here byte, char, and short values are converted to int values. The int values are converted to long values, and the long and float values are converted to double values.

Q23). Java Interview Questions -What is false sharing in the context of multi-threading?

Ans:- On multi-core systems, false sharing is one of the well-known performance issues. Here each process has its local cache. When threads on a different processor, modify variables false sharing occurs, that resides on the same cache line as shown in the following images

As the thread may access different global variables completely, false sharing can be hard to detect.

Q24). Java Interview Questions -What are the methods used to implement the key Object in HashMap?

Ans:- Equals and hashcode methods are to be implemented In order to use any object as Key in HashMap, in Java.

Read: What are the Benefits of loading jQuery from CDN

Q25). Java Interview Questions -What is an immutable object?

Ans:- Java classes whose objects cannot be modified once they are created are known as Immutable classes. Any modification of Immutable object results formation of the new object.

Q26). Java Interview Questions -Differentiate between StringBuffer and StringBuilder in Java?

Ans:- The only difference between StringBuffer and StringBuilder is that StringBuffer methods are synchronized while StringBuilder is not synchronized. StringBuilder in Java was introduced in Java 5.

Q27). Java Interview Questions -What is the difference between a factory and abstract factory patterns?

Ans:- Abstract Factory provides one more level of abstraction. Consider different factories each extended from an Abstract Factory and responsible for the creation of different hierarchies of objects based on the type of factory. E.g. Abstract Factory extended by Automobile Factory, User Factory, Role Factory, etc. Each individual factory would be responsible for the creation of objects in that genre.

Q28). Java Interview Questions -Differentiate JAR and WAR files

Ans:- We are going to share the Difference between the JAR & WAR File:

JAR FILES	WAR FILES
Full form of JAR files is Java Archive Files.	Full form of WAR files is Web Archive Files.
Aggregating many files into one is allowed in JAR files	XML, Java classes, and JavaServer Pages are stored in WAR
The JAR is usually used to hold Java classes in a library.	Mainly used for Web Application purposes.`

Q29). Java Interview Questions -What is a JIT compiler?

Ans:- Just-In-Time(JIT) compiler is used to improve the performance. JIT compiles parts of the bytecode that has similar functionality which in turn reduces the amount of time needed for compilation. The term “compiler” here refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

Q30). Java Interview Questions -What is the multi-catch block in Java?

Ans:- Multi-catch blocks make the code shorter and cleaner when every catch block has a similar code. We can catch multiple exceptions in a single catch block using this feature.

Q31). Java interview questions - What is a package in Java? 

Ans:- The collection of related classes and interfaces grouped together is Package in Java. It enables developers to easily modularize codes. Packages help in providing easier access control on the code.

Q32). Java interview questions - What is the final keyword in Java?

Ans:- Final is a special keyword in Java being used as a non-access modifier. It is used in various contexts like Final variables, Final Methods, and Final Class. Final class refers to a class being declared as final in Java. 

Q33). Java interview questions - What are the various types of inheritance in Java?

Ans:- Java has four types of inheritance namely:

• Single inheritance - Herein one class inherits the properties of another class.
• Multilevel inheritance - A class having more than one parent class but at different levels is termed as Multilevel inheritance.
• Hierarchical inheritance - When a class has more than one child class it is termed as hierarchical inheritance.
• Hybrid inheritance - It’s a combination of two or more types of inheritance.

Read:https://www.janbasktraining.com/blog/Java-programmer-resume-template/

Q34). Java interview questions - What is the most important feature of Java8?

Ans:- The most important feature of Java 8 is its functional interface and static methods. When adding a functional interface to the object-oriented interface, programming becomes less complicated and more efficient.

Q35). Java interview questions - What is the difference between path and classpath variables?

Ans:- Classpath is related to Java executables, used to locate class files. Path, on the other hand, exists in an operating system used to locate executables.

Q36).  Java interview questions - Can you explain serialization and deserialization?

Ans:- Both serialization and deserialization are crucial to stream handling in Java. Through serialization, you convert an object into a byte stream, and through deserialization, you simply undo the process.

Q37). Java interview questions - What is a ternary operator?

Ans:- A ternary operator is a conditional operator in Java used to decide what values to be assigned to a variable.

Read:https://www.janbasktraining.com/blog/how-long-does-it-take-to-learn-Java/

Q38). Java interview questions - What is platform independence?

Ans:- Platform independence is when you design a program that runs on any operating system making things more accessible. This is the reason why Java is ahead of other programming languages.

Q39). Java interview questions - Which is the latest Java release?

Ans:- As of Sep 2021, Java 17 is the latest release. Also, it is the next long-term support version (LTS) after Java 11.

These are some of the most asked interview questions you should prepare for your next Java Interview. Read our next section for the most essential interview tips.

Q40). Basic interview questions for Java - What are the differences between C++ and Java?

Ans:- C++ is not platform-independent; the main concept behind it, “write once, compile anywhere.”  Since the byte code generated by the Java compiler is platform-independent, it can run on any machine, Java programs are written once and run everywhere.

Most of the high-level languages are compatible with C++ but with Java they are incompatible.

Q41). Core Java interview questions for experienced - List the features of the Java Programming language?

Ans:- Some of the significant features of Java Programming Language are:

• Java is an easy-to-learn language. The Fundamental Concept of OOP Java has a catch to understand. 
• The secured feature of Java helps develop a virus-free and tamper-free system for the users.
• It is an OOP or Object-Oriented Programming language. OOP signifies that everything is considered an object in this language.
•  This programming language is not compiled into a platform-specific machine; instead, it is compiled into platform-independent bytecode. 

Q42). Core Java interview questions for experienced - What makes a HashSet different from a TreeSet?

Ans:- Here is the difference:

HashSet:  It is implemented through a hash table and permits the null object. 

TreeSet:  implements SortedSet Interface that uses trees for storing data. 

HashSet: It permits the null object.

TreeSet: It does not allow null objects.  

HashSet:  It does not maintain elements in an ordered way.  

TreeSet: It maintains elements in a sorted way.  

Q43). Core Java interview questions for experienced: What is a package in Java? List down its major advantages.

Ans:- It is a collection of related classes and interfaces which are bundled together. It allows developers to easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused.

• Packages are helpful in avoiding name clashes 
• They provide easier access control on the code 
• They can also carry hidden classes that are not visible to the outer classes 

Q44). Core Java interview questions for experienced: What is the importance of reflection in Java?

Ans:- It is a runtime API used to inspect and change the behavior of methods, classes, and interfaces. Java Reflection is a powerful tool that allows you to analyze classes, interfaces, fields, and methods during runtime without knowing what they are called at compile time. It also helps in creating new objects, call methods, and getting/set field values. 

Q45). Java programming questions and answers: What is Java String Pool?

Ans:- A collection of Strings stored in heap memory is called Java String Pool. Whenever a new object is formed, first of all, the String pool identifies whether the object is already present in the pool or not. If it is present, then the same reference is returned to the variable; otherwise, a new object is created.

Q46). Core Java interview questions and answers for experienced: What is a Map in Java? 

Ans:- A map in Java is an interface of the Util package that maps unique keys to values. The Map interface is not a subset of the main Collection interface and hence it performs a little differently from the other collection types.

Q47). Java interview questions for experienced: Why is Inheritance used in Java?

Ans:- Inheritance in Java allows for the reuse of code

Without using inheritance, runtime polymorphism cannot be achieved

It also provides data hiding

Method overriding is not possible without inheritance. 

Q48). Core Java interview questions for experienced: Why is Java dynamic? 

Ans:- Java is dynamic in nature, it is designed to adapt to an evolving environment. It includes a large amount of runtime information that is used to solve access to objects in real-time.

Q49). Core Java interview questions for experienced: What is a JSP page? 

Ans:- JSP stands for Java Servlet Page. The JSP page contains two types of text- Static Data and JSP elements.

Q50). Java interview questions and answers: What is the difference between systems.out, System. err, and System. in?  

Ans:- System. out and System. err both represent the monitor by default and so you can use both to send data to the monitor. 

System. out is used to display normal messages and results, System.eerr for error messages, and System.in represents InputStream object which by default represents standard input device, i.e., keyboard.

Q51). Core Java interview questions for experienced: Explain the different authentications in Java Servlets.

Ans:- These are the four different options for authentication in servlet: 

Basic Authentication: For basic authentication, the client provides usernames and passwords to authenticate the user. 

Form-based authentication: In this, the login form is made by the programmer by using HTML. 

Digest Authentication: It is like basic authentication, the only difference is that here the passwords are encrypted using the Hash formula to make it more secure. 

Client certificate Authentication: For client certificate authentication, it needs that each client accessing the resource has a certificate that it sends to authenticate itself. 

5 Essential Tips For Your Java Interview Preparation

• Java is undoubtedly the most fast-growing and in-demand programming language and many of the product-based companies like Facebook, Amazon, Google, Microsoft look for the best talent when hiring Java experts. Here’s how I may prepare for our next Java interview:
• Prepare all the essential topics like Java fundamentals, Data structure & algorithms, object-oriented programming & concept, Java collection framework, data structures, Java best practices, XML processing in Java, Garbage collection, SOLID design principles, Java IO and NIO JDBC, JUnit, to mention a few. Check the complete curriculum.
• Check out questions asked by top companies like Google, Morgan Stanley to brush up your knowledge.
• Enroll in an online certification course to get the foundation and advanced knowledge of Java along with preparation-related guidance.
• Look for the best online Java tutorial to practice what you have learned. 
• Have command over the commonly used technologies like spring framework, JPA, and hibernate. 

Read: https://www.janbasktraining.com/blog/Java-developer-role/

We are sure that these 39 Java interview questions and answers and our preparation tips will help you put your best foot forward. 

Conclusion!

Java is a very popular programming language that can be found in the technology stack of every company. No matter if you are heading for the Java developer interview or full-stack developer interview, knowing Interview Questions for Java with their answers is important for you to give your best and get hired in your dream company.

These are core Java interview questions for experienced and entry-level candidates that are often asked on repeat, so prepare them and give your best shot in an upcoming interview.

In case you are looking for complete preparation of Java Career, get along and explore our Java Training course online, led by real-programmers with real-time industry projects, along with complete resume feedback and impactful preparation of java interview questions and answers for experienced & freshers.

Let us know in the comments below if you have any queries or questions related to these core Java interview questions for experienced & beginners!

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