[轉]下面是20個很是有用的Java程序片斷，但願能對你有用

時間  2019-12-06
標籤 下面 很是 有用 java 程序 片斷 但願 欄目 Java 简体版
原文   原文鏈接

下面是20個很是有用的Java程序片斷，但願能對你有用。html

　　1. 字符串有整型的相互轉換java

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

　　2. 向文件末尾添加內容node

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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. 獲得當前方法的名字sql

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

　　4. 轉字符串到日期shell

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

　　或者是：apache

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

　　5. 使用JDBC連接Oraclejson

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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 轉成 sql.Dateapi

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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進行快速的文件拷貝數組

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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 = 0 ; 
             while ( position < size ) 
             { 
                position += inChannel.transferTo( position, maxCount, outChannel ); 
             } 
         } 
         finally
         { 
             if ( inChannel != null ) 
             { 
                inChannel.close(); 
             } 
             if ( outChannel != null ) 
             { 
                 outChannel.close(); 
             } 
         } 
     }

　　8. 建立圖片的縮略圖session

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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, 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. 建立 JSON 格式的數據

　　請先閱讀這篇文章 瞭解一些細節，

　　並下面這個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

　　閱讀這篇文章 瞭解更多細節

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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 代理設置

　　閱讀這篇 文章 瞭解更多細節。

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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. 單實例Singleton 示例

　　請先閱讀這篇文章 瞭解更多信息

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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; 
     } 
}

　　另外一種實現

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

　　13. 抓屏程序

　　閱讀這篇文章 得到更多信息。

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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. 列出文件和目錄

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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= 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. 建立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[ 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/讀取XML 文件

　　XML文件

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<?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代碼

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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() > 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 轉換成 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. 發送郵件

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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 = 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. 發送代數據的HTTP 請求

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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. 改變數組的大小

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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 > 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]); 
}
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