1500+ Java Developer Interview Questions and Answers

Immutable classes have advantages like thread safety, security, and simplicity, but also have disadvantages like memory usage and performance overhead.

• Advantages of immutable classes:

• - Thread safety: Immutable objects can be safely shared among multiple threads without the need for synchronization.

• - Security: Immutable objects cannot be modified, which can prevent unauthorized changes to sensitive data.

• - Simplicity: Immutable objects are easier to reason about and debug since...read more

Java certification is not mandatory for a Java Developer role.

• Certification is not a measure of practical skills and experience.

• Experience and skills are more important than certification.

• Certification can be helpful in some cases, but not mandatory.

• Certification can be expensive and time-consuming.

• Employers may value certification differently.

• Examples of Java certifications: Oracle Certified Professional Java SE 11 Developer, Java SE 8 Programmer, etc.

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.

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

To change a div background color in Java, use the setBackground() method and pass the desired color as a parameter.

• Create a reference to the div element using its id or class

• Use the setBackground() method to set the background color

• Pass the desired color as a parameter to the setBackground() method

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

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

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

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