Showing posts with label java. Show all posts
Showing posts with label java. Show all posts

5 class design principles [S.O.L.I.D.] in Java

Classes are the building blocks of your java application. If these blocks are not strong, your building (i.e. application) is going to face the tough time in future. This essentially means that not so well-written can lead to very difficult situations when the application scope goes up or application faces certain design issues either in production or maintenance.

On the other hand, set of well designed and written classes can speed up the coding process by leaps and bounds, while reducing the number of bugs in comparison.

In this post, I will list down 5 most recommended design principles, you should keep in mind, while writing your classes. These design principles are called SOLID, in short. They also form the best practices to be followed for designing your application classes.
  1. Single Responsibility Principle
  2. Open Closed Principle
  3. Liskov’s Substitution Principle
  4. Interface Segregation Principle
  5. Dependency Inversion Principle

Lets drill down all of them one by one.

Single Responsibility Principle

The name of the principle says it all:

"One class should have one and only one responsibility"

In other words, you should write, change and maintain a class for only one purpose. If it is model class then it should strictly represent only one actor/ entity. This will give you the flexibility to make changes in future without worrying the impacts of changes for another entity.

Similarly, If you are writing service/manager class then it should contain only that part of method calls and nothing else. Not even utility global functions related to module. Better separate them in another globally accessible class file. This will help in maintaining the class for that particular purpose, and you can decide the visibility of class to specific module only.

Open Closed Principle

This is second important rule which you should keep in mind while designing your application. It says:

"Software components should be open for extension, but closed for modification"

What does it mean?? It means that your classes should be designed such a way that whenever fellow developers wants to change the flow of control in specific conditions in application, all they need to extend your class and override some functions and that’s it.

If other developers are not able to design desired behavior due to constraints put by your class, then you should reconsider changing your class. I do not mean here that anybody can change the whole logic of your class, but he/she should be able to override the options provided by software in unharmful way permitted by software.

For example, if you take a look into any good framework like struts or spring, you will see that you can change their core logic and request processing, BUT you modify the desired application flow just by extending some classes and plugin them in configuration files.

Liskov’s Substitution Principle

This principle is a variation of previously discussed open closed principle. It says:

"Derived types must be completely substitutable for their base types"

It means that the classes fellow developer created by extending your class should be able to fit in application without failure. I.e. if a fellow developer poorly extended some
part of your class and injected into framework/ application then it should not break the application or should not throw fatal exceptions.

This can be insured by using strictly following first rule. If your base class is doing one thing strictly, the fellow developer will override only one feature incorrectly in worst case. This can cause some errors in one area, but whole application will not do down.

Interface Segregation Principle

This principle is my favorite one. It is applicable to interfaces as single responsibility principle holds to classes. It says:

"Clients should not be forced to implement unnecessary methods which they will not use"

Take an example. Developer Alex created an interface “Reportable” and added two methods generateExcel() and generatedPdf(). Now client ‘A’ wants to use this interface but he intend to use reports only in PDF format and not in excel. Will he achieve the functionality easily.

NO. He will have to implement two methods, out of which one is extra burden put on him by designer of software. Either he will implement another method or leave it blank. So are not desired cases, right??

So what is the solution? Solution is to create two interfaces by breaking the existing one. They should be like PdfReportable and ExcelReportable. This will give the flexibility to user to use only required functionality only.

Dependency Inversion Principle

Most of us are already familiar with the words used in principle’s name. It says:

"Depend on abstractions, not on concretions"

In other words. you should design your software in such a way that various modules can be separated from each other using an abstract layer to bind them together. The classical use of this principle of BeanFactory inspring framework. In spring framework, all modules are provided as separate components which can work together by simply injected dependencies in other module. They are so well closed in their boundaries that you can use them in other software modules apart from spring with same ease.

This has been achieved by dependency inversion and open closed principles. All modules expose only abstraction which is useful in extending the functionality or plugin in another module.

These were five class design principle which makes the best practices to be followed to design your application classes. Let me know of your thoughts.

Happy Learning !!

20 very useful Java code snippets for Java Developers

Following are few very useful Java code snippets for Java developers. Few of them are written by me and few are taken from other code reference. Feel free to comment about the code and also add your code snippet.

1. Converting Strings to int and int to String

String a = String.valueOf(2);   //integer to numeric string
int i = Integer.parseInt(a); //numeric string to an int

2. Append text to file in Java

Updated: Thanks Simone for pointing to exception. I have changed the code.
BufferedWriter out = null;
try {
    out = new BufferedWriter(new FileWriter(”filename”, true));
} catch (IOException e) {
    // error processing code
} finally {
    if (out != null) {

3. Get name of current method in Java

String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4. Convert String to Date in Java

java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date = format.parse( myString );

5. Connecting to Oracle using Java JDBC

public class OracleJdbcTest
    String driverClass = "oracle.jdbc.driver.OracleDriver";
    Connection con;
    public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException
        Properties props = new Properties();
        String url = props.getProperty("db.url");
        String userName = props.getProperty("db.user");
        String password = props.getProperty("db.password");
        con=DriverManager.getConnection(url, userName, password);
    public void fetch() throws SQLException, IOException
        PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");
        ResultSet rs = ps.executeQuery();
        while (
            // do the thing you do
    public static void main(String[] args)
        OracleJdbcTest test = new OracleJdbcTest();

6. Convert Java util.Date to sql.Date

This snippet shows how to convert a java util Date into a sql Date for use in databases.
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. Java Fast File Copy using NIO

public static void fileCopy( File in, File out )
            throws IOException
        FileChannel inChannel = new FileInputStream( in ).getChannel();
        FileChannel outChannel = new FileOutputStream( out ).getChannel();
//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows
            // magic number for Windows, 64Mb - 32Kb)
            int maxCount = (64 * 1024 * 1024) - (32 * 1024);
            long size = inChannel.size();
            long position = 0;
            while ( position < size )
               position += inChannel.transferTo( position, maxCount, outChannel );
            if ( inChannel != null )
            if ( outChannel != null )

8. Create Thumbnail of an image in Java

private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)
        throws InterruptedException, FileNotFoundException, IOException
        // load image from filename
        Image image = Toolkit.getDefaultToolkit().getImage(filename);
        MediaTracker mediaTracker = new MediaTracker(new Container());
        mediaTracker.addImage(image, 0);
        // use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny());
        // determine thumbnail size from WIDTH and HEIGHT
        double thumbRatio = (double)thumbWidth / (double)thumbHeight;
        int imageWidth = image.getWidth(null);
        int imageHeight = image.getHeight(null);
        double imageRatio = (double)imageWidth / (double)imageHeight;
        if (thumbRatio < imageRatio) {
            thumbHeight = (int)(thumbWidth / imageRatio);
        } else {
            thumbWidth = (int)(thumbHeight * imageRatio);
        // draw original image to thumbnail image object and
        // scale it to the new size on-the-fly
        BufferedImage thumbImage = new BufferedImage(thumbWidth, thumbHeight, BufferedImage.TYPE_INT_RGB);
        Graphics2D graphics2D = thumbImage.createGraphics();
        graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        graphics2D.drawImage(image, 0, 0, thumbWidth, thumbHeight, null);
        // save thumbnail image to outFilename
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(outFilename));
        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
        JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(thumbImage);
        quality = Math.max(0, Math.min(quality, 100));
        param.setQuality((float)quality / 100.0f, false);

9. Creating JSON data in Java

import org.json.JSONObject;
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
String output = json.toString();

10. PDF Generation in Java using iText JAR

import java.util.Date;
import com.lowagie.text.Document;
import com.lowagie.text.Paragraph;
import com.lowagie.text.pdf.PdfWriter;
public class GeneratePDF {
    public static void main(String[] args) {
        try {
            OutputStream file = new FileOutputStream(new File("C:\\Test.pdf"));
            Document document = new Document();
            PdfWriter.getInstance(document, file);
            document.add(new Paragraph("Hello Kiran"));
            document.add(new Paragraph(new Date().toString()));
        } catch (Exception e) {

11. HTTP Proxy setting in Java

System.getProperties().put("http.proxyHost", "someProxyURL");
System.getProperties().put("http.proxyPort", "someProxyPort");
System.getProperties().put("http.proxyUser", "someUserName");
System.getProperties().put("http.proxyPassword", "somePassword");

12. Java Singleton example

public class SimpleSingleton {
    private static SimpleSingleton singleInstance =  new SimpleSingleton();
    //Marking default constructor private
    //to avoid direct instantiation.
    private SimpleSingleton() {
    //Get instance for class SimpleSingleton
    public static SimpleSingleton getInstance() {
        return singleInstance;
One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.
public enum SimpleSingleton {
    public void doSomething() {
//Call the method from Singleton:

13. Capture screen shots in Java

import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
public void captureScreen(String fileName) throws Exception {
   Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
   Rectangle screenRectangle = new Rectangle(screenSize);
   Robot robot = new Robot();
   BufferedImage image = robot.createScreenCapture(screenRectangle);
   ImageIO.write(image, "png", new File(fileName));

14. Files-Directory listing in Java

File dir = new File("directoryName");
  String[] children = dir.list();
  if (children == null) {
      // Either dir does not exist or is not a directory
  } else {
      for (int i=0; i < children.length; i++) {
          // Get filename of file or directory
          String filename = children[i];
  // It is also possible to filter the list of returned files.
  // This example does not return any files that start with `.'.
  FilenameFilter filter = new FilenameFilter() {
      public boolean accept(File dir, String name) {
          return !name.startsWith(".");
  children = dir.list(filter);
  // The list of files can also be retrieved as File objects
  File[] files = dir.listFiles();
  // This filter only returns directories
  FileFilter fileFilter = new FileFilter() {
      public boolean accept(File file) {
          return file.isDirectory();
  files = dir.listFiles(fileFilter);

15. Creating ZIP and JAR Files in Java

public class ZipIt {
    public static void main(String args[]) throws IOException {
        if (args.length < 2) {
            System.err.println("usage: java ZipIt file1 file2 file3");
        File zipFile = new File(args[0]);
        if (zipFile.exists()) {
            System.err.println("Zip file already exists, please try another");
        FileOutputStream fos = new FileOutputStream(zipFile);
        ZipOutputStream zos = new ZipOutputStream(fos);
        int bytesRead;
        byte[] buffer = new byte[1024];
        CRC32 crc = new CRC32();
        for (int i=1, n=args.length; i < n; i++) {
            String name = args[i];
            File file = new File(name);
            if (!file.exists()) {
                System.err.println("Skipping: " + name);
            BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream(file));
            while ((bytesRead = != -1) {
                crc.update(buffer, 0, bytesRead);
            // Reset to beginning of input stream
            bis = new BufferedInputStream(
                new FileInputStream(file));
            ZipEntry entry = new ZipEntry(name);
            while ((bytesRead = != -1) {
                zos.write(buffer, 0, bytesRead);

16. Parsing / Reading XML file in Java

Sample XML file.
xml version="1.0"?>
Java code to parse above XML.
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class XMLParser {
    public void getAllUserNames(String fileName) {
        try {
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            DocumentBuilder db = dbf.newDocumentBuilder();
            File file = new File(fileName);
            if (file.exists()) {
                Document doc = db.parse(file);
                Element docEle = doc.getDocumentElement();
                // Print root element of the document
                System.out.println("Root element of the document: "
                        + docEle.getNodeName());
                NodeList studentList = docEle.getElementsByTagName("student");
                // Print total student elements in document
                        .println("Total students: " + studentList.getLength());
                if (studentList != null && studentList.getLength() > 0) {
                    for (int i = 0; i < studentList.getLength(); i++) {
                        Node node = studentList.item(i);
                        if (node.getNodeType() == Node.ELEMENT_NODE) {
                            Element e = (Element) node;
                            NodeList nodeList = e.getElementsByTagName("name");
                            System.out.println("Name: "
                                    + nodeList.item(0).getChildNodes().item(0)
                            nodeList = e.getElementsByTagName("grade");
                            System.out.println("Grade: "
                                    + nodeList.item(0).getChildNodes().item(0)
                            nodeList = e.getElementsByTagName("age");
                            System.out.println("Age: "
                                    + nodeList.item(0).getChildNodes().item(0)
                } else {
        } catch (Exception e) {
    public static void main(String[] args) {
        XMLParser parser = new XMLParser();

17. Convert Array to Map in Java

import java.util.Map;
import org.apache.commons.lang.ArrayUtils;
public class Main {
  public static void main(String[] args) {
    String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" },
        { "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } };
    Map countryCapitals = ArrayUtils.toMap(countries);
    System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));
    System.out.println("Capital of France is " + countryCapitals.get("France"));

18. Send Email using Java

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
    boolean debug = false;
     //Set the host smtp address
     Properties props = new Properties();
     props.put("", "");
    // create some properties and get the default Session
    Session session = Session.getDefaultInstance(props, null);
    // create a message
    Message msg = new MimeMessage(session);
    // set the from and to address
    InternetAddress addressFrom = new InternetAddress(from);
    InternetAddress[] addressTo = new InternetAddress[recipients.length];
    for (int i = 0; i < recipients.length; i++)
        addressTo[i] = new InternetAddress(recipients[i]);
    msg.setRecipients(Message.RecipientType.TO, addressTo);
    // Optional : You can also set your custom headers in the Email if you Want
    msg.addHeader("MyHeaderName", "myHeaderValue");
    // Setting the Subject and Content Type
    msg.setContent(message, "text/plain");

19. Send HTTP request & fetching data using Java

public class Main {
    public static void main(String[] args)  {
        try {
            URL my_url = new URL("");
            BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));
            String strTemp = "";
            while(null != (strTemp = br.readLine())){
        } catch (Exception ex) {

20. Resize an Array in Java

* Reallocates an array with a new size, and copies the contents
* of the old array to the new array.
* @param oldArray  the old array, to be reallocated.
* @param newSize   the new array size.
* @return          A new array with the same contents.
private static Object resizeArray (Object oldArray, int newSize) {
   int oldSize = java.lang.reflect.Array.getLength(oldArray);
   Class elementType = oldArray.getClass().getComponentType();
   Object newArray = java.lang.reflect.Array.newInstance(
   int preserveLength = Math.min(oldSize,newSize);
   if (preserveLength > 0)
      System.arraycopy (oldArray,0,newArray,0,preserveLength);
   return newArray;
// Test routine for resizeArray().
public static void main (String[] args) {
   int[] a = {1,2,3};
   a = (int[])resizeArray(a,5);
   a[3] = 4;
   a[4] = 5;
   for (int i=0; i
      System.out.println (a[i]);