20非常有用的Java程序片段

来源：互联网发布：淘宝十天不发货怎么办编辑：程序博客网时间：2024/04/28 13:21

转自：http://coolshell.cn/articles/889.html#more-889

1. 字符串有整型的相互转换

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

2. 向文件末尾添加内容

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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

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

4. 转字符串到日期

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java.util.Date = java.text.DateFormat.getDateInstance().parse(date String); 

或者是：

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SimpleDateFormat format =new SimpleDateFormat( "dd.MM.yyyy" );  
Date date = format.parse( myString ); 

5. 使用JDBC链接Oracle

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publicclass OracleJdbcTest  
{  
    String driverClass ="oracle.jdbc.driver.OracleDriver";  
  
    Connection con;  
  
    publicvoid init(FileInputStream fs) throwsClassNotFoundException, 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);  
    }  
  
    publicvoid 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();  
    }  
  
    publicstatic void main(String[] args)  
    {  
        OracleJdbcTest test =new OracleJdbcTest();  
        test.init();  
        test.fetch();  
    }  
} 

6. 把 Java util.Date 转成 sql.Date

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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

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publicstatic void fileCopy( File in, File out )  
            throwsIOException  
    {  
        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)  
            intmaxCount = (64* 1024* 1024) - (32* 1024);  
            longsize = inChannel.size();  
            longposition = 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

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privatevoid createThumbnail(String filename, intthumbWidth, intthumbHeight, intquality, String outFilename)  
        throwsInterruptedException, 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  
        doublethumbRatio = (double)thumbWidth / (double)thumbHeight;  
        intimageWidth = image.getWidth(null);  
        intimageHeight = image.getHeight(null);  
        doubleimageRatio = (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)

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importorg.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

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importjava.io.File;  
importjava.io.FileOutputStream;  
importjava.io.OutputStream;  
importjava.util.Date;  
  
importcom.lowagie.text.Document;  
importcom.lowagie.text.Paragraph;  
importcom.lowagie.text.pdf.PdfWriter;  
  
publicclass GeneratePDF {  
  
    publicstatic 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(newParagraph("Hello Kiran"));  
            document.add(newParagraph(newDate().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。

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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

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publicclass SimpleSingleton {  
    privatestatic SimpleSingleton singleInstance =  newSimpleSingleton();  
  
    //Marking default constructor private  
    //to avoid direct instantiation.  
    privateSimpleSingleton() {  
    }  
  
    //Get instance for class SimpleSingleton  
    publicstatic SimpleSingleton getInstance() {  
  
        returnsingleInstance;  
    }  
} 

另一种实现

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publicenum SimpleSingleton {  
    INSTANCE;  
    publicvoid 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。

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importjava.awt.Dimension;  
importjava.awt.Rectangle;  
importjava.awt.Robot;  
importjava.awt.Toolkit;  
importjava.awt.image.BufferedImage;  
importjavax.imageio.ImageIO;  
importjava.io.File;  
  
...  
  
publicvoid captureScreen(String fileName) throwsException {  
  
   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

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File dir =new File("directoryName");  
  String[] children = dir.list();  
  if(children == null) {  
      // Either dir does not exist or is not a directory  
  }else {  
      for(inti=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() {  
      publicboolean 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() {  
      publicboolean accept(File file) {  
          returnfile.isDirectory();  
      }  
  };  
  files = dir.listFiles(fileFilter); 

15. 创建ZIP和JAR文件

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importjava.util.zip.*;  
importjava.io.*;  
  
publicclass ZipIt {  
    publicstatic void main(String args[]) throwsIOException {  
        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);  
        intbytesRead;  
        byte[] buffer =new byte[1024];  
        CRC32 crc =new CRC32();  
        for(inti=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(  
                newFileInputStream(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(  
                newFileInputStream(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

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<?xmlversion="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

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packagenet.viralpatel.java.xmlparser;  
  
importjava.io.File;  
importjavax.xml.parsers.DocumentBuilder;  
importjavax.xml.parsers.DocumentBuilderFactory;  
  
importorg.w3c.dom.Document;  
importorg.w3c.dom.Element;  
importorg.w3c.dom.Node;  
importorg.w3c.dom.NodeList;  
  
publicclass XMLParser {  
  
    publicvoid 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(inti = 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);  
        }  
    }  
    publicstatic void main(String[] args) {  
  
        XMLParser parser =new XMLParser();  
        parser.getAllUserNames("c:\\test.xml");  
    }  
} 

17. 把 Array 转换成 Map

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importjava.util.Map;  
importorg.apache.commons.lang.ArrayUtils;  
  
publicclass Main {  
  
  publicstatic 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

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importjavax.mail.*;  
importjavax.mail.internet.*;  
importjava.util.*;  
  
publicvoid postMail( String recipients[ ], String subject, String message , String from) throws MessagingException  
{  
    booleandebug = 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(inti = 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

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importjava.io.BufferedReader;  
importjava.io.InputStreamReader;  
importjava.net.URL;  
  
publicclass Main {  
    publicstatic 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

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/** 
* 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. 
*/ 
privatestatic Object resizeArray (Object oldArray, intnewSize) {  
   intoldSize = java.lang.reflect.Array.getLength(oldArray);  
   Class elementType = oldArray.getClass().getComponentType();  
   Object newArray = java.lang.reflect.Array.newInstance(  
         elementType,newSize);  
   intpreserveLength = Math.min(oldSize,newSize);  
   if(preserveLength > 0)  
      System.arraycopy (oldArray,0,newArray,0,preserveLength);  
   returnnewArray;  
}  
  
// Test routine for resizeArray().  
publicstatic void main (String[] args) {  
   int[] a = {1,2,3};  
   a = (int[])resizeArray(a,5);  
   a[3] =4;  
   a[4] =5;  
   for(inti=0; i<a.length; i++)  
      System.out.println (a[i]);  
}

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