20 πολύ χρήσιμα Java code snippets

[kreach]

Ρίχτε μια ματιά εδώ: http://viralpatel.net/blogs/2009/05/20-useful-java-code-snippets-for-java-developers.html

Παραθέτω τα 10 πρώτα...

1. Converting Strings to int and int to String

[LIST=1]
[*]String a = String.valueOf(2);   //integer to numeric string
[*]int i = Integer.parseInt(a); //numeric string to an int
[/LIST]

2. Append text to file in Java

[LIST=1]
[*]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();
[*]    }
[*]}
[/LIST]

3. Get name of current method in Java

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

4. Convert String to Date in Java

[LIST=1]
[*]java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
[/LIST]

ή

[LIST=1]
[*]SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
[*]Date date = format.parse( myString );
[/LIST]

5. Connecting to Oracle using Java JDBC

[LIST=1]
[*]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();
[*]    }
[*]}
[/LIST]

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.

[LIST=1]
[*]java.util.Date utilDate = new java.util.Date();
[*]java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
[/LIST]

7. Java Fast File Copy using NIO

[LIST=1]
[*]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();
[*]            }
[*]        }
[*]    }
[/LIST]

8. Create Thumbnail of an image in Java

[LIST=1]
[*]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();
[*]    }
[/LIST]

9. Creating JSON data in Java

Read this article for more details.

Download JAR file json-rpc-1.0.jar (75 kb)

[LIST=1]
[*]import org.json.JSONObject;
[*]...
[*]...
[*]JSONObject json = new JSONObject();
[*]json.put("city", "Mumbai");
[*]json.put("country", "India");
[*]...
[*]String output = json.toString();
[*]...
[/LIST]

10. PDF Generation in Java using iText JAR

Read this article for more details.

[LIST=1]
[*]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();
[*]        }
[*]    }
[*]}
[/LIST]