Java Interview Q&A

Q. What is a class?

Ans. A class is a blueprint or prototype from which objects are created. A class models the state and behaviour of real world object.

Q. What is an object?

Ans. Object is an instance of class that stores its state in fields(variables) and exposes its behaviour through methods(functions). Methods operate on an object’s internal state and serve as the primary mechanism for object-to-object communication.

Q. What is data encapsulation?

Ans. Data encapsulation is a fundamental principle of object-oriented programming. It is a mechanism to hide internal state and ensures that all interaction are performed through an object’s method.

Q. Explain different types of variable in Java.

Ans. In java field or variable is used to store state of an object. The Java programming language defines the following four kinds of variable.

• Instance Variables (Non-Static Fields) – Non-static fields(fields declared without the static keyword) are also known as instance variables because their values are unique to each instance of class.
• Class Variables (Static Fields) – A class variable is any field declared with static modifier. Only one copy of this variable exists regardless of how many times a class is instantiated.
• Local Variables – A method in java stores its temporary state in local variable. The syntax for declaring a local variable is similar to declaring a field (for example, int count = 0;). A local variable is declared between opening and closing braces of the method.
• Parameters – Parameters are always classified as variables not fields. For example – the signature for the main method is public static void main(String[] args) and the variable args is a parameter to this method.

Q. What is inheritance in java?

Ans. Inheritance is an important feature of Object-oriented programming which allows classes to inherit commonly used state and behaviour from other class called base class or superclass using the keyword extends.

In the Java programming language, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses.

Q. What is interface?

Ans. An interface is an abstract type in Java that contains a collection of methods and constants. It is one of the core concepts in Java used to achieve abstraction, polymorphism, and multiple inheritance. An interface is defined using interface keyword.

Q. How to create and send zip file as attachment in HTTP Response without creating file physically in hard drive?

Ans. Suppose the data that needs to be sent in HTTP response is in StringBuffer, then we can use following code snippet.

final StringBuffer stringBuffer = new StringBuffer().append("response data ................");
try{
    // response is instance of HttpServletResponse
    final OutputStream stream = response.getOutputStream(); 
    ZipOutputStream zos = new ZipOutputStream(stream);
    String fileName = String.format("%s_%s.fileExtensionName", "fileName", 
                      System.currentTimeMillis());

    String zipFileName = String.format("%s_%s.zip", "fileName", 
                         System.currentTimeMillis());

     ZipEntry entry = new ZipEntry(fileName);
     zos.putNextEntry(entry);
     zos.write(buffer.toString().getBytes());
     zos.flush();
     zos.closeEntry();
     zos.close();
     response.setContentType("application/zip");
     response.setHeader("Content-Disposition", "attachment; filename=" + zipFileName);   
}catch (){
     response.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
}