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Difference between DOM and SAX parsers in Java

A difference between the SAX and the DOM Parser is a very popular Java interview and frequently asked when interviewing with Java and XML. The DOM and SAX analyzers are used extensively to read and analyze the XML file in java and have their own set of advantages and disadvantages that we will address in this article. Although there is another way to read the XML file using XPath in Java, which is the more selective approach, such as SQL statements, people tend to stick with XML parsers. The Parser vs SAX analyzers are also often seen in terms of speed, memory consumption and their ability to process large XML files.

 Difference between the DOM XML parser and SAX in Java

DOM XML parser in Java

DOM Stands for Document Object Model and represents an XML document in tree format, each element representing tree branches. The DOM parser creates an in-memory tree representation of the XML file and then parses it, so it requires more memory and it is advisable to increase the heap size for the DOM parser to avoid Java.lang.OutOfMemoryError: java heap space.

 Analyzing the XML file using the DOM parser is quite fast if the XML file is small, but if you try to read a large XML file using the DOM parser, there is a greater chance that it will be delayed a lot or until you cannot fully load it because it requires a lot of memory to create XML Dom Tree. Java provides support for DOM analysis and can analyze XML files in Java using the DOM analyzer. The DOM classes are in the w3c.dom package, while the DOM Parser for Java is in the JAXP package (Java API for XML Parsing).

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 SAX XML parser in Java

SAX Stands for simple API for XML analysis. It is an XML analysis based on events and analyzes the XML file step by step, very suitable for large XML files. The SAX XML Parser triggers an event when the tag, element or opening attribute was found and the analysis works accordingly. It is recommended to use the SAX XML parser to analyze large XML files in Java, since it is not necessary to load the entire XML file into Java and can read a large XML file in small parts. Java provides support for SAX Parser and you can analyze any XML file in Java using SAX Parser, I have covered an example of reading XML file using SAX Parser here. One disadvantage of using the SAX Parser in java is that reading the XML file in Java using the SAX Parser requires more code in the comparison of the DOM analyzer.

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 Difference between the XML DOM and SAX parser

Here are some high-level differences between the DOM analyzer and the SAX Parser in Java:

1) The DOM analyzer loads the entire XML document into memory while the SAX only loads a small part of the XML file into memory.

2) The DOM analyzer is faster than the SAX because it accesses the entire XML document in memory.

3) The SAX analyzer in Java is more suitable for a large XML file than the DOM analyzer because it does not require much memory.

4) The DOM analyzer works in Document Object Model, while the SAX is an XML analyzer based on events.

This is all about the difference between SAX and DOM analyzers in Java, now it's up to you to choose in which XML analyzer you will choose. I recommend using the DOM parser on the SAX parser if the XML file is small enough and using the SAX parser if you do not know the size of the XML files that are processed or if they are large.

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Java Servlet lifecycle

A servlet is managed through a well-defined life cycle that defines how it is loaded and instantiated, initialized, handled customer requests and removed from service. This  life cycle is expressed in the API by the init, service and destroy methods of the javax.servlet.Servlet interface that all servlets must implement directly or indirectly through the abstract classes GenericServlet or HttpServlet.

Stages of Servlet Lifecycle:

A servlet goes through the following stages in its life.

• Initialize

• Service

• Destroy

Initialize

When the servlet is created for the first time, it is in the start phase. The web server invokes the init () method of the servlet at this stage. It should be noted here that the init () is called only once and is not called for each request. As there is no constructor available in the servlet, this makes it be used for a single boot, as well as the init () method of the applet

Initialize stage has the following characteristics and usage

• Run once when the servlet loads for the first time

• Each customer request is not called

• The two previous points make the ideal location to perform the boot tasks performed in the constructor in a normal class.

Service

The service () method is the servlet mechanism, which actually processes the client's request. In each client request, the server generates a new subprocess and calls the service () method, as shown in the following figure. This makes it more efficient compared to technologies that become a single subprocess to respond to requests.

The following figure shows the version of the service cycle implementation. At the top of the diagram, we assume that the servlet is subclassing in GenericServlet (Remember, GenericServlet is used to build protocol-independent servlets). To provide the desired functionality, the service () method is replaced. The client sends a request to the Web server, a new chain is created to satisfy this request, followed by the call to the service () method. Finally, a response is prepared and sent back to the user according to the request. 

The second part of the figure illustrates a situation in which the servlet is made using the HttpServlet class. Now, this servlet can only serve HTTP requests. In these servlets, doGet () and doPost () are replaced to provide the desired behaviors. When a request is sent to the web server, the web server after creating a chain passes this request to the service () method. The Service () method checks the type of HTTP requests (GET, POST) and calls the doGet () or doPost method, depending on how the request is sent. After forming the response by the method doGet () or doPost (), the response is sent back to the service () method that is finally sent to the user by the web server.

 Destroy

The web server may decide to remove a previously loaded servlet instance, perhaps because it is explicitly requested by the server administrator, or the servlet container may be shut down or the servlet idle for a long time or the server may be overloaded. Before that, however, call the servlets destroy () method. This makes it a perfect place to release the acquired resources.

The following figure can help to summarize the life cycle of the Servlet

 The sever web creates a servlet instance. After the correct creation, the servlet enters the boot phase. Here, the init () method is invoked once. If the Web server fails in the previous two stages, the servlet instance is downloaded from the server. After the start stage, the servlet becomes available to satisfy customer requests and generate responses accordingly. Finally, the servlet is destroyed and downloaded from the web server.

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Introduction to Java

Java is one of the world's most important and widely used computer languages, and it has held this distinction for many years. Unlike some other computer languages whose influence has weared with passage of time, while Java's has grown.

As of 2015, Java is one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers using and working on it.

Creation of Java

Java was developed by James Ghosling, Patrick Naughton, Mike Sheridan at Sun Microsystems Inc. in 1991. It took 18 months to develop the first working version.

The initial name was Oak but it was renamed to Java in 1995 as OAK was a registered trademark of another Tech company.

Evolution of Java

Java was initially launched as Java 1.0 but soon after its initial release, Java 1.1 was launched. Java 1.1 redefined event handling, new library elements were added.

In Java 1.2 Swing and Collection framework was added and suspend(), resume() and stop()methods were deprecated from Thread class.

No major changes were made into Java 1.3 but the next release that was Java 1.4 contained several important changes. Keyword assert, chained exceptions and channel based I/O System was introduced. 

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 Java 1.5 was called J2SE 5, it added following major new features :

·         Generics

·         Annotations

·         Autoboxing and autounboxing

·         Enumerations

·         For-each Loop

·         Varargs

·         Static Import

·         Formatted I/O

·         Concurrency utilities

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Next major release was Java SE 7 which included many new changes, like :

·         Now String can be used to control Switch statement.

·         Multi Catch Exception

·         try-with-resource statement

·         Binary Integer Literals

·         Underscore in numeric literals, etc.

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And the latest addition to the lot is, Java SE 8, it was released on March 18, 2014. Some of the major new features introduced in JAVA 8 are,

·         Lambda Expressions

·         New Collection Package java.util.stream to provide Stream API.

·         Enhanced Security

·         Nashorn Javascript Engine included

·         Parallel Array Sorting

·         The JDBC-ODBC Bridge has been removed etc.

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Application of Java

Java is widely used in every corner of world and of human life. Java is not only used in softwares but is also widely used in designing hardware controlling software components. There are more than 930 million JRE downloads each year and 3 billion mobile phones run java.

Following are some other usage of Java :

1.      Developing Desktop Applications

2.      Web Applications like Linkedin.com, Snapdeal.com etc

3.      Mobile Operating System like Android

4.      Embedded Systems

5.      Robotics and games etc.

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Inheritance in Java

Inheritance is an important pillar of OOP(Object Oriented Programming). It is the mechanism in java by which one class is allow to inherit the features (fields and methods) of another class.

Important terminology:

 Super Class: The class whose features are inherited is known as super class(or a base class or a parent class).

 Sub Class: The class that inherits the other class is known as sub class(or a derived class, extended class, or child class). The subclass can add its own  fields and methods in addition to the superclass fields and methods.

 Reusability: Inheritance supports the concept of “reusability”, i.e. when we want to create a new class and there is already a class that includes some of the code that we want, we can derive our new class from the existing class. By doing this, we are reusing the fields and methods of the existing class.

Types of Inheritance in Java

Below are the different types of inheritance which is supported by Java.

 Single Inheritance: In single inheritance, subclasses inherit the features of one superclass. The class A serves as a base class for the derived class B.

Multilevel Inheritance: In Multilevel Inheritance, a derived class will be inheriting a base class and as well as the derived class also act as the base class to other class. In below image, the class A serves as a base class for the derived class B, which in turn serves as a base class for the derived class C. In Java, a class cannot directly access the members.

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Hierarchical Inheritance: In Hierarchical Inheritance, one class serves as a superclass (base class) for more than one sub class. The class A serves as a base class for the derived class B, C and D.

 Multiple Inheritance (Through Interfaces): In Multiple inheritance , one class can have more than one superclass and inherit features from all parent classes. Please note that Java does not support multiple inheritance with classes. In java, we can achieve multiple inheritance only through interface. Class C is derived from interface A and B.

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Hybrid Inheritance (Through Interfaces) : It is a mix of two or more of the above types of inheritance. Since java doesn’t support multiple inheritance with classes, the hybrid inheritance is also not possible with classes. In java, we can achieve hybrid inheritance only through interfaces .

Important facts about inheritance in Java

 Default superclass: Except Object class, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object class.

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Superclass can only be one: A superclass can have any number of subclasses. But a subclass can have only one superclass. This is because Java does not support multiple inheritance with classes. Although with interfaces, multiple inheritance is supported by java.

Inheriting Constructors: A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

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Amazing Things to Do With a Hadoop-Based Data Lake

This is an engineering for a Business Data Lake, and it is revolved around Hadoop-based capacity. It incorporates devices and segments for ingesting information from various types of information sources, preparing information for examination and experiences, and for supporting applications that use information, execute bits of knowledge, and contribute information back to the information lake as wellsprings of new information. In this introduction, we will take a gander at the different segments of a business information lake design, and show how when assembled these innovations help amplify the estimation of your organization's information.

 1. Store Massive Data Sets

 Apache hadoop, and the basic Apache Hadoop File System, or HDFS , is a circulated record framework that backings subjectively expansive groups and scales out on ware equipment. This implies your information stockpiling can hypothetically be as expansive as required and fit any need at a sensible cost. You basically include more groups as you require more space. Apache Hadoop groups additionally unite registering assets near capacity, encouraging quicker preparing of the substantial put away informational indexes.

2. Blend Disparate Data Sources

 HDFS is likewise construction less, which implies it can bolster records of any kind and organization. This is incredible for putting away unstructured or semi-organized information, and also non-social information organizations, for example, paired streams from sensors, picture information, or machine logging. It's likewise fine and dandy for putting away organized, social forbidden information. There was a current illustration where one of our information science groups blended organized and unstructured information to break down the reasons for understudy achievement.

Putting away these distinctive sorts of informational indexes basically isn't conceivable in customary databases, and prompts siloed information sources, not supporting the joining of informational indexes.

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3. Ingest Bulk Data

Ingesting build information truly appears in two structures—standard clusters and small scale clumps. There are three adaptable, open source instruments that would all be able to be utilized relying upon the situation.Scoop, for instance, is awesome for taking care of huge information group stacking and is intended to pull information from inheritance databases.

On the other hand, organizations would prefer not to simply stack the information, yet they need to likewise accomplish something with the information as it is stacked.

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For instance, now and again a stacking task needs extra preparing, organizations may should be changed, metadata must be made as the information is stacked, or investigation, for example, for tallies and ranges, must be caught as the information is ingested. In these cases, Spring XD gives a lot of scale and adaptability.

 4. Ingest High Velocity Data

Gushing high-speed information into Apache hadoop is an alternate test by and large. At the point when there is an extensive volume to consider at speed, you require devices that can catch and line information at any scale or volume until the Apache hadoop group can store.

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 5. Apply Structure to Unstructured/Semi-Structured Data

It's awesome that one can get any sort of information into a HDFS information store. To have the capacity to direct progressed examination on it, you regularly need to make it available to organized based investigation devices.

This sort of preparing may include coordinate change of record composes, changing words into checks or classifications, or essentially breaking down and making meta information about a file. For instance, retail site information can be parsed and transformed into diagnostic data and applications.

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