20个非常有用的Java程序片段

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);

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();  
        }  
    }  
}

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

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]);  
}