20个非常有用的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

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

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（需下载IText.jar）

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

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

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

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

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]);   }