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20个常用java代码段

Javascript 作者：ii_chengzi 时间：2019-07-18 16:18:19 0 删除 编辑

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

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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

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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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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

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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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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 = ; 
             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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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, ); 
         mediaTracker.waitForID( ); 
         // 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, , , 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( , 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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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

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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。

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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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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

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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。

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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

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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   ( int   i= ; 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

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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[ ]); 
         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, , 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, , 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

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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() > ) { 
                     for   ( int   i = ; 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( ).getChildNodes().item( ) 
                                             .getNodeValue()); 
 
                             nodeList = e.getElementsByTagName( "grade" ); 
                             System.out.println( "Grade: "
                                     + nodeList.item( ).getChildNodes().item( ) 
                                             .getNodeValue()); 
 
                             nodeList = e.getElementsByTagName( "age" ); 
                             System.out.println( "Age: "
                                     + nodeList.item( ).getChildNodes().item( ) 
                                             .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 

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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

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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 = ; 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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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

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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.
*/
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 > ) 
       System.arraycopy (oldArray, ,newArray, ,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= ; i<a.length; i++) 
       System.out.println (a[i]); 
}

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