20个常用的Java程序块

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//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 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. 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(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 

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", new File(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.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 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 

{ 

 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. 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 (int i=0; i<a.length; i++) 

 System.out.println (a[i]); 

} 

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