非常有用的20个Java程序片段

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, Mb - Kb) 
                    int maxCount = ( *  * ) - ( * ); 
                    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, 0)); 
                param.setQuality((float)quality / 0.0f, 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 ( 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[]; 
                CRC crc = new CRC(); 
                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></age>
            </student>
            <student>
                <name>Mary</name>
                <grade>A</grade>
                <age></age>
            </student>
            <student>
                <name>Simon</name>
                <grade>A</grade>
                <age></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]); 
        }