Useful Java Code Snippet

Convert String to Date in Java

1. java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);

java.util.Date = java.text.DateFormat.getDateInstance().parse(date String); 

or

1. SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
2. Date date = format.parse( myString )

Connecting to Oracle using Java JDBC

1. public class OracleJdbcTest
2. {
3. String driverClass = "oracle.jdbc.driver.OracleDriver";
4. 
   
5. Connection con;
6. 
   
7. public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException
8. {
9. Properties props = new Properties();
10. props.load(fs);
11. String url = props.getProperty("db.url");
12. String userName = props.getProperty("db.user");
13. String password = props.getProperty("db.password");
14. Class.forName(driverClass);
15. 
    
16. con=DriverManager.getConnection(url, userName, password);
17. }
18. 
    
19. public void fetch() throws SQLException, IOException
20. {
21. PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");
22. ResultSet rs = ps.executeQuery();
23. 
    
24. while (rs.next())
25. {
26. // do the thing you do
27. }
28. rs.close();
29. ps.close();
30. }
31. 
    
32. public static void main(String[] args)
33. {
34. OracleJdbcTest test = new OracleJdbcTest();
35. test.init();
36. test.fetch();
37. }
38. }
    

Java Fast File Copy using NIO

1. public static void fileCopy( File in, File out )
2. throws IOException
3. {
4. FileChannel inChannel = new FileInputStream( in ).getChannel();
5. FileChannel outChannel = new FileOutputStream( out ).getChannel();
6. try
7. {
8. // inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows
9. 
   
10. // magic number for Windows, 64Mb - 32Kb)
11. int maxCount = (64 * 1024 * 1024) - (32 * 1024);
12. long size = inChannel.size();
13. long position = 0;
14. while ( position <>
15. {
16. position += inChannel.transferTo( position, maxCount, outChannel );
17. }
18. }
19. finally
20. {
21. if ( inChannel != null )
22. {
23. inChannel.close();
24. }
25. if ( outChannel != null )
26. {
27. outChannel.close();
28. }
29. }
30. }
    

PDF Generation in Java using iText JAR

Read this article for more details.

1. import java.io.File;
2. import java.io.FileOutputStream;
3. import java.io.OutputStream;
4. import java.util.Date;
5. 
   
6. import com.lowagie.text.Document;
7. import com.lowagie.text.Paragraph;
8. import com.lowagie.text.pdf.PdfWriter;
9. 
   
10. public class GeneratePDF {
11. 
    
12. public static void main(String[] args) {
13. try {
14. OutputStream file = new FileOutputStream(new File("C:\\Test.pdf"));
15. 
    
16. Document document = new Document();
17. PdfWriter.getInstance(document, file);
18. document.open();
19. document.add(new Paragraph("Hello Kiran"));
20. document.add(new Paragraph(new Date().toString()));
21. 
    
22. document.close();
23. file.close();
24. 
    
25. } catch (Exception e) {
26. 
    
27. e.printStackTrace();
28. }
29. }
30. }
    

HTTP Proxy setting in Java

Read this article for more details.

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

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.

1. public class SimpleSingleton {
2. private static SimpleSingleton singleInstance = new SimpleSingleton();
3. 
   
4. //Marking default constructor private
5. //to avoid direct instantiation.
6. private SimpleSingleton() {
7. }
8. 
   
9. //Get instance for class SimpleSingleton
10. public static SimpleSingleton getInstance() {
11. 
    
12. return singleInstance;
13. }
14. }

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.

1. public enum SimpleSingleton {
2. INSTANCE;
3. public void doSomething() {
4. }
5. }
6. 
   
7. //Call the method from Singleton:
8. SimpleSingleton.INSTANCE.doSomething();
   

Capture screen shots in Java

Read this article for more details.

1. import java.awt.Dimension;
2. import java.awt.Rectangle;
3. import java.awt.Robot;
4. import java.awt.Toolkit;
5. import java.awt.image.BufferedImage;
6. import javax.imageio.ImageIO;
7. import java.io.File;
8. 
   
9. ...
10. 
    
11. public void captureScreen(String fileName) throws Exception {
12. 
    
13. Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
14. Rectangle screenRectangle = new Rectangle(screenSize);
15. Robot robot = new Robot();
16. BufferedImage image = robot.createScreenCapture(screenRectangle);
17. ImageIO.write(image, "png", new File(fileName));
18. 
    
19. }
20. ...

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));  } ... 

14. Files-Directory listing in Java

1. File dir = new File("directoryName");
2. String[] children = dir.list();
3. if (children == null) {
4. // Either dir does not exist or is not a directory
5. } else {
6. for (int i=0; i <>
7. // Get filename of file or directory
8. String filename = children[i];
9. }
10. }
11. 
    
12. // It is also possible to filter the list of returned files.
13. // This example does not return any files that start with `.'.
14. FilenameFilter filter = new FilenameFilter() {
15. public boolean accept(File dir, String name) {
16. return !name.startsWith(".");
17. }
18. };
19. children = dir.list(filter);
20. 
    
21. // The list of files can also be retrieved as File objects
22. File[] files = dir.listFiles();
23. 
    
24. // This filter only returns directories
25. FileFilter fileFilter = new FileFilter() {
26. public boolean accept(File file) {
27. return file.isDirectory();
28. }
29. };
30. files = dir.listFiles(fileFilter);
    

Creating ZIP and JAR Files in Java

1. import java.util.zip.*;
2. import java.io.*;
3. 
   
4. public class ZipIt {
5. public static void main(String args[]) throws IOException {
6. if (args.length < 2) {
7. System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
8. System.exit(-1);
9. }
10. File zipFile = new File(args[0]);
11. if (zipFile.exists()) {
12. System.err.println("Zip file already exists, please try another");
13. System.exit(-2);
14. }
15. FileOutputStream fos = new FileOutputStream(zipFile);
16. ZipOutputStream zos = new ZipOutputStream(fos);
17. int bytesRead;
18. byte[] buffer = new byte[1024];
19. CRC32 crc = new CRC32();
20. for (int i=1, n=args.length; i <>
21. String name = args[i];
22. File file = new File(name);
23. if (!file.exists()) {
24. System.err.println("Skipping: " + name);
25. continue;
26. }
27. BufferedInputStream bis = new BufferedInputStream(
28. new FileInputStream(file));
29. crc.reset();
30. while ((bytesRead = bis.read(buffer)) != -1) {
31. crc.update(buffer, 0, bytesRead);
32. }
33. bis.close();
34. // Reset to beginning of input stream
35. bis = new BufferedInputStream(
36. new FileInputStream(file));
37. ZipEntry entry = new ZipEntry(name);
38. entry.setMethod(ZipEntry.STORED);
39. entry.setCompressedSize(file.length());
40. entry.setSize(file.length());
41. entry.setCrc(crc.getValue());
42. zos.putNextEntry(entry);
43. while ((bytesRead = bis.read(buffer)) != -1) {
44. zos.write(buffer, 0, bytesRead);
45. }
46. bis.close();
47. }
48. zos.close();
49. }
50. }
    

Parsing / Reading XML file in Java

1. package net.viralpatel.java.xmlparser;
2. 
   
3. import java.io.File;
4. import javax.xml.parsers.DocumentBuilder;
5. import javax.xml.parsers.DocumentBuilderFactory;
6. 
   
7. import org.w3c.dom.Document;
8. import org.w3c.dom.Element;
9. import org.w3c.dom.Node;
10. import org.w3c.dom.NodeList;
11. 
    
12. public class XMLParser {
13. 
    
14. public void getAllUserNames(String fileName) {
15. try {
16. DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
17. DocumentBuilder db = dbf.newDocumentBuilder();
18. File file = new File(fileName);
19. if (file.exists()) {
20. Document doc = db.parse(file);
21. Element docEle = doc.getDocumentElement();
22. 
    
23. // Print root element of the document
24. System.out.println("Root element of the document: "
25. + docEle.getNodeName());
26. 
    
27. NodeList studentList = docEle.getElementsByTagName("student");
28. 
    
29. // Print total student elements in document
30. System.out
31. .println("Total students: " + studentList.getLength());
32. 
    
33. if (studentList != null && studentList.getLength() > 0) {
34. for (int i = 0; i <>
35. 
    
36. Node node = studentList.item(i);
37. 
    
38. if (node.getNodeType() == Node.ELEMENT_NODE) {
39. 
    
40. System.out
41. .println("=====================");
42. 
    
43. Element e = (Element) node;
44. NodeList nodeList = e.getElementsByTagName("name");
45. System.out.println("Name: "
46. + nodeList.item(0).getChildNodes().item(0)
47. .getNodeValue());
48. 
    
49. nodeList = e.getElementsByTagName("grade");
50. System.out.println("Grade: "
51. + nodeList.item(0).getChildNodes().item(0)
52. .getNodeValue());
53. 
    
54. nodeList = e.getElementsByTagName("age");
55. System.out.println("Age: "
56. + nodeList.item(0).getChildNodes().item(0)
57. .getNodeValue());
58. }
59. }
60. } else {
61. System.exit(1);
62. }
63. }
64. } catch (Exception e) {
65. System.out.println(e);
66. }
67. }
68. public static void main(String[] args) {
69. 
    
70. XMLParser parser = new XMLParser();
71. parser.getAllUserNames("c:\\test.xml");
72. }
73. }
    

Convert Array to Map in Java

1. import java.util.Map;
2. import org.apache.commons.lang.ArrayUtils;
3. 
   
4. public class Main {
5. 
   
6. public static void main(String[] args) {
7. String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" },
8. { "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } };
9. 
   
10. Map countryCapitals = ArrayUtils.toMap(countries);
11. 
    
12. System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));
13. System.out.println("Capital of France is " + countryCapitals.get("France"));
14. }
15. }
    

Send Email using Java

1. import javax.mail.*;
2. import javax.mail.internet.*;
3. import java.util.*;
4. 
   
5. public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
6. {
7. boolean debug = false;
8. 
   
9. //Set the host smtp address
10. Properties props = new Properties();
11. props.put("mail.smtp.host", "smtp.example.com");
12. 
    
13. // create some properties and get the default Session
14. Session session = Session.getDefaultInstance(props, null);
15. session.setDebug(debug);
16. 
    
17. // create a message
18. Message msg = new MimeMessage(session);
19. 
    
20. // set the from and to address
21. InternetAddress addressFrom = new InternetAddress(from);
22. msg.setFrom(addressFrom);
23. 
    
24. InternetAddress[] addressTo = new InternetAddress[recipients.length];
25. for (int i = 0; i <>
26. {
27. addressTo[i] = new InternetAddress(recipients[i]);
28. }
29. msg.setRecipients(Message.RecipientType.TO, addressTo);
30. 
    
31. // Optional : You can also set your custom headers in the Email if you Want
32. msg.addHeader("MyHeaderName", "myHeaderValue");
33. 
    
34. // Setting the Subject and Content Type
35. msg.setSubject(subject);
36. msg.setContent(message, "text/plain");
37. Transport.send(msg);
38. }
    

Send HTTP request & fetching data using Java

1. import java.io.BufferedReader;
2. import java.io.InputStreamReader;
3. import java.net.URL;
5. public class Main {
6. public static void main(String[] args) {
7. try {
8. URL my_url = new URL("http://www.viralpatel.net/blogs/");
9. BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));
10. String strTemp = "";
11. while(null != (strTemp = br.readLine())){
12. System.out.println(strTemp);
13. }
14. } catch (Exception ex) {
15. ex.printStackTrace();
16. }
17. }
18. }
    

Resize an Array in Java

1. /**
2. * Reallocates an array with a new size, and copies the contents
3. * of the old array to the new array.
4. * @param oldArray the old array, to be reallocated.
5. * @param newSize the new array size.
6. * @return A new array with the same contents.
7. */
8. private static Object resizeArray (Object oldArray, int newSize) {
9. int oldSize = java.lang.reflect.Array.getLength(oldArray);
10. Class elementType = oldArray.getClass().getComponentType();
11. Object newArray = java.lang.reflect.Array.newInstance(
12. elementType,newSize);
13. int preserveLength = Math.min(oldSize,newSize);
14. if (preserveLength > 0)
15. System.arraycopy (oldArray,0,newArray,0,preserveLength);
16. return newArray;
17. }
19. // Test routine for resizeArray().
20. public static void main (String[] args) {
21. int[] a = {1,2,3};
22. a = (int[])resizeArray(a,5);
23. a[3] = 4;
24. a[4] = 5;
25. for (int i=0; i
26. System.out.println (a[i]);
27. }