What are the differences between SAX and DOM parser.

SAX	DOM
Both SAX and DOM are used to parse the XML document. Both has advantages and disadvantages and can be used in our programming depending on the situation.
Parses node by node	Stores the entire XML document into memory before processing
Doesn’t store the XML in memory	Occupies more memory
We cant insert or delete a node	We can insert or delete nodes
Top to bottom traversing	Traverse in any direction.
SAX is an event based parser	DOM is a tree model parser
SAX is a Simple API for XML	Document Object Model (DOM) API
import javax.xml.parsers.*; import org.xml.sax.*; import org.xml.sax.helpers.*;	import javax.xml.parsers.*; import org.w3c.dom.*;
doesn’t preserve comments	preserves comments
SAX generally runs a little faster than DOM	SAX generally runs a little faster than DOM
If we need to find a node and doesn’t need to insert or delete we can go with SAX itself otherwise DOM provided we have more memory.

Interview Question

JSP Interview Question

1.      After hitting submit button, a JSP page processes a database transaction. You have accidentally closed the browser window. Will the database transaction continue or will get over?

2.      What is the difference between http GET and POST methods ?

3.      There are 10 servlets in your web application, and they are frequently used (sticky Servlet). How will you optimize their loading?

4.      How will you communicate across Servlets? (or, how will you achieve Inter Servlet communication?)

5.      Hope, you know the difference between RequestDispatcher.forward() and RequestDispatcher.sendRedirect(). So, which one is desirable to use?

6.      What is the class path hierarchy for a Servlet?

7.      You can’t read files (located out of tomcat directory) from JSP and Servlets for security purpose. But, how will you access those files from JSP or Servlets, in case if you need them?

Answers:

The database transaction will still continue; Because a Servlet thread was created when you hit the JSP page and that runs the database transaction. Closing the browser window will not kill the Servlet thread.

a) GET method passes the form fields as part of URL, but POST method wraps or encrypts the form fields in request data; b) Also, you can pass only 255 characters to the action page with GET method.

Add <load-on-startup> tag to the Servlet in web.xml. This will load your Servlet on startup, and doXXX() method will be ready for any further requests.

Use RequestDispatcher.forward() and RequestDispatcher.include();

RequestDispatcher.sendRedirect() is desirable. Because, forward() can’t work across different JVM.

First, lookup in WEB-INF/classes, then WEB-INF/lib then from tomcat’s central library folder (<tomcat-home>/lib)

Any class from WEB-INF/classes or WEB-INF/lib can access files. So, use those classes to do the job for your JSP and Servlets.

Design Pattern:

1.      How will you design your Singleton Patten class to be thread-safe? (a standard interview question!)

2.      What Design Pattern can be used for classical Producer Consumer problem? (a simple question)

3.      M.S Word can store million lines of text. Each character in the text will have set of properties like font-color, font-size, font-style and etc. So, M.S Word application has to maintain a property structure for each and every character and that may lead to heavy storage. What design pattern will you apply to overcome this problem?

4.      Your application uses 1000 classes and they have interaction among each other. This will lead to complex communication across objects, so what design pattern can solve this problem?

5.      An application reads input from various clients in multiple stages and creates a binary tree structure out of all received inputs. Which design pattern can be used to design such application?

6.      You have to design a budget management application for global users. But, have to provide an option to configure their native tax rules. Which design pattern, do you think, can solve this?

7.      You have a core application class. But, you have to be able assign extra functionalities to the core class on the fly (at runtime). Which patterns deserves this purpose?

1. ANSWER: Adding synchronized keyword tothe getSingleton() method is inefficient solution. Initialize the static private Singleton object within Synchronized block. In this way you can achieve it.

2.ANSWER: Use observer pattern (publisher subscriber model)

3. ANSWER: You have to create property group for each possible styles (font-size, font-color, font-face & etc). So, common characters will be part of a property group, there by reducing the storage of each character. This is called FlyWeight Pattern.

4. ANSWER: Use Mediator pattern to separate the direct communication across objects.

5. ANSWER: Building an object in multiple set - you call it Builder pattern! The realworld example is XSDBuilder class in Java.

6. ANSWER: Use Template Pattern. It provides templates of methods and allowing you to override certain portion of logic.

7. ANSWER: Decorator Pattern. This is similar to the pattern designed for java.io.* classes(BufferedReader, DataInputStream and etc.)

Association, Aggregation, Composition, Abstraction, Generalization, Realization, Dependency

26/06/2010

These terms signify the relationships between classes. These are the building blocks of object oriented programming and very basic stuff. But still for some, these terms look like Latin and Greek. Just wanted to refresh these terms and explain in simpler terms.

Association

Association is a relationship between two objects. In other words, association defines the multiplicity between objects. You may be aware of one-to-one, one-to-many, many-to-one, many-to-many all these words define an association between objects. Aggregation is a special form of association. Composition is a special form of aggregation.

Example: A Student and a Faculty are having an association.

Aggregation

Aggregation is a special case of association. A directional association between objects. When an object ‘has-a’ another object, then you have got an aggregation between them. Direction between them specified which object contains the other object. Aggregation is also called a “Has-a” relationship.

Composition

Composition is a special case of aggregation. In a more specific manner, a restricted aggregation is called composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition.

Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

Difference between aggregation and composition

Composition is more restrictive. When there is a composition between two objects, the composed object cannot exist without the other object. This restriction is not there in aggregation. Though one object can contain the other object, there is no condition that the composed object must exist. The existence of the composed object is entirely optional. In both aggregation and composition, direction is must. The direction specifies, which object contains the other object.

Example: A Library contains students and books. Relationship between library and student is aggregation. Relationship between library and book is composition. A student can exist without a library and therefore it is aggregation. A book cannot exist without a library and therefore its a composition. For easy understanding I am picking this example. Don’t go deeper into example and justify relationships!

Abstraction

Abstraction is specifying the framework and hiding the implementation level information. Concreteness will be built on top of the abstraction. It gives you a blueprint to follow to while implementing the details. Abstraction reduces the complexity by hiding low level details.

Example: A wire frame model of a car.

Generalization

Generalization uses a “is-a” relationship from a specialization to the generalization class. Common structure and behaviour are used from the specializtion to the generalized class. At a very broader level you can understand this as inheritance. Why I take the term inheritance is, you can relate this term very well. Generalization is also called a “Is-a” relationship.

Example: Consider there exists a class named Person. A student is a person. A faculty is a person. Therefore here the relationship between student and person, similarly faculty and person is generalization.

Realization

Realization is a relationship between the blueprint class and the object containing its respective implementation level details. This object is said to realize the blueprint class. In other words, you can understand this as the relationship between the interface and the implementing class.

Example: A particular model of a car ‘GTB Fiorano’ that implements the blueprint of a car realizes the abstraction.

Dependency

Change in structure or behaviour of a class affects the other related class, then there is a dependency between those two classes. It need not be the same vice-versa. When one class contains the other class it this happens.

Example: Relationship between shape and circle is dependency.