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]); 
}