20 very useful code snippets from the Java program
- 2020-05-05 11:17:55
- OfStack
Here are 20 useful Java snippets that I hope you found useful.
1. Strings have integer conversions
String a = String.valueOf(2); //integer to numeric string
int i = Integer.parseInt(a); //numeric string to an int
2. Add
to the end of the file
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();
}
}
Get the name of the current method,
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
4. Transfer string to date
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
Or:
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date = format.parse( myString );
5. Link Oracle
using 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();
}
}
6. Convert Java util.Date to sql.Date
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
7. Quick 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();
}
}
}
8. Create thumbnails of images
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();
}
9. Create
data in JSON format
And the following JAR file: json-rpc-1.0.jar (75 kb)
import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...
10. Generate PDF
using iText JAR
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();
}
}
}
11. HTTP agent sets
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. Single instance Singleton sample
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();
}
}
0
Another implementation is
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();
}
}
1
13. Screen capture program
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();
}
}
2
14. Lists files and directories
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. Create ZIP and JAR files
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();
}
}
4
16. Parse/read XML file
XML file
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();
}
}
5
Java 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();
}
}
6
17. Convert Array to Map
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();
}
}
7
18. Send email
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);
}
19. Request
for HTTP sending generation data
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();
}
}
9
20. Change the size of the array
/**
* 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<a.length; i++)
System.out.println (a[i]);
}
I hope this article is helpful for you to learn java programming.