1. Why Java is platform independent?
Platform independent practically means “write once run anywhere”. Java is called so because of its byte codes which can run on any system irrespective of its underlying operating system.

2. Why java is not 100% Object-oriented?
Java is not 100% Object-oriented because it makes use of eight primitive datatypes such as boolean, byte, char, int, float, double, long, short which are not objects.

3. What are wrapper classes?
Wrapper classes converts the java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class. Refer to the below image which displays different primitive type, wrapper class and constructor argument. 

4. What are constructors in Java?
In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

5. What is singleton class and how can we make a class singleton?
Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Also Read:- Top 30 Java Interview Questions and Answers

6. What is the difference between equals() and == ?
Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.
“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

7. What is Polymorphism?
Polymorphism is briefly described as “one interface, many implementations”. Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts – specifically, to allow an entity such as a variable, a function, or an object to have more than one form. 

8. What is association?
Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationship can be one to one, One to many, many to one and many to many.

9. What is a servlet?
Java Servlet is server side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.
The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing our own servlets.
All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services.
Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image.

10. What is Request Dispatcher?
RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in same application. We can also use this to include the content of another resource to the response.

11. How does cookies work in Servlets?
Cookies are text data sent by server to the client and it gets saved at the client local machine.
Servlet API provides cookies support through javax.servlet.http.Cookie class that implements Serializable and Cloneable interfaces.
HttpServletRequest getCookies() method is provided to get the array of Cookies from request, since there is no point of adding Cookie to request, there are no methods to set or add cookie to request.
Similarly HttpServletResponse addCookie(Cookie c) method is provided to attach cookie in response header, there are no getter methods for cookie.

12. What are the different methods of session management in servlets?
Session is a conversational state between client and server and it can consists of multiple request and response between client and server. Since HTTP and Web Server both are stateless, the only way to maintain a session is when some unique information about the session (session id) is passed between server and client in every request and response.

13. What is the role of JDBC DriverManager class?
The DriverManager class manages the registered drivers. It can be used to register and unregister drivers. It provides factory method that returns the instance of Connection.

14. What is JDBC Connection interface?
The Connection interface maintains a session with the database. It can be used for transaction management. It provides factory methods that returns the instance of Statement, PreparedStatement, CallableStatement and DatabaseMetaData.

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15. What is the purpose of JDBC ResultSet interface?
The ResultSet object represents a row of a table. It can be used to change the cursor pointer and get the information from the database.

16. What is JDBC ResultSetMetaData interface?
The ResultSetMetaData interface returns the information of table such as total number of columns, column name, column type etc.

17. What is JDBC DatabaseMetaData interface?
The DatabaseMetaData interface returns the information of the database such as username, driver name, driver version, number of tables, number of views etc.

18. What do you mean by batch processing in JDBC?
Batch processing helps you to group related SQL statements into a batch and execute them instead of executing a single query. By using batch processing technique in JDBC, you can execute multiple queries which makes the performance faster.

19. What is a Spring?
Wikipedia defines the Spring framework as “an application framework and inversion of control container for the Java platform. The framework’s core features can be used by any Java application, but there are extensions for building web applications on top of the Java EE platform.” Spring is essentially a lightweight, integrated framework that can be used for developing enterprise applications in java.

20. Name the different modules of the Spring framework.
Some of the important Spring Framework modules are:

Spring Context – for dependency injection.
Spring AOP – for aspect oriented programming.
Spring DAO – for database operations using DAO pattern
Spring JDBC – for JDBC and DataSource support.
Spring ORM – for ORM tools support such as Hibernate
Spring Web Module – for creating web applications.
Spring MVC – Model-View-Controller implementation for creating web applications, web services etc.

Also Read:- Top 90 Java Interview Questions and Answers


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