下面是20个非常有用的Java程序片段，希望能对你有用。

1. 字符串有整型的相互转换

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

2. 向文件末尾添加内容

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();        }    }

3. 得到当前方法的名字

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

4. 转字符串到日期

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 );

5. 使用JDBC链接Oracle

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. 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();                }            }        }

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();        }

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();            }        }    }

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;        }    }

另一种实现

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

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));      }    ...

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);

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();        }    }

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");        }    }

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"));      }    }

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);    }

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/");                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();            }        }    }

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]);    }

来源： 酷壳