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();  
}  

}</pre>
 6. 把 Java util.Date 转成 sql.Date 
java.util.Date utilDate = new java.util.Date();  
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
 7. 使用NIO进行快速的文件拷贝 
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();  
        }  
    }  
}</pre> 

 8. 创建图片的缩略图 
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();  
}</pre> 

 9. 创建 JSON 格式的数据 
 请先阅读这篇文章 了解一些细节， 
 并下面这个JAR 文件：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. 使用iText JAR生成PDF 
 阅读这篇文章 了解更多细节 
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();  
    }  
}  

}</pre>
 11. HTTP 代理设置 
 阅读这篇 文章 了解更多细节。 
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. 单实例Singleton 示例 
 请先阅读这篇文章 了解更多信息 
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;  
}  

}</pre>
 另一种实现 
public enum SimpleSingleton {

    INSTANCE;

    public void doSomething() {

    }

}
//Call the method from Singleton:

SimpleSingleton.INSTANCE.doSomething();</pre>
 13. 抓屏程序 
 阅读这篇文章 获得更多信息。 
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));
}

...</pre>
 14. 列出文件和目录 
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);</pre>
 15. 创建ZIP和JAR文件 
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();

    }

}</pre>
 16. 解析/读取XML 文件 
 XML文件 
<?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>
 Java代码 
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");  
}  

}</pre>
 17. 把 Array 转换成 Map  
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"));  

}

}</pre>
 18. 发送邮件 
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);  

}</pre>
 19. 发送代数据的HTTP 请求 
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/&quot;);

            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();

        }

    }

}</pre>
 20. 改变数组的大小 
/**

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]);

}</pre> 来源：
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