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40 Frequently asked Advanced Java Interview Questions

Java is a high-level, object-oriented, and robust programming language. It is also a platform that happens to be a hardware or software domain that helps software run a program. The language was developed at Sun Microsystems by James Arthur Gosling, also known as Dr Java, in 1995. Java was previously known as Oak, but due to Oak already being a registered name in the market, James Gosling changed it to Java. 

Java, albeit, was not the first computer programming language to be developed, it has, however, changed the way the world saw computer programming. It opened new territories and furthered tremendous development in the field. Java was the first language to use the Unicode in the local spring type and the source code. 

Advanced Java is the language version that deals with standard topics such as networking, servlet, and Web-services. Advanced Java is primarily used to develop Web-based or enterprise applications.

Programming interview questions are an integral part of a software analyst or a programmer’s professional interview. When you apply for a job, you are expected to acquire a familiarity with the work and the language you are about to apply for.

The Java Programming Interview has become a common procedure to include both basic and advanced level questions instead of relying solely on theoretical knowledge.

In this article, we will be discussing the advanced java interview questions. 

Advanced Java Interview Questions

1. What are the Variable Arguments (Varargs) in Java?

This is a frequently asked advanced java interview question.

Varargs is the acronym for variable-length arguments. JDK 5 has featured a component that clarifies the formation of the methods required to take variable numbers of arguments. The Varargs allows the Method to accept multiple or zero arguments. Prior to the usage of the Varargs, either an overloaded method was used, or an array method was taken as a parameter, which was not considered suitable due to the maintenance issues. In case the user has no idea about the number of the required arguments to pass in the Method, Vargas is deemed to be a better option. 

2. What Is a Servlet, and where is it needed?

A Servlet is a type of Java Programming Language required to further the proficiency of a server hosting applications run by the request/response programming model. Servlets are essentially used to work on web servers. However, they can function and respond to any requests on the web. Servlets devise the controller section of the MVC design pattern with the management of requests/responses by Java Server Pages (JSPs).

Servlets are used at different levels for their respective frameworks. 

The servlets can be used to accept input while simultaneously generating the HTML pages on the web. 

It is used in the enterprise networks to connect the Standard Query Language database (SQL) via JDBC.

However, with the growing popularity of Model-View-Controller (MVC) frameworks such as Strut, Spring, etc., the servlet is slowly losing its monopoly in the industry due to the variety, and potential the frameworks provide. 

3. What is the Struts Framework in Java? 

For this Java programming interview question, you could answer;

Struts, made by Apache, is an open-sourced framework, and the company provides an easy and free source to install the applications for its users. Struts replicate the Model-View-Controller patterns, with MVC offering three models. The usage of the application is to develop the web application software based on the servlets and Java Server Program. Struts provides two versions, the former of which is scarcely used because the second version offers various additional features. 



4. How would you explain synchronization in terms of multithreading? 

Synchronization, with respect to multithreading, is the ability to manage and control resource accessibility by thread requests. The objective of synchronization is to keep threads from intercession. Whenever more than one thread is expected to access a shared resource, it must be assured that it uses only one resource at that given time. With no proper synchronization, one thread can reorganize a particular variable from the other thread while the latter is under the process of updating or working on a similar variable as the former one. This can result in significant errors in the program. 

5. How would you define a Static Initializer Block? 

This is a frequently asked advanced java interview question.

A Static Initializer Block is basically a block that runs prior to the main( ) Method. In Java, initializer blocks can be used to initialize instance variables. A simple form of Static Initializer is:




When a static field or a static method is used for the first time, every static initializer in the class is executed, considering an instance has already not been created for the given class. 

The example of the class with Static Initializer is as follows:

Class StaticInit


Public static int x;


6. Does Java pass by reference or pass by value? 

This is an important advanced java interview question.

Java is a pass by value. While dealing with the objects, the user is dealing with the object-handles known as a reference. It goes the following way:

Public static void main(String[] args) {

    Dog aDog = new Dog(“Max”);

    Dog oldDog = aDog;

    // we pass the object to

Pass by reference stands for passing address instead of passing the value, while passing the value means passing the copy of the value that is to be passed. 

7. How would you differentiate between ArrayList and Vector?










1Because of not being synchronous, ArrayList is fast.



During the multithreading process, the vector holds the threads in their runnable or the contrary state until the thread under work releases the object’s lock.



2ArrayList is not synchronized, which means that multiple threads can function in ArrayList simultaneously.



Vector is synchronized; hence one thread at a given time can work on the code.



3ArrayList is a new and fast application, and it is preferred to vector.



Vector has been considered outdated due to the lack of certain features that ArrayList hosts.



4Because of being unsynchronized, ArrayList is not considered thread-safe.



In the case of multithreading, the vector is a preferred choice because only one thread at a time functions.



5ArrayList is not a legacy class.Vector is a legacy class.



8. What is an Iterator? 

This is a frequently asked advanced java interview question.

In a computer programming language, an Iterator is an object that allows a user to negotiate with a grouping of a variable number of data items. It is an object that one can loop through collections such as ArrayList and HashSet. 

9. What are Access Modifiers in Java, and how many types are there? 

For this java interview question, you could answer;

As the name suggests, Access Modifiers in Java are used to hamper a class range, field, constructor, variable, and method. Applying the access modifier, one can decide the access level of domains, constructors, and methods. 

There are four types of Access Modifiers. 

  • Private 
  • Default 
  • Protected 
  • Public 

To understand Access Modifier in simple terms, the following table is to be addressed. 

Access Modifier















Within Class



Within Package



Outside Package by Subclass



Outside Package




10. What is the difference between Core Java and Advanced Java?

Core Java Core Java
Core Java is used to build general applications.



Advanced Java is used to develop mobile applications and websites.



Core Java is the basic building block of the Java Programming Language.



Advanced Java deals with the specifications including the web, networking, and



OOP, data types, operators, functions, loops, and threading are its elements



Other than the basics, advanced Java has the specifics like servlets, web services etc.



Core Java deals with Swings, socket, awt, thread concept, etc.It is used for web-based applications and software.



11. How would you differentiate between Map and HashMap? 

This is an important advanced java interview question.

A map is an interface describing the functioning of the associative array. It is a broader term and is used to manage the elements. It can be used only with an implementing class such as HashMap, TreeMap, Hashtable, ConcurrentHashMap, and Linked HashMap. Whereas a HashMap is a class and is an implementation of the map. Hashmap allows a programmer to save null keys and null values.

12. What do you understand about Static Keywords in Java? 

This is a frequently asked advanced java interview question.

The static keyword is a modifier in Java suitable for block, variable, Method, nested class. If a member (block, variable, Method, nested class) is declared static, it is accessible prior to the formation of any object in its class without referring to an object. The primary use of static Java is the management of the memory.

13. Why is the difference between Java Heap Space and Stack Memory? 

For the efficient running of the application, memory is divided into two parts, known as stack and heap memory. Whenever new variables, objects, strings are declared, or one calls a new method, these memories are assigned to these functions either from Stack Memory or Heap Space.

The primary use of Stack Memory is for the execution of a thread and allocating static memory. It contains the temporary method-specific value and can be alluded to other specific values and objects that are referred from the Method.

Heap space in Java is used by java runtime dynamics for memory allocation of Java objects and JRE classes. Whenever a new object is created, it is created in heap space, and the references for the objects are saved in the stack memory.

14. What is the difference between Instance Variable and a Local Variable? 

Instance Variable

An instance variable in Java is a variable declared outside a method but in class. There is no limitation to the instance variable. These variables pronounce the functions of an object while remaining bound to it at all costs. All the things in the class get their copy of the utilization, and whenever a change is made in the variables, only the instance is affected by it.

Local Variable

On the other hand, a local variable is a variable defined within a method or a constructor. It is declared inside a method with a specific block in which the variable is legalized. The usage of the local variable is limited only to the block scope. In case of a variable being declared inside the Method, the other class methods do not keep any information regarding the local variable. 

This is an important advanced java interview question.

Just-in-Time compiler (JIT) is a part of JVM used to enhance the performance of the program. It compiles the parts of the bytecode with similar functionality at the same time, thus magnificently reducing the time needed for compilation. The JIT compiler is a translator of the source code to the machine-friendly code that can be executed without any hassle. 

15. What are the drawbacks of JIT? 

Just-in-Time compiler escalates the level of complexity as it adds another layer of the program, which is harder to understand for a programmer. 

The lack of an inline assembler is one of the major drawbacks of the Just-in-Time compiler. 

16. What is the difference between an interface class and an abstract class? 

For this java interview question, could answer;

An interface is a contract, whereas, on the contrary, an abstract class is a class. Since one has to look up what to do, the abstract class is more expensive than the two. Implementation of an interface class requires little of the CPU.

17. What are objects and a class in Java? 

This is an important advanced java interview question.

Java is an object-oriented language and supports several basic concepts, including class and objects. Objects are the elements that have behaviour and a state. Quite like the real world, the object is defined by its physical attributes. Class, in Java, is a layout that creates individual objects. A class can contain either Local Variables, Instance Variables, or Class Variables.

18. What does the term ‘final’ mean in regards to a method? 

When you include ‘final’ at the end of a method, it can no longer be hidden or extended by the Subclass. It is used to stop an unprecedented exploit from a subclass that could be essential for the consistency of the class.

19. How does Method defer from Constructor? 

A method can have a user-defined name, while a constructor can only be a class name. This is because a compiler does not provide a method, whereas, in the absence of a constructor, the Java compiler provides a default constructor. Another difference between Method and Constructor is that Method should have a result type while Constructor should not have a result type, not even avoid.

20. What is a default constructor? 

If a constructor never takes any parameter, a constructor is known as a default constructor. In the case of a class not containing a minimal one user-defined Constructor, the system, while compiling, creates a default constructor. The primary task of a default constructor, depending on its type, is to provide a default value to the object such as null, 0, 0, etc.

21. What is List and Set in Java?

A list is a tool that manages the ordered collection in Java. It consists of the index-based system to remove, install, search, and update an element. It can contain a duplicate component as well, and null elements can also be stored in the List.

It is a disarrayed collection of unique elements and does not have any duplicate elements or objects. The elements in a set do not follow any particular order. They are added to set spontaneously from the programmers’ side, and it is not deemed very relevant to monitor them.

22. What is marshalling and unmarshalling in Java? 

This is an important advanced java interview question.

Marshalling is similar to serialization, where an object’s memory representation is transformed into a data format ideal for transference or storage. It is usually brought into use whenever data is to be transferred from one program to another or separate parts of the computer program. Since marshalling uses objects instead of primitive objects, it rationalizes the complicated communication in the program. 

Unmarshalling or demarshalling is the reverse process of marshalling. It uses an arrayed object and turns it into an internal data structure, which is considered to be executable.

23. Explain double brace initialization in Java. 

Double brace initialization in Java is a blend of two separate processes. Two braces ({ ) are involved in the process, and the usage of two curly braces, consecutively indicates the use of double brace initialization. This is used to initialize collections. 

24. How is synchronized block different from synchronized Method?

For this advanced java interview question, you could answer;

When threads communicate accessing the fields, although the communication is efficient, it can cause two errors. Thread interference and memory consistency error. A healthy number of synchronization constructs is required to prevent these errors. Method synchronization allows a specific policy to stop the thread interference and memory consistency errors. In the case of an object being visible to more than one thread, all the necessary changes are brought in by the synchronized Method.

If only a handful of succeeding lines of a code is to be executed, one should synchronize the block of the code. 

25. How do you create an immutable class in Java?

An Immutable class is that the contents of an object cannot be changed once it has been created. Wrapper classes such as Integer, Boolean, Byte, Short etc., are Immutable. One can create their own immutable class. 

26. What are objects and classes in Java? 

This is an important advanced java interview question.

Objects are the fundamental entities of the language and are defined by real-life objects. A java program creates objects which interact by putting the effect into the methods. Objects consist of State, Behaviour, and Identity. On the other hand, a class is a user-defined prototype that creates the objects. It is representative of the common methods and properties in all similar objects.

27. What is garbage collection, and where is it required in Java? 

For this Java programming interview question, you could answer;

Garbage Collection in Java provides many conflicting views with both certainty and uncertainty involved. The java applications collect the objects in memory depending on their requirements. The process of garbage collection requires the Java virtual machine to determine which memory is not being used by the java application and recycle it for other uses. Two types of garbage collections activities are there:

A minor or incremental garbage collection

A major or full garbage collection

Along with this, there are a total of four ways of conducting garbage collection activities:



Concurrent Mark Sweep (CMS)


One of the major importances of garbage collection is that it deletes the unreferenced objects from the heap memory in the application. Moreover, since it is an automatic process, the users do not have to make extra efforts to run garbage collections. 

28. What is the reason char [] preferred over String for passwords?

Strings are unyielding and Immutable, which means if a further process adversely affects the memory, unless garbage collection kicks in, the data in the program cannot be overridden or destroyed. At the same time, a character array (char[]) can be easily wiped out by turning the setting to zero. A character array can be overridden by user-defined data. It is for security reasons that char[] is preferred to string when passwords are concerned. 

29. What is the importance of return type in Java?

This is an important advanced java interview question.

The return type is used as a reserve in Java and is used to exit a method whether it is with a value or without it. Its primary purpose is to complete the execution of a method, and its value is dependent on the method return type. For example, an INT method would always return an integer value. 

It is used in two types. 

Methods returning a value. 

Method not returning a value. 

30. What is a socket in Java, and how many types are there? 

For this java interview question, could answer;

A server works on a system and consists of a socket bound to a certain port number. The server answers the call of the user, as the user requests for the connection to be established. By definition, a socket is a finishing point (or endpoint) of a two-way communication link lying between two programs available on a particular network. A socket is tied to a port number to let the TCP layer identify the application that is expected to receive the data. 

There are three types of sockets that Java provides. First, Connection-oriented (TCP) is used with the Socket class. Connectionless sockets (UDP) work with the Datagramsocket class. The third sort is the Multicast socket class which happens to be the subordinate class of Datagramsocket class. 

31. Can you mention the advantages and drawbacks of sockets in Java? 

Advantages of Sockets in Java: Sockets are flexible and easily implementable for extensive communications. They are better in terms of providing low network traffic contrary to the HTML forms and CGI scripts that prompt and shift the entity of the web pages for every new request.

Drawbacks of Sockets: The communication based on the socket allows to send chunks of raw data between applications on the computer. The client and server systems both need to allocate appliances to ensure the usefulness of the data in any way possible. 

32. What is a Singleton design pattern in Java, and what is the most efficient way to implement it?

A Singleton design pattern assures that only single instances should be formed, and shingle objects should be created in all the available classes. Basically, a Singleton design pattern in Java defines a class with a single instance and provides it with universal access. Singleton design pattern is available in two types – Early Instantiation and Lazy Instantiation. 

Because an object is not created with each request and a single instance is used again and again, the Singleton design pattern saves up a lot of memory and time. 

33. How to create/implement Singleton design pattern in Java:

Static member: Static Member uses the memory once due to static, and it consists of the instance of the Singleton class.

Private Constructor: Private Constructors stops instantiating the Singleton class from outside the class.

34. What is the difference between Soft Reference and Weak Reference in Java?

Weak Reference

A weak reference, in simple words, is a reference that is not powerful enough to compel a memory to stay in the memory. If garbage collection finds an object that is weakly reachable, it will clear the object immediately. Weak reference allows the user to grasp the garbage collector’s ability to define the accessibility for the user, so the user does not have to do it himself. Following is the way a weak reference is created:

Soft Reference:

A soft reference is similar to a weak reference; however, it does not get rid of the object referred to. An object weakly reachable will be disposed of by the further garbage collection activity, whereas a softly reachable object will remain stuck for a more extended period of time.

Both softly reachable and weakly reachable objects are not expected to behave in a similar fashion, and the later objects are held onto until the memory is sloppy enough.

35. What is dynamic method dispatch or Runtime Polymorphism in Java? 

Method overriding is one of the features that allow Java to support Runtime Polymorphism. Dynamic method dispatch is a technique that enables a call to a superseded method to be reconciled at the runtime instead of compiled time. At the runtime, it is the type of the object and not the type of the reference variable that decides the version of the overridden Method to be executed. When a superclass reference is used to call through the overridden Method, Java conjectures the version of the Method that is to be executed depending upon the kind of object being referred to.

36. What are Spring Beans in Java? 

The Spring Framework in Java is an application framework for Java. Its features are used for the applications, but it is also used to build Web applications and Web services as an extension. The objects forming the foundation of the application and handled by the Spring IoC container are known as Spring Beans. Bean, in other words, is amongst the various objects in the application.

37. Despite the pointers being popularly used in C and C++, why does Java not use the pointers? 

Pointers are slightly complicated and quite risky to use for beginners. Java dominantly emphasizes the simplicity of the coding. Using pointers can prompt it into a challenging mode considering its chances of error. Unregulated usage of the pointers might also cause unprecedented errors, consuming time. Furthermore, security is also a major issue with the pointers since with the help of the pointers. A user can access the memory without any navigation.

Therefore, using the pointers is avoided in Java. Also, using the pointers adversely impacts the process of garbage collection rendering it slow and faulty to a great degree. Since references cannot be manipulated, Java makes sure to include them in its functions.

38. What is Data encapsulation in Java? 

Data encapsulation is also known as data hiding. In this mechanism, the details of a class are concealed from the user. The user can only work on a sparse set of the available functions on the hidden members of the given class by executing the singular operatives usually known as methods. It is used to build data and operate on it l, encapsulating it into a single unit. A number of programming languages use encapsulation as a class. It can also be referred to as the technique of restraining access to some of the data. Encapsulation is also used to hide the data members and data functions associated with an object or a class. 

39. What is the Super Keyword, and when do you use it?

Super Keyword is a superclass object which is used to call the superclass methods and access the superclass constructor. Thus, it removed the clutter and confusion from superclass and Subclass with similar names. It is mainly used to access the abrogated methods and hidden fields or aspects of the parent class.

Super Keyword can be used in the following cases:

Acquiring the data members of the parent class when the essential details, including the names of the class and its child subclasses, are the same.

To call the default and stipulate the Constructor of the parent class within the child class.

Accessing the parent class methods along with the child classes having revoked them.

40. What is the importance of constructors in Java? 

A constructor in Java is a cluster of code resembling a method that is automatically called whenever the programmer creates an instance of an object. It is used for object initialization. A constructor has no return, and it is not required to call Constructor manually. Constructors are used to initialize the object with the initial state. One of the most important features of the Constructor is that it apprises the user of the dependencies, which means, with the help of the Constructor, one can request the user of the other class for the necessary dependencies.


Java has been eternally contributing to the computer and software development program. Not only does it require you to learn its basics, but also to master its advanced form, which allows you to practice it in real-time. Java is one of the most important and effective computer programming languages needed for the development of web-based applications. It’s easy syntax, platform, and secure environment, along with the impressive built-in features, contribute tremendously to its rigorously growing popularity. The language has evolved over time, and its popularity comes from the fact that it is a most practical computer programming language that has outperformed its predecessors magnificently. 

More resources : Java Technical Architect Interview questions | Java Interview questions and answers for 2 to 3 years experience | 10 frequently asked Advanced Java Interview Questions | Javascript Interview Questions and Answers

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