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Top Java Interview Questions & Answers

Jay Mishra| Java | 6 months, 1 week



 

Here is a list of Java interview Questions and Answers

 

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. There are two types of polymorphism:

  1. Compile time polymorphism
  2. Run time polymorphism

 

Compile time polymorphism is method overloading whereas Runtime time polymorphism is done using inheritance and interface.

 

What is multiple inheritance? Is it supported by Java?

If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes.

The problem with multiple inheritance is that if multiple parent classes have a same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class.

Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred as Diamond Problem.

 

What is composition in Java?

Composition is again specialized form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete.

 

What is a Java 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.

 

What is the lifecycle of a servlet in Java?

  1. Servlet is loaded
  2. Servlet is instantiated
  3. Servlet is initialized
  4. Service the request
  5. Servlet is destroyed

 

How does cookies work in Java servlet ?

  • 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.

 

 



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