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. [ad#singleblogtop-linkad]

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
Code language: Java (java)

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));
	out.write(”aString”);
} catch (IOException e) {
	// error processing code
} finally {
	if (out != null) {
		out.close();
	}
}
Code language: Java (java)

3. Get name of current method in Java

String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
Code language: Java (java)

4. Convert String to Date in Java

java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
Code language: Java (java)

or

SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date = format.parse( myString );
Code language: Java (java)

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();
		props.load(fs);
		String url = props.getProperty("db.url");
		String userName = props.getProperty("db.user");
		String password = props.getProperty("db.password");
		Class.forName(driverClass);

		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 (rs.next())
		{
			// do the thing you do
		}
		rs.close();
		ps.close();
	}

	public static void main(String[] args) 
	{
		OracleJdbcTest test = new OracleJdbcTest();
		test.init();
		test.fetch();
	}
}
Code language: Java (java)

6. Convert Java util.Date to sql.Date

[ad] 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());
Code language: Java (java)

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();
        try
        {
//          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 );
            }
        }
        finally
        {
            if ( inChannel != null )
            {
               inChannel.close();
            }
            if ( outChannel != null )
            {
                outChannel.close();
            }
        }
    }
[/code]

<h2>8. Create Thumbnail of an image in Java</h2>
<!-- wp:code {"language": "java"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
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);
		mediaTracker.waitForID(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);
		encoder.setJPEGEncodeParam(param);
		encoder.encode(thumbImage);
		out.close();
	}
[/code]

<h2>9. Creating JSON data in Java</h2>
Read <a href="https://www.viralpatel.net/creating-parsing-json-data-with-java-servlet-struts-jsp-json/">this article</a> for more details.
Download JAR file <a href="https://www.viralpatel.net/download/json/json-rpc-1.0.jar">json-rpc-1.0.jar (75 kb)</a>
<!-- wp:code {"language": "java"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...
Code language: Java (java)

10. PDF Generation in Java using iText JAR

Read this article for more details.

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
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.open();
            document.add(new Paragraph("Hello Kiran"));
            document.add(new Paragraph(new Date().toString()));

            document.close();
            file.close();

        } catch (Exception e) {

            e.printStackTrace();
        }
    }
}
Code language: Java (java)

11. HTTP Proxy setting in Java

Read this article for more details.

System.getProperties().put("http.proxyHost", "someProxyURL");
System.getProperties().put("http.proxyPort", "someProxyPort");
System.getProperties().put("http.proxyUser", "someUserName");
System.getProperties().put("http.proxyPassword", "somePassword");
Code language: Java (java)

12. Java Singleton example

Read this article for more details. Update: Thanks Markus for the comment. I have updated the code and changed it to more robust implementation.

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;
	}
}
Code language: Java (java)

One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.

public enum SimpleSingleton {
	INSTANCE;
	public void doSomething() {
	}
}

//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
Code language: Java (java)

13. Capture screen shots in Java

Read this article for more details.

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;
import java.io.File;

...

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));

}
...
Code language: Java (java)

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);
[/code]


<h2>15. Creating ZIP and JAR Files in Java</h2>
<!-- wp:code {"language": "java"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
import java.util.zip.*;
import java.io.*;

public class ZipIt {
    public static void main(String args[]) throws IOException {
        if (args.length < 2) {
            System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
            System.exit(-1);
        }
        File zipFile = new File(args[0]);
        if (zipFile.exists()) {
            System.err.println("Zip file already exists, please try another");
            System.exit(-2);
        }
        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);
                continue;
            }
            BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream(file));
            crc.reset();
            while ((bytesRead = bis.read(buffer)) != -1) {
                crc.update(buffer, 0, bytesRead);
            }
            bis.close();
            // Reset to beginning of input stream
            bis = new BufferedInputStream(
                new FileInputStream(file));
            ZipEntry entry = new ZipEntry(name);
            entry.setMethod(ZipEntry.STORED);
            entry.setCompressedSize(file.length());
            entry.setSize(file.length());
            entry.setCrc(crc.getValue());
            zos.putNextEntry(entry);
            while ((bytesRead = bis.read(buffer)) != -1) {
                zos.write(buffer, 0, bytesRead);
            }
            bis.close();
        }
        zos.close();
    }
}
[/code]



<h2>16. Parsing / Reading XML file in Java</h2>
Sample XML file.
<!-- wp:code {"language": "xml"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
<?xml version="1.0"?>
<students>
	<student>
		<name>John</name>
		<grade>B</grade>
		<age>12</age>
	</student>
	<student>
		<name>Mary</name>
		<grade>A</grade>
		<age>11</age>
	</student>
	<student>
		<name>Simon</name>
		<grade>A</grade>
		<age>18</age>
	</student>
</students>
Code language: Java (java)

Java code to parse above XML.

package net.viralpatel.java.xmlparser;

import java.io.File;
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
				System.out
						.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) {

							System.out
									.println("=====================");

							Element e = (Element) node;
							NodeList nodeList = e.getElementsByTagName("name");
							System.out.println("Name: "
									+ nodeList.item(0).getChildNodes().item(0)
											.getNodeValue());

							nodeList = e.getElementsByTagName("grade");
							System.out.println("Grade: "
									+ nodeList.item(0).getChildNodes().item(0)
											.getNodeValue());

							nodeList = e.getElementsByTagName("age");
							System.out.println("Age: "
									+ nodeList.item(0).getChildNodes().item(0)
											.getNodeValue());
						}
					}
				} else {
					System.exit(1);
				}
			}
		} catch (Exception e) {
			System.out.println(e);
		}
	}
	public static void main(String[] args) {

		XMLParser parser = new XMLParser();
		parser.getAllUserNames("c:\\test.xml");
	}
}
[/code]

<h2>17. Convert Array to Map in Java</h2>
<!-- wp:code {"language": "java"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
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"));
  }
}
Code language: Java (java)

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("mail.smtp.host", "smtp.example.com");

    // create some properties and get the default Session
    Session session = Session.getDefaultInstance(props, null);
    session.setDebug(debug);

    // create a message
    Message msg = new MimeMessage(session);

    // set the from and to address
    InternetAddress addressFrom = new InternetAddress(from);
    msg.setFrom(addressFrom);

    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.setSubject(subject);
    msg.setContent(message, "text/plain");
    Transport.send(msg);
}
[/code]

<h2>19. Send HTTP request & fetching data using Java</h2>
<!-- wp:code {"language": "java"} --><pre class="wp-block-code"><code></code></pre><!-- /wp:code -->
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;

public class Main {
	public static void main(String[] args)  {
		try {
			URL my_url = new URL("http://www.viralpatel.net/");
			BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));
			String strTemp = "";
			while(null != (strTemp = br.readLine())){
			System.out.println(strTemp);
		}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
}
Code language: Java (java)

20. Resize an Array in Java

Code language: Java (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( elementType,newSize); 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