20个常用的Java程序块

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, 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 create Thumbnail(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(newContainer()); 

    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

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(newFile("C:\\Test.pdf"));

  

            Document document = new Document(); 

            PdfWriter.getInstance(document, file); 

            document.open(); 

            document.add(new Paragraph("Hello Kiran")); 

            document.add(new Paragraph(newDate().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",newFile(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.new Instance(); 

 　　　　DocumentBuilder db = dbf.new DocumentBuilder(); 

 　　　　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); 

  } 

 } 

 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 { 

　　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 

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://cocre.com/"); 

 　　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(inti=0; i<a.length; i++)  

 　　System.out.println (a[i]); 

}