200+ Java Developer Interview Questions and Answers for Freshers

The four pillars of OOPS are Abstraction, Encapsulation, Inheritance, and Polymorphism.

• Abstraction: Hiding implementation details and showing only necessary information.

• Encapsulation: Binding data and methods together to protect data from outside interference.

• Inheritance: Creating new classes from existing ones, inheriting properties and methods.

• Polymorphism: Ability of objects to take on multiple forms or behaviors.

A Spring Boot project can be created to develop an API by setting up a new Spring Boot project, defining API endpoints, and implementing the necessary logic.

• Create a new Spring Boot project using Spring Initializr

• Define API endpoints using @RestController annotation

• Implement the logic to return specified data in the API endpoints

• Use @GetMapping, @PostMapping, @PutMapping, @DeleteMapping annotations to map HTTP methods to controller methods

Use nested loops to print a right-angled triangle of numbers based on input.

• Use two nested loops to control the rows and columns of the triangle.

• Increment the number to be printed in each row.

• Example: If input is 5, the output would be: 1, 12, 123, 1234, 12345

Security management involves implementing measures to protect APIs from unauthorized access and ensure data integrity.

• Implement authentication mechanisms such as OAuth or API keys to control access to APIs

• Use encryption to secure data transmission between clients and APIs

• Implement rate limiting to prevent abuse and protect against denial of service attacks

• Regularly update and patch API security vulnerabilities to stay protected

• Monitor API traffic and logs for suspicious activ...read more

Collection framework in Java is a set of classes and interfaces that provide a way to store and manipulate groups of objects.

• Collection framework provides interfaces like List, Set, and Map for storing and manipulating groups of objects.

• It includes classes like ArrayList, LinkedList, HashSet, and HashMap that implement these interfaces.

• Collections in Java provide methods for adding, removing, and accessing elements in a collection.

• It allows for easy iteration over elements us...read more

No, Set does not allow duplicates.

• Set is a collection that does not allow duplicates.

• If you try to add a duplicate element to a Set, it will not be added.

• You can use a List if you need to allow duplicates.

Yes, duplicates can be added to an array.

• Arrays in Java can contain duplicate elements.

• Duplicate elements can be added to an array using the add() method of ArrayList class.

• For example, int[] arr = {1, 2, 3, 3, 4}; or ArrayList<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 3, 4));

HashMap is a data structure in Java that stores key-value pairs. It is mutable while iterating through it.

• HashMap is part of the Java Collections Framework.

• It allows null values and at most one null key.

• It provides constant-time performance for basic operations like get and put.

• While iterating through a HashMap, it is mutable and can throw ConcurrentModificationException if modified.

• To avoid this, use an Iterator or ConcurrentHashMap.

I prefer Java because of its versatility, strong community support, and extensive libraries.

• Java is a versatile language that can be used for a wide range of applications.

• It has a strong community support with a large number of developers and resources available.

• Java has extensive libraries that provide ready-made solutions for various tasks.

• It offers platform independence, allowing the code to run on different operating systems.

• Java's object-oriented nature promotes code reu...read more

OOP in Java organizes code into objects, enhancing modularity, reusability, and maintainability.

• Encapsulation: Bundling data and methods. Example: A 'Car' class with attributes like 'speed' and methods like 'accelerate()'.

• Inheritance: Reusing code through parent-child relationships. Example: 'ElectricCar' inherits from 'Car', adding features like 'chargeBattery()'.

• Polymorphism: Methods can take many forms. Example: A method 'start()' can behave differently for 'Car' and 'Bike...read more

Encryption and decryption methods can be applied at various layers of the OSI model to secure data transmission.

• Application Layer: Use of SSL/TLS for encrypting HTTP traffic (HTTPS).

• Transport Layer: Implementation of secure protocols like DTLS for UDP traffic.

• Network Layer: IPsec can encrypt data packets for secure communication over IP.

• Data Link Layer: WPA2 encrypts data in wireless networks to protect against eavesdropping.

Data Structures and Algorithms optimize data management and processing, enhancing performance and efficiency in programming tasks.

• Efficient data storage: Arrays, linked lists, and trees help organize data for quick access.

• Optimized searching: Algorithms like binary search reduce time complexity when finding elements in sorted data.

• Improved performance: Using appropriate data structures can significantly speed up operations, e.g., hash tables for fast lookups.

• Resource manageme...read more

Various annotations used in SpringBoot

• 1. @SpringBootApplication: Marks the main class of a Spring Boot application

• 2. @RestController: Marks a class as a RESTful controller

• 3. @RequestMapping: Maps HTTP requests to handler methods

• 4. @Autowired: Injects dependencies automatically

• 5. @Component: Marks a class as a Spring component

• 6. @Service: Marks a class as a service component

• 7. @Repository: Marks a class as a repository component

• 8. @Configuration: Marks a class as a source of b...read more

protected is a Java access modifier that allows access within the same package and subclasses.

• Protected members can be accessed within the same package and subclasses.

• It is used to provide encapsulation and prevent direct access from outside the class.

• Protected members can be accessed using the dot operator.

• Example: protected int num; can be accessed in a subclass using obj.num;

Collections are objects that group multiple elements into a single unit.

• Collections are used to store, retrieve, manipulate and communicate aggregate data.

• Java provides a set of interfaces and classes that implement various types of collections.

• Examples of collections include List, Set, Map, Queue, etc.

• Collections can be used to improve code readability, reusability, and performance.

Program to find numbers divisible by 3 in a string and reverse their indexes.

• Iterate through the string and check if each character is a number divisible by 3.

• Store the divisible numbers in an array and reverse their indexes.

• Join the array back into a string and return the result.

I transitioned from mechanical to IT because of my passion for technology and problem-solving.

• Passion for technology and problem-solving led me to pursue a career in IT

• Enjoyment of coding and creating software applications

• Opportunities for growth and learning in the IT field

• Mechanical background provides a unique perspective and problem-solving skills in IT

• Example: I found coding more fulfilling and challenging than mechanical engineering tasks

Threads are lightweight processes that enable multitasking. Inheritance is a mechanism of creating a new class from an existing class.

• Threads allow multiple tasks to run concurrently within a single program

• Types of threads include user threads, daemon threads, and system threads

• Inheritance allows a new class to inherit properties and methods from an existing class

• Types of inheritance include single inheritance, multiple inheritance, and hierarchical inheritance

Compile time polymorphism is achieved through method overloading, while runtime polymorphism is achieved through method overriding.

• Compile time polymorphism is also known as static polymorphism.

• It is achieved by having multiple methods in the same class with the same name but different parameters.

• Example: method overloading in Java.

• Runtime polymorphism is also known as dynamic polymorphism.

• It is achieved by having a method in a superclass and a method with the same signature ...read more

Java is a widely used programming language known for its platform independence and extensive libraries.

• Java is platform independent, meaning it can run on any operating system

• Java has a large standard library with pre-built classes and methods for common tasks

• Java is object-oriented, allowing for modular and reusable code

• Java supports multithreading, enabling concurrent execution of tasks

• Java has automatic memory management through garbage collection

• Java is widely used in ent...read more

Error is a serious issue that cannot be handled at runtime, while exception is a runtime issue that can be handled.

• Errors are caused by the environment in which the application is running, while exceptions are caused by the application code.

• Errors cannot be caught or handled, while exceptions can be caught and handled using try-catch blocks.

• Examples of errors include OutOfMemoryError and StackOverflowError, while examples of exceptions include NullPointerException and ArrayIn...read more

Yes, we can have multiple try blocks in Java.

• Multiple try blocks can be used to handle different exceptions separately.

• Each try block must have at least one catch or finally block.

• Nested try blocks can also be used.

• Example: try { //code } catch (Exception e) { //code } try { //code } catch (IOException e) { //code }

• Example of nested try blocks: try { try { //code } catch (Exception e) { //code } } catch (Exception e) { //code }

Method overloading is when multiple methods have the same name but different parameters.

• Method overloading allows for more flexibility in method calls.

• The methods must have different parameters, such as different data types or different numbers of parameters.

• Example: public void print(int num) and public void print(String str)

• Overloading constructors is also common in Java.

I have learned Java programming language.

• Java is a widely used programming language for developing various applications.

• It is an object-oriented language known for its platform independence.

• Java is used for developing web, desktop, mobile, and enterprise applications.

• It has a large standard library and supports multithreading, networking, and database connectivity.

• Some examples of Java frameworks are Spring, Hibernate, and JavaFX.

Sonarqube is a static code analysis tool used to detect code smells and bugs in code. It is integrated into CI/CD pipelines for continuous code quality checks.

• Sonarqube is a static code analysis tool that identifies code smells, bugs, and security vulnerabilities in code.

• Code smells are common programming issues that may indicate a deeper problem in the code. Examples include duplicated code, long methods, and complex conditionals.

• Sonarqube can be integrated into CI/CD pipeli...read more

There are multiple ways to create objects in Java.

• Using the 'new' keyword

• Using object cloning

• Using object deserialization

• Using reflection

• Using factory methods

• Using dependency injection frameworks

Authentication and authorization in a Java Spring Boot app involves using Spring Security for managing user access.

• Use Spring Security to handle authentication and authorization

• Configure security settings in the application.properties file

• Implement custom authentication providers if needed

• Use annotations like @PreAuthorize and @Secured for method-level security

Call by value passes a copy of the value while call by reference passes the address of the value.

• Call by value passes a copy of the value while call by reference passes the address of the value.

• In call by value, any changes made to the parameter inside the function do not affect the original value outside the function.

• In call by reference, any changes made to the parameter inside the function affect the original value outside the function.

• Java uses call by value for primitive...read more

Exception handling is the process of handling errors and exceptions that occur during program execution.

• Exceptions are objects that represent errors or exceptional situations that occur during program execution.

• Exception handling involves catching and handling these exceptions to prevent program crashes.

• Java provides try-catch-finally blocks for handling exceptions.

• Examples of exceptions include NullPointerException, ArrayIndexOutOfBoundsException, and FileNotFoundException.

throw is used to explicitly throw an exception while throws is used to declare the exception that a method may throw.

• throw is used to handle exceptional situations in the code

• throws is used to declare the exceptions that a method may throw

• throw is followed by an instance of an exception class

• throws is followed by the list of exceptions that a method may throw

• throw is used inside a method

• throws is used in the method signature

The final keyword in Java is used to restrict the user from modifying the value of a variable, the definition of a method, or the inheritance of a class.

• When applied to a variable, the final keyword makes it a constant that cannot be changed.

• When applied to a method, the final keyword prevents it from being overridden by subclasses.

• When applied to a class, the final keyword prevents it from being extended by other classes.

• Final variables must be initialized and cannot be reas...read more

To rollback a transaction in Spring Boot, use @Transactional annotation and throw an exception

• Use @Transactional annotation on the method where the transaction needs to be rolled back

• Throw an exception within the method to trigger the rollback process

• Spring will automatically rollback the transaction when an exception is thrown

Use reflection to access and test private methods

• Use reflection to access the private method in the test case

• Set the method accessible using setAccessible(true)

• Invoke the method with the necessary parameters and assert the expected result

Checked exceptions are checked at compile time, while unchecked exceptions are not.

• Checked exceptions must be either caught or declared in the method signature using 'throws' keyword

• Unchecked exceptions do not need to be caught or declared

• Examples of checked exceptions: IOException, SQLException

• Examples of unchecked exceptions: NullPointerException, ArrayIndexOutOfBoundsException

Inheritance in Java allows a class to inherit properties and methods from another class.

• Inheritance is achieved using the 'extends' keyword.

• The class that is being inherited from is called the superclass or parent class.

• The class that inherits from the superclass is called the subclass or child class.

• Subclasses can access the public and protected members of the superclass.

• Inheritance promotes code reusability and allows for the creation of hierarchical relationships.

Find duplicate element in array using Java 8 features.

• Convert array to stream using Arrays.stream()

• Use Collectors.groupingBy() to group elements by their occurrence

• Filter the grouped elements to find duplicates

Java and JavaScript are two different programming languages with different use cases.

• Java is used for building server-side applications, Android apps, and enterprise software.

• JavaScript is used for building interactive web applications and front-end development.

• Java is a compiled language while JavaScript is an interpreted language.

• Java is statically typed while JavaScript is dynamically typed.

• Java code runs on the Java Virtual Machine (JVM) while JavaScript code runs on web ...read more

Arrays are fixed in size, while lists can grow dynamically. Lists can hold objects of different types.

• Arrays are of fixed size, while lists can grow dynamically.

• Arrays can hold only elements of the same type, while lists can hold objects of different types.

• Arrays are faster for accessing elements, while lists are faster for adding or removing elements.

• Arrays are implemented as a simple object, while lists are implemented as a linked list.

• Examples: int[] arr = new int[5]; List...read more

The different bean scopes in Spring are singleton, prototype, request, session, and application.

• Singleton scope creates a single instance of a bean per Spring IoC container.

• Prototype scope creates a new instance of a bean every time it is requested.

• Request scope creates a new instance of a bean for each HTTP request.

• Session scope creates a new instance of a bean for each HTTP session.

• Application scope creates a single instance of a bean per ServletContext.

Create a package in Java and call a class from that package in another class using object-oriented programming principles.

• Create a package by using the 'package' keyword at the beginning of the Java file.

• Define a class within the package and create an object of that class in another class.

• Import the package in the class where you want to use the class from the package.

• Access the class from the package using the object created in the other class.

OOPs concept is a programming paradigm based on the concept of objects, Spring Boot is a framework for building Java applications.

• OOPs concept stands for Object-Oriented Programming, which focuses on creating objects that interact with each other to solve problems.

• It involves concepts like inheritance, encapsulation, polymorphism, and abstraction.

• Spring Boot is a framework that simplifies the development of Java applications by providing pre-configured settings and tools.

• It a...read more

Difficulties faced as a Java Developer

• Debugging complex code

• Handling legacy code

• Working with outdated technologies

• Managing project timelines and expectations

• Collaborating with non-technical stakeholders

Use the Strategy design pattern for complex design.

• Strategy pattern allows you to define a family of algorithms, encapsulate each one, and make them interchangeable.

• It helps in separating the behavior of a class from the class itself, making it easier to switch between different algorithms at runtime.

• Examples of using Strategy pattern include implementing different sorting algorithms in a class or defining various payment methods in an e-commerce application.

Creating a number plate using a multidimensional array of strings.

• Create a 2D array of strings to represent the number plate

• Use loops to populate the array with the required characters

• Print the array to display the number plate