1500+ Java Developer Interview Questions and Answers

Comparator is used to compare objects for sorting, while Comparable is implemented by objects to define their natural ordering.

• Comparator is an external class, while Comparable is implemented by the object itself.

• Comparator can compare objects of different classes, while Comparable can only compare objects of the same class.

• Comparator uses the compare() method, while Comparable uses the compareTo() method.

Implemented Executor service in the backend for parallel processing.

• Implemented ExecutorService in the backend to handle multiple requests concurrently.

• Used ExecutorService to execute multiple tasks in parallel.

• Implemented ExecutorService to improve the performance of the application.

• Used ExecutorService to manage thread pools and execute tasks asynchronously.

Object is an instance of a class while object reference is a variable that holds the memory address of the object.

• Object is a real-world entity while object reference is a pointer to the memory location of the object.

• Object reference variable is a variable that holds the reference of an object.

• Object reference can be null while object cannot be null.

• Multiple object references can refer to the same object.

• Object reference can be reassigned to another object while object cannot...read more

Use CSS to change the background color of a div element.

• Use the 'background-color' property in CSS to specify the color.

• You can use color names, hex codes, RGB values, or HSL values to set the background color.

• Example: div { background-color: blue; }

Microservices communicate with each other through APIs and messaging protocols.

• Microservices use APIs to communicate with each other.

• Messaging protocols like HTTP, AMQP, and MQTT are used for asynchronous communication.

• Service discovery mechanisms like Eureka and Consul are used to locate services.

• API gateways like Zuul and Kong are used to manage API traffic.

• Event-driven architecture is used for real-time communication between services.

We can use composition, aggregation, or association to connect two classes.

• Composition is a strong form of aggregation where the lifetime of the contained object is controlled by the container object.

• Aggregation is a weaker form of composition where the contained object has an independent lifecycle.

• Association is a relationship between two classes where one class uses the functionality of another class.

• Examples of association are dependency, inheritance, and realization.

• Examp...read more

Memory is allocated when an instance of a class is created.

• Memory allocation happens when an object is created using the 'new' keyword.

• Static variables are allocated memory when the class is loaded.

• Memory is released when the object is no longer referenced or when the program terminates.

The interview questions cover a wide range of topics including design patterns, exception handling, Hibernate, MongoDB, Java collections, Spring framework, and microservices.

• Design patterns like markers interface, functional interface, and Singleton pattern are important in Java development.

• Understanding exception handling with try-catch-finally blocks is crucial for handling errors in Java applications.

• Knowing the differences between load and get in Hibernate, as well as the...read more

Local, static, and instance variables differ in their scope and lifetime.

• Local variables are declared inside a method and have a limited scope.

• Static variables belong to the class and are shared among all instances.

• Instance variables are unique to each object and can be accessed using the object reference.

Program for different types of books using Inheritance Concept

• Create a Book class as the parent class

• Create Paperbook, E-book, and Audiobook classes as child classes

• Inherit properties and methods from the Book class

• Add unique properties and methods to each child class

• Example: Paperbook class can have a property for number of pages

• Example: E-book class can have a property for file format

Hashtable is synchronized and does not allow null keys or values, while HashMap is not synchronized and allows null keys and values.

• Hashtable is thread-safe, while HashMap is not.

• Hashtable does not allow null keys or values, while HashMap allows null keys and values.

• Hashtable is slower than HashMap due to synchronization.

• Hashtable is a legacy class, while HashMap is part of the Java Collections Framework.

• Hashtable is recommended to be used in multi-threaded environments, whil...read more

Stored procedures can be called in Java using JDBC API.

• Create a CallableStatement object using Connection.prepareCall() method.

• Set the input parameters using setXXX() methods.

• Execute the stored procedure using execute() or executeUpdate() method.

• Retrieve the output parameters using getXXX() methods.

• Close the CallableStatement object using close() method.

To make a class immutable, we need to ensure that its state cannot be modified after creation.

• Make all fields private and final

• Do not provide any setters

• Ensure that mutable objects are not returned from methods

• Make the class final or use a private constructor

• Consider using defensive copying

Daemon thread is a low priority thread that runs in the background and provides services to other threads.

• Daemon threads are used for tasks that don't require user interaction or input.

• They are automatically terminated when all non-daemon threads have completed.

• Examples include garbage collection, logging, and monitoring.

• They can be created using setDaemon() method.

• Daemon threads should not be used for tasks that require data consistency or integrity.

• They can help in improvin...read more

Session in Java is a way to store information about a user across multiple requests.

• Session is used to maintain stateful information about a user.

• It is created when a user first accesses a web application and remains active until the user logs out or the session times out.

• Session data is stored on the server and can be accessed by multiple requests from the same user.

• It is commonly used for user authentication, shopping carts, and personalization.

• The HttpSession interface in ...read more

This program sorts the characters of a string in descending order.

• Convert the string to a character array

• Sort the character array in descending order

• Convert the sorted character array back to a string

Answering questions related to OOPs concepts, Java8 features, functional interface, Spring Boot annotations, and microservices.

• OOPs concepts include encapsulation, inheritance, abstraction, and polymorphism.

• Java8 features include lambda expressions, streams, default methods, and functional interfaces.

• Functional interface is an interface with only one abstract method and can be used with lambda expressions.

• Spring Boot annotations include @RestController, @RequestMapping, @Auto...read more

Internal partitions of Java Virtual Machine

• Java Virtual Machine has three internal partitions: Heap, Stack, and Method Area

• Heap is used for dynamic memory allocation of objects and arrays

• Stack is used for storing method frames and local variables

• Method Area is used for storing class-level data such as method code and static variables

Implicit objects in JSP servlets are pre-defined objects that can be accessed without any declaration.

• Implicit objects are created by the container and are available to the JSP page.

• Some of the implicit objects are request, response, session, application, out, pageContext, config, exception, etc.

• These objects can be used to perform various operations like accessing request parameters, setting attributes, forwarding requests, etc.

Exception is an event that occurs during the execution of a program and disrupts the normal flow of instructions.

• Exception is a subclass of Throwable class.

• Exception Handling is a mechanism to handle runtime errors and prevent program termination.

• try-catch block is used to handle exceptions.

• Multiple catch blocks can be used to handle different types of exceptions.

• finally block is used to execute code after try-catch block.

• Example: int a = 10/0; will throw ArithmeticException....read more

Using Java stream, find the sum of all odd numbers in a list.

• Use the filter() method to filter out the odd numbers from the list.

• Use the mapToInt() method to convert the filtered numbers to integers.

• Use the sum() method to calculate the sum of the filtered odd numbers.

Entry point for Java programs.

• public: Access modifier that allows the method to be called from anywhere.

• static: Method belongs to the class and not to any instance of the class.

• void: Method does not return any value.

• main: Method name that is recognized by JVM as the entry point for the program.

• String args[]: Command line arguments passed to the program as an array of strings.

Java is a high-level programming language known for its portability and object-oriented programming support. OOPs stands for Object-Oriented Programming. HashMap and HashSet are both data structures in Java, but HashMap is used to store key-value pairs while HashSet is used to store unique elements.

• Java is a high-level programming language used for developing applications and software.

• OOPs (Object-Oriented Programming) is a programming paradigm based on the concept of objects...read more

Dependency injection is a design pattern that allows objects to receive dependencies rather than creating them.

• Dependency injection is a way to achieve loose coupling between objects.

• It allows for easier testing and maintenance of code.

• There are three types of dependency injection: constructor injection, setter injection, and interface injection.

• Inversion of control is a principle that states that the control of object creation and lifecycle should be handed over to a contain...read more

Microservices offer advantages such as scalability, flexibility, and easier maintenance.

• Scalability: Microservices allow for individual components to be scaled independently, making it easier to handle high traffic.

• Flexibility: Microservices can be developed and deployed independently, allowing for faster development and deployment cycles.

• Easier maintenance: Microservices are smaller and more focused, making it easier to identify and fix issues.

• Example: Netflix uses microserv...read more

HashMap is not synchronized and allows null values, while Hashtable is synchronized and does not allow null values.

• HashMap is faster than Hashtable due to lack of synchronization.

• Hashtable is thread-safe while HashMap is not.

• HashMap allows null values for both key and value, while Hashtable does not.

• HashMap is part of the Java Collections Framework while Hashtable is a legacy class.

• HashMap uses Iterator while Hashtable uses Enumeration.

• Example: HashMap map = new HashMap<>(); ...read more

Threads in Java allow concurrent execution of multiple tasks. They can be created in multiple ways.

• Using the Thread class

• Implementing the Runnable interface

• Using the Executor framework

• Using the Callable interface with ExecutorService

The question is about exception handling, OOPs, and the difference between abstraction and encapsulation in a project.

• Exception handling is a mechanism to handle runtime errors and prevent program termination.

• Abstraction is the process of hiding implementation details and showing only necessary information to the user.

• Encapsulation is the process of binding data and methods together in a single unit.

• In a project, abstraction can be achieved by using interfaces and abstract cl...read more