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 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]);

}