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

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(); 
  } 
}

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

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

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(); 
  } 
}

java.util.Date utilDate = new java.util.Date(); 
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

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(); 
      } 
    } 
  }

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(); 
  }

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

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(); 
    } 
  } 
}

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; 
  } 
}

public enum SimpleSingleton { 
  INSTANCE; 
  public void doSomething() { 
  } 
} 
 
//Call the method from Singleton: 
SimpleSingleton.INSTANCE.doSomething();

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)); 
 
} 
...

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);

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(); 
  } 
}

<?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>

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"); 
  } 
}

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")); 
 } 
}

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); 
}

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://coolshell.cn/"); 
      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(); 
    } 
  } 
}

/** 
* 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]); 
}