However sometime while working with Collection API, lot of developers end up writing unnecessary and mostly inefficient code. For example, to convert an Java Array to ArrayList, I have seen people writing loops instead of simple Arrays.asList().

Here is a simple writeup on Converting Java Arrays to ArrayList that I wrote a while ago.

One of such simple requirement is to convert Java Arrays to Set. While working with Hibernate, I once had to convert a Java Arrays that we used to populate from UI and convert it into Set. While this is a simple task, most often one may end up writing for loop.

Here is a dead simple trick. Use below code to Convert Arrays to Set in Java.

Set<T> mySet = new HashSet<T>(Arrays.asList(someArray));

Tanaa!!! Simple isn’t it. Its like “I already knew that” stuff.

Notice how we have used Generics also in above code snippet. Thus if you have an ArrayList than you can convert it to Set with one simple line of code.

Checkout below example:

Example: Java Array to Set

String [] countires = {"India", "Switzerland", "Italy"}; 

Set<String> set = new HashSet<String>(Arrays.asList(countires));
System.out.println(set);

Output:

[Italy, Switzerland, India]

Hope this is useful.

Let us see a quick overview of Quick Response (QR) codes and also how to generate these codes in Java.

Introduction to QR Codes

A QR code (abbreviated from Quick Response code) is a type of matrix barcode (or two-dimensional code) first designed for the automotive industry. More recently, the system has become popular outside of the industry due to its fast readability and comparatively large storage capacity. The code consists of black modules arranged in a square pattern on a white background. The information encoded can be made up of four standardized kinds (“modes”) of data (numeric, alphanumeric, byte/binary, Kanji), or by supported extensions virtually any kind of data.

Created by Toyota subsidiary Denso Wave in 1994 to track vehicles during the manufacturing process, the QR code is one of the most popular types of two-dimensional barcodes. It was designed to allow its contents to be decoded at high speed.

Hello World QR Code in Java

Zebra Crossing (ZXing) is an awesome open source library that one can use to generate / parse QR Codes in almost all the platforms (Android, JavaSE, IPhone, RIM, Symbian etc). But if you have to generate simple QR Codes, I found it a bit clumsy to implement.

However QRGen is a good library that creates a layer on top of ZXing and makes QR Code generation in Java a piece of cake. It has a dependency on ZXing, so you would need ZXing jar files along with QRGen to create QR Codes in Java.

On the download page of ZXing, you will not find the JAR files. Instead we have to create JAR files using the source code. I have already generated these JAR files. Here are the links:

zxing-core-1.7.jar (346 KB)
zxing-javase-1.7.jar (21 KB)

Also download the QRGen JAR File from their download page.

Include these JAR files in your Classpath and execute following Java code to generate QR Code.

package net.viralpatel.qrcode;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import net.glxn.qrgen.QRCode;
import net.glxn.qrgen.image.ImageType;

public class Main {
	public static void main(String[] args) {
		ByteArrayOutputStream out = QRCode.from("Hello World")
										.to(ImageType.PNG).stream();

		try {
			FileOutputStream fout = new FileOutputStream(new File(
					"C:\\QR_Code.JPG"));

			fout.write(out.toByteArray());

			fout.flush();
			fout.close();

		} catch (FileNotFoundException e) {
			// Do Logging
		} catch (IOException e) {
			// Do Logging
		}
	}
}

The code is pretty straight forward. We used QRCode class to generate QR Code Stream and write the byte stream to a file C:\QR_Code.JPG.

Download Source Code

QR_Code_Java.zip (339 KB)

If you open this JPEG file and scan using your iPhone or Android QR scanner, you’ll find a cool “Hello World” in it

Apart from generating Sterams of data using QRGen API, we can also use below APIs to create QR Codes:

// get QR file from text using defaults
File file = QRCode.from("Hello World").file();
// get QR stream from text using defaults
ByteArrayOutputStream stream = QRCode.from("Hello World").stream();

// override the image type to be JPG
QRCode.from("Hello World").to(ImageType.JPG).file();
QRCode.from("Hello World").to(ImageType.JPG).stream();

// override image size to be 250x250
QRCode.from("Hello World").withSize(250, 250).file();
QRCode.from("Hello World").withSize(250, 250).stream();

// override size and image type
QRCode.from("Hello World").to(ImageType.GIF).withSize(250, 250).file();
QRCode.from("Hello World").to(ImageType.GIF).withSize(250, 250).stream();

Website Link (URLs) QR Code in Java

One of the most common use of a QR Code is to bring traffic to a particular webpage or download page of website. Thus QR Code encodes a URL or website address which a user can scan using phone camera and open in their browser. URLs can be straight forward included in QR Codes. In above Java Hello World example, just replace “Hello World” string with the URL you want to encode in QR Code. Below is the code snippet:

ByteArrayOutputStream out = QRCode.from("http://viralpatel.net")
				.to(ImageType.PNG).stream();

QR Code in Servlet

Most of the time you would need to generate QR Codes dynamically in some website. We already saw how easy it is to generate QR code in Java. Now we will see how to integrate this QR Code generation in a Java Servlet.

Following is a simple Http Servlet that creates QR Code using QRGen and ZXing library. User provides the text for which QR Code is generated.

The index jsp file contains a simple html form with a textbox and submit button. User can enter the text that she wishes to generate QR code of and presses submit.

File: index.jsp

<html>
<head>
<title>QR Code in Java Servlet - viralpatel.net</title>
</head>
<body>

	<form action="qrservlet" method="get">
		<p>Enter Text to create QR Code</p>
		<input type="text" name="qrtext" />
		<input type="submit" value="Generate QR Code" />
	</form>
</body>
</html>

The magic happens in QRCodeServlet.java. Here we uses QRGen library along with ZXing and generates QR Code for given text (Text we get from request.getParameter). Once the QR Stream is generated, we write this to response and set appropriate content type.

File: QRCodeServlet.java

package net.viralpatel.qrcodes;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import net.glxn.qrgen.QRCode;
import net.glxn.qrgen.image.ImageType;

public class QRCodeServlet extends HttpServlet {
	@Override
	protected void doGet(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {

		String qrtext = request.getParameter("qrtext");

		ByteArrayOutputStream out = QRCode.from(qrtext).to(
				ImageType.PNG).stream();

		response.setContentType("image/png");
		response.setContentLength(out.size());

		OutputStream outStream = response.getOutputStream();

		outStream.write(out.toByteArray());

		outStream.flush();
		outStream.close();
	}
}

The below web.xml simply maps QRCodeServlet.java with /qrservlet URL.

File: web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xmlns="http://java.sun.com/xml/ns/javaee"
		xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
		xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
		id="WebApp_ID" version="2.5">

  <display-name>QR_Code_Servlet</display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>

	<servlet>
		<servlet-name>QRCodeServlet</servlet-name>
		<servlet-class>net.viralpatel.qrcodes.QRCodeServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>QRCodeServlet</servlet-name>
		<url-pattern>/qrservlet</url-pattern>
	</servlet-mapping>

</web-app>

Download Source Code

QR_Code_Servlet.zip (340 KB)

Output

Conclusion

Generating QR Codes in Java is not only easy, but quite straight forward. Integrating this functionality with any existing Java based app is just a piece of cake! In this tutorial we saw how to generate these QR codes in Java and also with Servlet.

Hope you’ll like this

Tools and Technologies used:

1. Java JDK 5 or above
2. Eclipse IDE 3.2 or above
3. Maven 3.0 or above
4. Hibernate 3.0 or above
5. MySQL 5.0 or above

1. Database Creation

For this tutorial, we will create a simple table “employee” in MySQL. Use following script to create the table.

CREATE TABLE `employee` (
    `id` BIGINT(10) NOT NULL AUTO_INCREMENT,
    `firstname` VARCHAR(50) NULL DEFAULT NULL,
    `lastname` VARCHAR(50) NULL DEFAULT NULL,
    `birth_date` DATE NOT NULL,
    `cell_phone` VARCHAR(15) NOT NULL,
    PRIMARY KEY (`id`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB
ROW_FORMAT=DEFAULT
AUTO_INCREMENT=606

2. Create Project Structure

Follow the steps in Hibernate Maven MySQL hello world example (XML Mapping) to create project structure.

Or you can directly download the source code from above tutorial in Eclipse.

Download: HibernateHelloWorldXML.zip (8 kb)

3. Update Hibernate dependency in Maven pom.xml

We are going to use Annotation mapping in Hibernate. Update the maven’s pom.xml file and add following dependencies.

<?xml version="1.0" encoding="UTF-8"?><project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>HibernateCache</groupId>
  <artifactId>HibernateCache</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <description></description>
  <dependencies>
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>ejb3-persistence</artifactId>
      <version>1.0.1.GA</version>
    </dependency>
    <dependency>
      <groupId>org.hibernate</groupId>
      <artifactId>hibernate-annotations</artifactId>
      <version>3.3.1.GA</version>
    </dependency>
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.10</version>
    </dependency>
  </dependencies>
</project>

Once we update the Hibernate dependency, run following command in your project folder to update classpath in eclipse to that errors are removed.

mvn eclipse:eclipse

4. Delete unused Employee.hbm.xml

The Employee.hbm.xml file under /src/main/java/net/viralpatel/hibernate is no more required. Delete this file from project.

5. Update Employee entity

The Employee class is simple POJO. We will add Annotations in this class. Update it as follows:

package net.viralpatel.hibernate;

import java.sql.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="EMPLOYEE")
public class Employee {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name="firstname")
	private String firstname;

	@Column(name="lastname")
	private String lastname;

	@Column(name="birth_date")
	private Date birthDate;

	@Column(name="cell_phone")
	private String cellphone;

	public Employee() {

	}

	public Employee(String firstname, String lastname, Date birthdate, String phone) {
		this.firstname = firstname;
		this.lastname = lastname;
		this.birthDate = birthdate;
		this.cellphone = phone;

	}

	// Getter and Setter methods
}

6. Update Hibernate Utility

In our previous example, we were using Configuration class to generate SessionFactory object. In HibernateUtil we had following line of code:

return new Configuration()
            		.configure()
                    .buildSessionFactory();

Change this line and use AnnotationConfiguration instead of Configuration(). Replace this line with below code.

// import org.hibernate.cfg.AnnotationConfiguration;

return new AnnotationConfiguration()
            		.configure()
                    .buildSessionFactory();

7. Update Hibernate Configuration file

The hibernate.cfg.xml configuration file needs to be changed to add the new Annotation based entity class Employee.java instead of old XML Mapping Employee.hbm.xml.

Open /src/main/resources/hibernate.cfg.xml and replace following line:

<mapping resource="net/viralpatel/hibernate/Employee.hbm.xml"/>

Replace above line with following:

<mapping class="net.viralpatel.hibernate.Employee"/>

8. Review Final Project Structure

Review your project structure. It should be like:

9. Execute project

Execute the Main.java class and see output.

******* READ *******
Total Employees: 200
******* WRITE *******
201 Jack Bauer
******* UPDATE *******
Name Before Update:Paula
Name Aftere Update:James
******* DELETE *******
Object:null

That’s All Folks

Today we saw how to write our first Hibernate hello world example using Annotations. We used Maven to generate Java project and added Hibernate dependencies into it. Also we saw how to do different CRUD operations in Hibernate.

Download Source

HibernateHelloWorldAnnotation.zip (15 kb)

Here is a small code snippet which you might not know. Its very easy to iterate Lists using JSTL. For example:

//Java
List<String> cityList = new ArrayList<String>();
cityList.add("Washington DC");
cityList.add("Delhi");
cityList.add("Berlin");
cityList.add("Paris");
cityList.add("Rome");

request.setAttribute("cityList", cityList);

//JSP
<c:forEach var="city" items="cityList">
	<b> ${city} </b>
</c:forEach>

But what if you want to iterate a Hashmap? Well that too is piece of cake. Here is the example:

//Java
Map<String, String> countryCapitalList = new HashMap<String, String>();
countryCapitalList.put("United States", "Washington DC");
countryCapitalList.put("India", "Delhi");
countryCapitalList.put("Germany", "Berlin");
countryCapitalList.put("France", "Paris");
countryCapitalList.put("Italy", "Rome");

request.setAttribute("capitalList", countryCapitalList);

//JSP
<c:forEach var="country" items="${capitalList}">
	Country: ${country.key}  - Capital: ${country.value}
</c:forEach>

Happy iterating

1. How would it be the process of recreating the same example but using Spring Roo.
2. What would it be the similarities/differences in the projects source code.

The article assumes the Reader is a Developer with some familiarity with Spring Roo. For introductory information about Spring Roo vist http://www.ibm.com/developerworks/java/library/os-springroo1/index.html?ca=drs-.

Project setup

Spring Roo brings alternatives for setting up a project: a) Entering project using a script file or b) Creating the database first and reverse engineering it using Roo DBRE commands.

For this article I am using option a), via an script file. Option b) is specially convenient for projects with an database already in place. I’ll vist Roo DBRE in a coming article in the near future.

Script File

project --topLevelPackage org.pragmatikroo.roodocman
persistence setup --provider HIBERNATE --database MYSQL --databaseName roodocman --userName <username> --password <password>
entity		--class ~.domain.Document
field string 	--fieldName name 		--notNull --sizeMax 30
field string 	--fieldName description 	--sizeMax 500
field string 	--fieldName filename 		--notNull
//field blob 	--fieldName content		--notNull //Type not supported by Roo
field string 	--fieldName contentType 	--notNull
field numbert	--fieldName size -
controller all 	--package ~.web
perform clean
perform eclipse

Notice content field is of type blob. This field would have to be entered manually into the entity class. You can use the IDE of your choice. I use STS. Either you use STS or the native Roo shell, the project would need to be synced by Roo after the change. More on this later.

Generate Entity Class

package org.pragmatikroo.roodocman;

@RooJavaBean
@RooToString
@RooEntity
public class Document {

    @NotNull
    @Size(max = 30)
    private java.lang.String name;

    @NotNull
    @Size(max = 500)
    private java.lang.String description;

    private java.lang.String filename;

    @NotNull
    @Lob
    @Basic(fetch = FetchType.LAZY)
    private byte[] content;

    private java.lang.String contentType;

    private java.lang.Long size;

    @Transient
    @Size(max = 100)
    private String url ;
}

Please review the seminal article for more information about the example entity fields. All highlighted code is manually added to the original file created by Roo. Why I added field size and url directly into the entity class?. I could have entered referred fields using the shell too. Right?. I did it in purpose to mentioned that Spring Roo is a round-trip tool. There are two more fields not shown in the entity class. They are the id and version fields, Roo automatically handle them using an ITD. Details about round-trip capabilities and how Roo uses ITDs, please visit Spring Roo website at http://www.springsource.org/roo. One more thing about the blob field. DBMS have different type of blobs. Verify that your project database is using the right one for your purposes.

As mention before: always make sure that Roo synced your latest changes. That would save you a lot of confusion.

It is a good time to deploy the application and verify that everything is working find to this point.

Required Source code changes

Well, to this point Spring Roo has done a lot for us. Properly setup the project. Created a Maven pom.xml with all required dependencies -well, almost; more on this below-. Generate all the code necessary for having a working web app. If this not enough, Roo is there in the background monitoring the project against any possible change.

With exception of the blob field -content- all the other fields are ready to go. Roo took care of them. In order to save/retrieve data from the blob field, we need to modify code in the client and in the server side for allowing file uploads.

Server Side Changes

Basically we need code that allows us to save/retrieve documents that include a blob field. The documents are entered using a web form. Typically Roo by default, generates client and server side code for having CRUD operations for every entity. I am not allowing updates in this version of the application. So, I disabled it, by setting the update=false in the @RooWebScaffold annotation. Please make sure next code is included in the DocumentController project file.

@RooWebScaffold(path = "documents", formBackingObject = Document.class, update=false)
@InitBinder
protected void initBinder(HttpServletRequest request,
    				   ServletRequestDataBinder binder)
				   throws ServletException {
	binder.registerCustomEditor(byte[].class,   	newByteArrayMultipartFileEditor());
    }
@RequestMapping(value="savedoc",  method = RequestMethod.POST)
public String createdoc(@Valid Document document,
    				BindingResult result,
    				Model model,
    				@RequestParam("content") MultipartFile content,
    					 	HttpServletRequest request) {

    	document.setContentType(content.getContentType());
    	document.setFilename(content.getOriginalFilename());
    	document.setSize(content.getSize());

    	log.debug("Document: ");
    	log.debug("Name: "+content.getOriginalFilename());
    	log.debug("Description: "+document.getDescription());
    	log.debug("File: " +content.getName());
    	log.debug("Type: "+content.getContentType());
    	log.debug("Size: "+content.getSize());
        if (result.hasErrors()) {
            model.addAttribute("document", document);
            return "documents/create";
        }
        document.persist();

        return "redirect:/documents?page=1&size=10" +   			encodeUrlPathSegment(document.getId().toString(), request);
    }

@RequestMapping(value = "/{id}", method = RequestMethod.GET)
    public String show(@PathVariable("id") Long id, Model model) {
    	Document doc = Document.findDocument(id);
    	doc.setU("/documents/showdoc/"+id);
        model.addAttribute("document", Document.findDocument(id));
        model.addAttribute("itemId", id);
        return "documents/show";
    }

@RequestMapping(value = "/showdoc/{id}", method = RequestMethod.GET)
public String showdoc(	@PathVariable("id") Long id,
    				HttpServletResponse response,
    				Model model) {
   Document doc = Document.findDocument(id);

   try {
          response.setHeader("Content-Disposition", "inline;filename=\"" +doc.getFilename()+ "\"");

          OutputStream out = response.getOutputStream();
          response.setContentType(doc.getContentType());
          IOUtils.copy( new ByteArrayInputStream(doc.getContent()),out);
            out.flush();

        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

The code above allows CRUD operations with all the document fields including the blob. Very-very important: all this source code must be placed in the DocumentControler.java file to avoid been removed by Roo when the project is synced for changes.

Client Side Changes

On client side, I created a create custom form. Please replace the content of the create.jspx, with:

Create.jpsx Form

<form:multi id="fc_org_pragmatikroo_roodocman_domain_Document" modelAttribute="document" path="/documents/savedoc" render="${empty dependencies}" z="O7jiRGyhKQOUnBT8yJ9W4XU6VrQ=">
      <field:input field="name" id="c_org_pragmatikroo_roodocman_domain_Document_name" max="30" required="true" z="K3YncHn7F+HtBX/zgCZMYM6VO7k="/>
      <field:textarea field="description" id="c_org_pragmatikroo_roodocman_domain_Document_description" required="true" z="/RLpJWAf9zVjHtRapWIFve8JB8Y="/>
      <field:input field="filename" id="c_org_pragmatikroo_roodocman_domain_Document_filename" render="false" z="u4EclOEjQnID4g34VPv9zu9+qPo="/>
      <field:file field="content" id="c_org_pragmatikroo_roodocman_domain_Document_content" required="true" z="GZHlfc+o4h7EBA/SZ8/yXMVenOw="/>
      <field:input field="contentType" id="c_org_pragmatikroo_roodocman_domain_Document_contentType" render="false" z="jtq5/DHhBTgNImjac/d4AIfpCzA="/>
      <field:input field="size" id="c_org_pragmatikroo_roodocman_domain_Document_size" render="false" validationMessageCode="field_invalid_integer" z="ONDmhU5Eg5pw1j8LgTz9sXjOm6E="/>
      <field:textarea field="url" id="c_org_pragmatikroo_roodocman_domain_Document_url" render="false" z="bQ3FUVGL01nfWselK7WDPUD65Rw="/>
</form:multi>

As you can see, the form depends on two custom tagx files -multi.tagx and file.tagx- not included in the Spring Roo tag library. These two tags handle the blob field input.

Create Form

Show.jpx Form

<page:show id="ps_org_pragmatikroo_roodocman_domain_Document" object="${document}" path="/documents" update="false" z="J8X2O2T06x6jYb3WAyaNiTPGVZ0=">
      <field:display field="name" id="s_org_pragmatikroo_roodocman_domain_Document_name" object="${document}" z="VZEPJgXqYkqaHpDDPTcYr6DAjsM="/>
      <field:display field="description" id="s_org_pragmatikroo_roodocman_domain_Document_description" object="${document}" z="CeXQhSyEDeLn1Iu2hblanNbVc+A="/>
      <field:display field="filename" id="s_org_pragmatikroo_roodocman_domain_Document_filename" object="${document}" z="2HT3+zT1+1ft5kN4KhvXFWT9QnM="/>
      <field:display field="contentType" id="s_org_pragmatikroo_roodocman_domain_Document_contentType" object="${document}" z="FgCNUQ2KCygzKpUbP06Nxyjyfd8="/>
      <field:display field="size" id="s_org_pragmatikroo_roodocman_domain_Document_size" object="${document}" z="WdQ4wob2LThzMpucODZRa275TBc="/>
      <field:frame field="url" id="s_org_pragmatikroo_roodocman_domain_Document_url" object="${document}" render="true" z="user-managed"/>
</page:show>

This form depends on a custom tag frame.tagx. Basically is a iframe html for rendering images.

Show Form

List Form

Little issue with a missed dependency: When you deploy the app -after the file upload changes- you will find a missing dependence error.

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.0.1</version>
</dependency>

This is it!.

Download Source

Click here to download source code (ZIP, 94kb).

Conclusion

Well, the Reader has been exposed to two pretty close but at the same time different ways of implementing a particular web application project. Both are using the same high quality open source code Java-based development stack: Spring 3.X. I believe both ways are good and valid. There is a significant difference in Spring Roo approach though. Roo advocates for removing the “unnecessary abstractions” from web development. So, you won’t find Daos or Services classes in a typical Roo project. This is totally fine with me. However, it is a subject still in revision by the community at large. Not to mention that nothing stops you for implementing them if you need/want them. If you allow me one final point, I like better Roo approach because it gives me more time for thinking and figuring our how to solve the problem.

For example : In following we are multiplying 2.35 with 10000 and the result is printed.

//Division example
Double a = 2.85d / 10000;
System.out.println("1) " + a.doubleValue());

//Multiplication example
a = 2.85d * 100000000;
System.out.println("2) " + a.doubleValue());

Result:

1)  2.85E-4
2)  2.85E8

Thus you can see the result is printed in exponential format. Now you may want to display the result in pure decimal format like: 0.000285 or 285000000. You can do this simply by using class java.math.BigDecimal. In following example we are using BigDecimal.valueOf() to convert the Double value to BigDecimal and than .toPlainString() to convert it into plain decimal string.

import java.math.BigDecimal;
//..
//..

//Division example
Double a = 2.85d / 10000;
System.out.println("1) " + BigDecimal.valueOf(a).toPlainString());

//Multiplication example
a = 2.85d * 100000000;
System.out.println("2) " + BigDecimal.valueOf(a).toPlainString());

Result:

1)  0.000285
2)  285000000

The only disadvantage of the above method is that it generates lonnnnggg strings of number. You may want to restrict the value and round off the number to 5 or 6 decimal point. For this you can use java.text.DecimalFormat class. In following example we are rounding off the number to 4 decimal point and printing the output.

import java.text.DecimalFormat;
//..
//..

Double a = 2.85d / 10000;
DecimalFormat formatter = new DecimalFormat("0.0000");
System.out.println(formatter .format(a));

Result:

0.0003

Happy converting

Whenever an ENUM is complied in Java, two static methods are added by compiler called valueOf() and values(). We can use valueOf() method to convert any String value to ENUM. For example lets say we have an ENUM called Weekdays.

package net.viralpatel.java.enum;

public enum Weekdays {
	Monday,
	Tuesday,
	Wednesday,
	Thursday,
	Friday,
	Saturday,
	Sunday
}

Now we want to get an instance of Weekdays enum from string values lets say “Monday”, “Tuesday” etc. We can get this as follow:

Weekdays weekday = Weekdays.valueOf("Monday");
System.out.println(weekday);

Output:

Monday

One thing we need to take care here is if we pass an invalid string to valueOf() method like “XYZ”, the method will give a runtime exception.

Weekdays weekday = Weekdays.valueOf("XYZ");
System.out.println(weekday);

Output:

Exception in thread "main" java.lang.IllegalArgumentException:
        No enum const class net.viralpatel.java.enum.Weekdays.XYZ
	at java.lang.Enum.valueOf(Enum.java:192)

Happy converting

Here is a simple tutorial which you can go through to create Dynamic Web Project having Maven enabled in Eclipse. This project can be used as base project and can be easily converted to most kind of project like Struts based, Spring MVC based etc.

Required Tools

For this tutorial, I assume you have following setup in your machine.
1. JDK 1.5 or above (download)
2. Eclipse 3.2 or above (download)
3. Maven 2.0 or above (download)
4. M2Eclipse Plugin (download)

Step 1: Create Maven Project in Eclipse

Create a new project in Eclipse. Goto File > New > Project.. and select Maven Project from the list. Click Next.

Enter “MavenWeb” as Project name and click Next. On Configuration screen, select war in Packaging and also check the checkbox for src/main/webapp.

Once done, click Finish. This will create a Maven project in Eclipse.

Step 2: Generate Eclipse Project with WTP

Let us convert the Maven project to Dynamic Web Project for Eclipse. For this we will use following maven command.

mvn eclipse:eclipse -Dwtpversion=1.5

Goto the folder where the new project is created and execute above command.

C:\Workspace\Test\MavenWeb>mvn eclipse:eclipse -Dwtpversion=1.5

[INFO] Scanning for projects...
[INFO] Searching repository for plugin with prefix: 'eclipse'.
[INFO] ------------------------------------------------------------------------
[INFO] Building Unnamed - MavenWeb:MavenWeb:war:0.0.1-SNAPSHOT
[INFO]    task-segment: [eclipse:eclipse]
[INFO] ------------------------------------------------------------------------
[INFO] Preparing eclipse:eclipse
[INFO] No goals needed for project - skipping
[INFO] [eclipse:eclipse]
[INFO] Adding support for WTP version 1.5.
[INFO] Using Eclipse Workspace: C:\Workspace\Test
[INFO] no substring wtp server match.
[INFO] Using as WTP server : Apache Tomcat v5.5
[INFO] Adding default classpath container: org.eclipse.jdt.launching.JRE_CONTAINER
[INFO] Not writing settings - defaults suffice
[INFO] Wrote Eclipse project for "MavenWeb" to C:\Workspace\Test\MavenWeb.
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4 seconds
[INFO] Finished at: Wed Jul 28 13:55:09 CEST 2010
[INFO] Final Memory: 7M/30M
[INFO] ------------------------------------------------------------------------

That’s it. We just created Dynamic Web Project from Maven project. Now refresh the project in Eclipse.

Step 3: Change Project Facet

The above step by default set the project facet to JDK 1.4. We need to modify this and set project facet to JDK 5. Right click on MavenWeb project and select Properties (shortcut: Alt+Enter). From the Properties dialog box, select Project Facets. Now click on “Modify Project…” button and change the project facet to Java 5.0

Step 4: Setting Build Path

We need to specify the Maven jar dependencies in Java Build Path of our MavenWeb project. Open Properties.. dialog box (Alt+Enter) and select Java Build Path. Click “Add Library..” button and select “Maven Managed Dependencies” and click Finish.

Step 5: Hello World Servlet

Our dynamic web project with maven support is done now. Let us add a small Hello World servlet to this project and see how it works.

Open pom.xml from root folder and copy following content into it.

File: pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project>
	<modelVersion>4.0.0</modelVersion>
	<groupId>MavenWeb</groupId>
	<artifactId>MavenWeb</artifactId>
	<packaging>war</packaging>
	<version>0.0.1-SNAPSHOT</version>
	<description></description>
	<build>
                <plugins>
                        <plugin>
                                <artifactId>maven-compiler-plugin</artifactId>
                                <configuration>
                                        <source>1.5</source>
                                        <target>1.5</target>
                                </configuration>
                        </plugin>
			<plugin>
				<artifactId>maven-war-plugin</artifactId>
				<version>2.0</version>
			</plugin>
		</plugins>
	</build>
	<dependencies>
		<dependency>
			<groupId>javax.servlet</groupId>
			<artifactId>servlet-api</artifactId>
			<version>2.5</version>
		</dependency>
	</dependencies>
</project>

Also create a servlet file HelloWorldServlet.java. We will add this servlet in net.viralpatel.maven package.

File: /src/main/java/net/viralpatel/maven/HelloWorldServlet.java

package net.viralpatel.maven;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorldServlet extends HttpServlet {

	private static final long serialVersionUID = 1031422249396784970L;

	public void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {

		resp.setContentType("text/html");

		PrintWriter out = resp.getWriter();
		out.print("Hello World from Servlet");
		out.flush();
		out.close();
	}
}

Once the servlet is created, let us configure this in web.xml. Note that in our maven project no web.xml is present. We will create one at /src/main/webapp/WEB-INF/ location.

File: /src/main/webapp/WEB-INF/web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns="http://java.sun.com/xml/ns/javaee"
	xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	id="WebApp_ID" version="2.5">
	<display-name>HelloWorldServlet</display-name>
	<welcome-file-list>
		<welcome-file>hello-world</welcome-file>
	</welcome-file-list>

	<servlet>
		<servlet-name>HelloWorldServlet</servlet-name>
		<servlet-class>
			net.viralpatel.maven.HelloWorldServlet
		</servlet-class>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<servlet-mapping>
		<servlet-name>HelloWorldServlet</servlet-name>
		<url-pattern>/hello-world</url-pattern>
	</servlet-mapping>
</web-app>

That’s All Folks

Our dynamic web project with Maven support in Eclipse is completed. Run the web project in eclipse (Alt+Shirt+X, R)

Download Source

Click here to download the source code (7.0 kb)

Here is a simple trick for Java developers to calculate free diskspace. We have used Apache Commons IO library to calculate this.

Apache Commons IO library contains a class org.apache.commons.io.FileSystemUtils which can be used to calculate the free disk space in any system. Let us see the Java code for this.

package net.viralpatel.java;

import java.io.IOException;

import org.apache.commons.io.FileSystemUtils;

public class DiskSpace {
	public static void main(String[] args) {
		try {

			//calculate free disk space
			double freeDiskSpace = FileSystemUtils.freeSpaceKb("C:"); 

			//convert the number into gigabyte
			double freeDiskSpaceGB = freeDiskSpace / 1024 / 1024;

			System.out.println("Free Disk Space (GB):" + freeDiskSpaceGB);
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}

Output:

Free Disk Space (GB): 40.145268

In above code we used FileSystemUtils.freeSpaceKb( ) method to get the free space in kilo byte. This method invokes the command line to calculate the free disk space. You may want to call this method in following way to get free disk space in Windows and Linux.

 FileSystemUtils.freeSpaceKb("C:");       // Windows
 FileSystemUtils.freeSpaceKb("/volume");  // *nix
 

The free space is calculated via the command line. It uses ‘dir /-c’ on Windows, ‘df -kP’ on AIX/HP-UX and ‘df -k’ on other Unix.

In order to work, you must be running Windows, or have a implementation of Unix df that supports GNU format when passed -k (or -kP). If you are going to rely on this code, please check that it works on your OS by running some simple tests to compare the command line with the output from this class.

import java.util.Arrays;
...
...
Arrays.sort (int [])
Arrays.sort (String [])
Arrays.sort (float [])
Arrays.sort (double [])
Arrays.sort (long [])
Arrays.sort (Object [])
...

Let us check an example were we will sort an array of String in ascending as well as descending order. Here is a string array we defined in Java.

String [] stringArray = {"ab", "aB", "c", "0", "2", "1Ad", "a10"};

System.out.println("****** Unsorted String Array *******");
for (String str : stringArray) {
	System.out.println(str);
}

Output:

****** unsorted string *******
ab
aB
c
0
2
1Ad
a10

In above code we simple define an array of String and printed its value. Now lets sort this array in ascending order using Arrays.sort() method.

Sort in Ascending Order

//Sort array in ascending order
Arrays.sort(stringArray);

System.out.println("****** Sorted String Array *******");
for (String str : stringArray) {
	System.out.println(str);
}

Output:

****** Sorted String Array *******
0
1Ad
2
a10
aB
ab
c

Note that we just sorted an array of String in ascending order using sort method. Wasn’t it easy..

Sort in Descending Order

Now lets try to sort the array in reverse order. For this we will use a different signature of sort method.

Arrays.sort (Object [], Comparator)

Following is the code to sort array in reverse order.

//Sort array in reverse order
Arrays.sort(stringArray, Collections.reverseOrder());

System.out.println("****** Reverse Sorted String Array *******");
for (String str : stringArray) {
	System.out.println(str);
}

Output:

****** Reverse Sorted String Array *******
c
ab
aB
a10
2
1Ad
0

Selective Sorting

Using Arrays.sort() method it is possible to sort an array selectively. i.e. if you want a subpart of array to be sorted, that is possible using following method.

Arrays.sort (Object [], int startIndex, int endIndex)

In following code we are sorting the array starting from index 3 till the end.

//Sorting array starting from index 3 till 6
Arrays.sort(stringArray, 3, 6);

System.out.println("****** Selective Sort String Array *******");
for (String str : stringArray) {
	System.out.println(str);
}

Output:

****** Selective Sort String Array *******
ab
aB
c
0
1Ad
2
a10

Happy sorting..