1. What is Java?
Java is a high-level, object-oriented programming language used for developing various applications, including web and enterprise-level applications.

2. What is the difference between JDK, JRE, and JVM?
– JDK (Java Development Kit) is a software development environment that provides tools for developing, debugging, and running Java applications.
– JRE (Java Runtime Environment) is the runtime environment where Java programs are executed.
– JVM (Java Virtual Machine) is a virtual machine that executes Java bytecode.

3. Explain the difference between an interface and an abstract class?
– An interface is a collection of abstract methods, which can be implemented by any class. It provides a contract for classes to follow.
– An abstract class is a class that cannot be instantiated and can contain both abstract and non-abstract methods. It serves as a base class for other classes.

4. What is the difference between equals() and == in Java?
– The ‘==’ operator is used to compare the reference equality of two objects.
– The equals() method is used to compare the content or value equality of two objects. It is overridden from the Object class by most of the Java classes.

5. What is a constructor in Java?
A constructor is a special method that is used to initialize an object of a class. It is called automatically when an object is created.

6. What is method overloading in Java?
Method overloading is the ability of a class to have multiple methods with the same name but different parameters. The method to be called is determined based on the parameters passed during the method call.

7. What is method overriding in Java?
Method overriding is the ability of a subclass to provide a specific implementation of a method that is already defined in its superclass. The method in the superclass is overridden with the same name and signature in the subclass.

8. What are the access modifiers in Java?
Java provides four types of access modifiers:
– public: accessible to all classes and packages.
– private: accessible only within the same class.
– protected: accessible within the same class and its subclasses, as well as within the same package.
– default (no modifier): accessible within the same package only.

9. What is the final keyword in Java?
The final keyword is used to make a variable, method, or class unchangeable or non-overridable. If a variable is declared as final, its value cannot be changed. If a method is declared as final, it cannot be overridden in the subclasses. If a class is declared as final, it cannot be inherited.

10. What are the static and non-static variables in Java?
– Static variables: Variables that are declared as static belong to the class and are shared among all instances of the class. They can be accessed using the class name.
– Non-static variables: Variables that are not declared as static belong to individual instances or objects of the class. Each instance has its own copy of non-static variables.

11. What are the different types of loops in Java?
Java provides three types of loops:
– for loop: It is used to execute a set of statements repeatedly, for a specific number of times.
– while loop: It is used to execute a set of statements repeatedly, as long as a condition is true.
– do-while loop: It is similar to the while loop, but the loop body is executed at least once before the condition is checked.

12. What is exception handling in Java?
Exception handling is a mechanism used to handle and recover from errors or exceptional conditions that occur during the execution of a program. It involves the use of try-catch blocks to catch and handle exceptions.

13. What is the difference between checked and unchecked exceptions?
– Checked exceptions: Checked exceptions are checked at compile-time. The compiler checks if the code handles or declares the checked exceptions using try-catch or throws clauses.
– Unchecked exceptions: Unchecked exceptions are not checked at compile-time. They occur at runtime and do not require explicit handling or declaration.

14. What are constructors and destructors in Java?
– Constructor: A constructor is a special method that is called when an object is created. It is used to initialize the object’s state.
– Destructor: In Java, there is no explicit destructor like in C++. Java garbage collector automatically handles the memory management, and objects are destroyed when they are no longer reachable.

15. What is the difference between a static method and an instance method?
– Static method: A static method belongs to the class rather than an instance of the class. It can be accessed using the class name without creating an object of the class.
– Instance method: An instance method belongs to an individual instance or object of the class. It can only be accessed using an object of the class.

16. What is multithreading in Java?
Multithreading is a feature of Java that allows the concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of the program is called a thread, and they run independently of each other.

17. What is the use of the ‘synchronized’ keyword in Java?
The ‘synchronized’ keyword is used to acquire a lock on an object, allowing only one thread to execute a synchronized block of code on the object at a time. It ensures thread safety in concurrent programming.

18. What is the difference between serialization and deserialization in Java?
– Serialization: Serialization is the process of converting an object into a byte stream, which can be stored in a file or transmitted over a network.
– Deserialization: Deserialization is the process of reconstructing an object from its serialized byte stream.

19. What is JDBC in Java?
JDBC (Java Database Connectivity) is an API that allows Java programs to interact with relational databases. It provides methods for executing SQL queries, retrieving and manipulating data, and handling transactions.

20. What is the difference between GET and POST methods in HTTP?
– GET method: It is used to request data from a specified resource. It sends the data in the URL query parameters.
– POST method: It is used to submit data to be processed by a specified resource. It sends the data in the body of the HTTP request.

1. What is the difference between Java and J2EE?
Java is a general-purpose programming language, while J2EE (Java 2 Enterprise Edition) is a platform specifically designed for developing enterprise applications.

2. What is Servlet Filter?
A Servlet Filter intercepts incoming web requests and outgoing responses and allows manipulating them before they reach the servlet or the client.

3. What is the Java Message Service (JMS)?
Java Message Service is an API for sending and receiving messages between distributed systems. It provides a way for Java programs to communicate asynchronously.

4. What is the purpose of JSP (JavaServer Pages)?
JSP is a technology used to create dynamic web pages by embedding Java code into HTML markup. It simplifies the development of web-based applications.

5. What are the different session management techniques in J2EE?
J2EE offers three session management techniques: URL Rewriting, Cookies, and HttpSession.

6. What is the purpose of Object Serialization in Java?
Object Serialization is the process of converting objects into a byte stream, which can be saved to a file or transmitted over a network. It allows objects to be stored, retrieved, and shared.

7. What is the difference between an abstract class and an interface?
An abstract class can have abstract and non-abstract methods, whereas an interface can only have abstract methods. A class can extend only one abstract class but can implement multiple interfaces.

8. What is the purpose of the Java Virtual Machine (JVM)?
JVM is responsible for executing Java bytecode. It provides a runtime environment that enables Java programs to run independently of the underlying hardware and operating system.

9. What is the difference between HashMap and Hashtable?
HashMap is not synchronized and allows null values, while Hashtable is synchronized and does not allow null values. HashMap is preferred for non-thread-safe operations.

10. What is the purpose of the finalize() method in Java?
The finalize() method is called by the garbage collector before reclaiming the memory occupied by an object. It can be overridden to perform specific cleanup operations.

11. What is the use of the transient keyword in Java?
The transient keyword is used to indicate that a variable should not be serialized when the object is converted into a byte stream using object serialization.

12. Explain the concept of design patterns in Java.
Design patterns are reusable solutions to common problems in software design. They provide a way to standardize and organize code, making it more maintainable and flexible.

13. What is multi-threading in Java?
Multi-threading allows concurrent execution of multiple threads within a single program. Each thread represents an independent path of execution, sharing the same memory space.

14. What is the purpose of the synchronized keyword in Java?
The synchronized keyword is used to create mutually exclusive access to shared resources among different threads. It ensures that only one thread can execute a synchronized block at a time.

15. What are the advantages of using JDBC (Java Database Connectivity) in J2EE?
JDBC provides a standard API for database connectivity in Java. It allows developers to interact with databases, execute SQL queries, and retrieve data, making it easier to develop database-driven applications.

16. What is the purpose of EJB (Enterprise JavaBeans) in J2EE?
EJB is a server-side component architecture for building scalable and distributed applications. It provides a set of services such as transaction management, persistence, and security.

17. What is the difference between stateful and stateless EJBs?
Stateful EJBs maintain a conversational state with the client, while stateless EJBs do not. Stateful EJBs store client-specific data throughout the conversation, while stateless EJBs handle each request independently.

18. What is the purpose of the Java Naming and Directory Interface (JNDI)?
JNDI provides a standardized way to access naming and directory services in Java. It allows applications to look up resources, such as data sources, EJBs, and JMS queues, in a distributed environment.

19. What is the difference between forward and sendRedirect in JSP?
Forwarding a request means transferring control to another resource on the server, while sendRedirect sends a response to the client, instructing it to request a new resource from the server.

20. What are the different types of J2EE containers?
J2EE containers include EJB containers, servlet containers, and JMS (Java Message Service) containers. These containers provide a runtime environment for executing J2EE components.