20个常用的java开发块

//1. 字符串有整型的相互转换  

002 String a = String.valueOf(2); //integer to numeric string  

003 int i = Integer.parseInt(a); //numeric string to an int 

004   

005 //2. 向文件末尾添加内容  

006 BufferedWriter out = null;  

007 try {  

008 out = new BufferedWriter(new FileWriter(”filename”, true));  

009 out.write(”aString”);  

010 } catch (IOException e) {  

011  // error processing code 

012   

013 } finally {  

014 if (out != null) {  

015 out.close();  

016 } 

017   

018 } 

019   

020 //3. 得到当前方法的名字  

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

022   

023 //4. 转字符串到日期  

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

025 //或者是：  

026 SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );  

027 Date date = format.parse( myString ); 

028   

029 //5. 使用JDBC链接Oracle  

030 public class OracleJdbcTest  

031 {  

032  String driverClass = "oracle.jdbc.driver.OracleDriver"; 

033   

034  Connection con; 

035   

036  public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException  

037  {  

038  Properties props = new Properties();  

039  props.load(fs);  

040  String url = props.getProperty("db.url");  

041  String userName = props.getProperty("db.user");  

042  String password = props.getProperty("db.password");  

043  Class.forName(driverClass); 

044   

045  con=DriverManager.getConnection(url, userName, password);  

046  } 

047   

048  public void fetch() throws SQLException, IOException  

049  {  

050  PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");  

051  ResultSet rs = ps.executeQuery(); 

052   

053  while (rs.next())  

054  {  

055  // do the thing you do  

056  }  

057  rs.close();  

058  ps.close();  

059  } 

060   

061  public static void main(String[] args)  

062  {  

063  OracleJdbcTest test = new OracleJdbcTest();  

064  test.init();  

065  test.fetch();  

066  }  

067 } 

068   

069 6. 把 Java util.Date 转成 sql.Date  

070 java.util.Date utilDate = new java.util.Date();  

071 java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime()); 

072   

073 //7. 使用NIO进行快速的文件拷贝  

074  public static void fileCopy( File in, File out )  

075  throws IOException  

076  {  

077  FileChannel inChannel = new FileInputStream( in ).getChannel();  

078  FileChannel outChannel = new FileOutputStream( out ).getChannel();  

079  try 

080  {  

081 // inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows 

082   

083  // magic number for Windows, 64Mb - 32Kb)  

084  int maxCount = (64 * 1024 * 1024) - (32 * 1024);  

085  long size = inChannel.size();  

086  long position = 0;  

087  while ( position < size )  

088  {  

089  position += inChannel.transferTo( position, maxCount, outChannel );  

090  }  

091  }  

092  finally 

093  {  

094  if ( inChannel != null )  

095  {  

096  inChannel.close();  

097  }  

098  if ( outChannel != null )  

099  {  

100  outChannel.close();  

101  }  

102  }  

103  } 

104   

105 //8. 创建图片的缩略图  

106 private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)  

107  throws InterruptedException, FileNotFoundException, IOException  

108  {  

109  // load image from filename  

110  Image image = Toolkit.getDefaultToolkit().getImage(filename);  

111  MediaTracker mediaTracker = new MediaTracker(new Container());  

112  mediaTracker.addImage(image, 0);  

113  mediaTracker.waitForID(0);  

114  // use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny()); 

115   

116  // determine thumbnail size from WIDTH and HEIGHT  

117  double thumbRatio = (double)thumbWidth / (double)thumbHeight;  

118  int imageWidth = image.getWidth(null);  

119  int imageHeight = image.getHeight(null);  

120  double imageRatio = (double)imageWidth / (double)imageHeight;  

121  if (thumbRatio < imageRatio) {  

122  thumbHeight = (int)(thumbWidth / imageRatio);  

123  } else {  

124  thumbWidth = (int)(thumbHeight * imageRatio);  

125  } 

126   

127  // draw original image to thumbnail image object and  

128  // scale it to the new size on-the-fly  

129  BufferedImage thumbImage = new BufferedImage(thumbWidth, thumbHeight, BufferedImage.TYPE_INT_RGB);  

130  Graphics2D graphics2D = thumbImage.createGraphics();  

131  graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);  

132  graphics2D.drawImage(image, 0, 0, thumbWidth, thumbHeight, null); 

133   

134  // save thumbnail image to outFilename  

135  BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(outFilename));  

136  JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);  

137  JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(thumbImage);  

138  quality = Math.max(0, Math.min(quality, 100));  

139  param.setQuality((float)quality / 100.0f, false);  

140  encoder.setJPEGEncodeParam(param);  

141  encoder.encode(thumbImage);  

142  out.close();  

143  } 

144   

145 //9. 创建 JSON 格式的数据  

146 import org.json.JSONObject;  

147 ...  

148 ...  

149 JSONObject json = new JSONObject();  

150 json.put("city", "Mumbai");  

151 json.put("country", "India");  

152 ...  

153 String output = json.toString();  

154 ... 

155   

156 //10. 使用iText JAR生成PDF  

157 import java.io.File;  

158 import java.io.FileOutputStream;  

159 import java.io.OutputStream;  

160 import java.util.Date; 

161   

162 import com.lowagie.text.Document;  

163 import com.lowagie.text.Paragraph;  

164 import com.lowagie.text.pdf.PdfWriter; 

165   

166 public class GeneratePDF { 

167   

168  public static void main(String[] args) {  

169  try {  

170  OutputStream file = new FileOutputStream(new File("C:\\Test.pdf")); 

171   

172  Document document = new Document();  

173  PdfWriter.getInstance(document, file);  

174  document.open();  

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

176  document.add(new Paragraph(new Date().toString())); 

177   

178  document.close();  

179  file.close(); 

180   

181  } catch (Exception e) { 

182   

183  e.printStackTrace();  

184  }  

185  }  

186 } 

187   

188 //11. HTTP 代理设置  

189 System.getProperties().put("http.proxyHost", "someProxyURL");  

190 System.getProperties().put("http.proxyPort", "someProxyPort");  

191 System.getProperties().put("http.proxyUser", "someUserName");  

192 System.getProperties().put("http.proxyPassword", "somePassword"); 

193   

194 //12. 单实例Singleton 示例  

195 public class SimpleSingleton {  

196  private static SimpleSingleton singleInstance = new SimpleSingleton(); 

197   

198  //Marking default constructor private  

199  //to avoid direct instantiation.  

200  private SimpleSingleton() {  

201  } 

202   

203  //Get instance for class SimpleSingleton  

204  public static SimpleSingleton getInstance() { 

205   

206  return singleInstance;  

207  }  

208 } 

209   

210 //另一种实现 

211   

212 public enum SimpleSingleton {  

213  INSTANCE;  

214  public void doSomething() {  

215  }  

216 } 

217   

218 //Call the method from Singleton:  

219 SimpleSingleton.INSTANCE.doSomething(); 

220   

221 //13. 抓屏程序  

222 import java.awt.Dimension;  

223 import java.awt.Rectangle;  

224 import java.awt.Robot;  

225 import java.awt.Toolkit;  

226 import java.awt.image.BufferedImage;  

227 import javax.imageio.ImageIO;  

228 import java.io.File; 

229   

230 ...  

231 public void captureScreen(String fileName) throws Exception { 

232   

233  Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();  

234  Rectangle screenRectangle = new Rectangle(screenSize);  

235  Robot robot = new Robot();  

236  BufferedImage image = robot.createScreenCapture(screenRectangle);  

237  ImageIO.write(image, "png", new File(fileName)); 

238   

239 }  

240 //14. 列出文件和目录  

241 File dir = new File("directoryName");  

242  String[] children = dir.list();  

243  if (children == null) {  

244  // Either dir does not exist or is not a directory  

245  } else {  

246  for (int i=0; i < children.length; i++) {  

247  // Get filename of file or directory  

248  String filename = children[i];  

249  }  

250  } 

251   

252  // It is also possible to filter the list of returned files.  

253  // This example does not return any files that start with `.’.  

254  FilenameFilter filter = new FilenameFilter() {  

255  public boolean accept(File dir, String name) {  

256  return !name.startsWith(".");  

257  }  

258  };  

259  children = dir.list(filter); 

260   

261  // The list of files can also be retrieved as File objects  

262  File[] files = dir.listFiles(); 

263   

264  // This filter only returns directories  

265  FileFilter fileFilter = new FileFilter() {  

266  public boolean accept(File file) {  

267  return file.isDirectory();  

268  }  

269  };  

270  files = dir.listFiles(fileFilter); 

271   

272 //15. 创建ZIP和JAR文件 

273   

274 import java.util.zip.*;  

275 import java.io.*; 

276   

277 public class ZipIt {  

278  public static void main(String args[]) throws IOException {  

279  if (args.length < 2) {  

280  System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");  

281  System.exit(-1);  

282  }  

283  File zipFile = new File(args[0]);  

284  if (zipFile.exists()) {  

285  System.err.println("Zip file already exists, please try another");  

286  System.exit(-2);  

287  }  

288  FileOutputStream fos = new FileOutputStream(zipFile);  

289  ZipOutputStream zos = new ZipOutputStream(fos);  

290  int bytesRead;  

291  byte[] buffer = new byte[1024];  

292  CRC32 crc = new CRC32();  

293  for (int i=1, n=args.length; i < n; i++) {  

294  String name = args[i];  

295  File file = new File(name);  

296  if (!file.exists()) {  

297  System.err.println("Skipping: " + name);  

298  continue;  

299  }  

300  BufferedInputStream bis = new BufferedInputStream(  

301  new FileInputStream(file));  

302  crc.reset();  

303  while ((bytesRead = bis.read(buffer)) != -1) {  

304  crc.update(buffer, 0, bytesRead);  

305  }  

306  bis.close();  

307  // Reset to beginning of input stream  

308  bis = new BufferedInputStream(  

309  new FileInputStream(file));  

310  ZipEntry entry = new ZipEntry(name);  

311  entry.setMethod(ZipEntry.STORED);  

312  entry.setCompressedSize(file.length());  

313  entry.setSize(file.length());  

314  entry.setCrc(crc.getValue());  

315  zos.putNextEntry(entry);  

316  while ((bytesRead = bis.read(buffer)) != -1) {  

317  zos.write(buffer, 0, bytesRead);  

318  }  

319  bis.close();  

320  }  

321  zos.close();  

322  }  

323 } 

324   

325 //16. 解析/读取XML 文件  

326 XML文件  

327 <?xml version="1.0"?>  

328 <students>  

329  <student>  

330  <name>John</name>  

331  <grade>B</grade>  

332  <age>12</age>  

333  </student>  

334  <student>  

335  <name>Mary</name>  

336  <grade>A</grade>  

337  <age>11</age>  

338  </student>  

339  <student>  

340  <name>Simon</name>  

341  <grade>A</grade>  

342  <age>18</age>  

343  </student>  

344 </students> 

345   

346 //Java代码  

347 package net.viralpatel.java.xmlparser; 

348   

349 import java.io.File;  

350 import javax.xml.parsers.DocumentBuilder;  

351 import javax.xml.parsers.DocumentBuilderFactory; 

352   

353 import org.w3c.dom.Document;  

354 import org.w3c.dom.Element;  

355 import org.w3c.dom.Node;  

356 import org.w3c.dom.NodeList; 

357   

358 public class XMLParser { 

359   

360  public void getAllUserNames(String fileName) {  

361  try {  

362  DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();  

363  DocumentBuilder db = dbf.newDocumentBuilder();  

364  File file = new File(fileName);  

365  if (file.exists()) {  

366  Document doc = db.parse(file);  

367  Element docEle = doc.getDocumentElement(); 

368   

369  // Print root element of the document  

370  System.out.println("Root element of the document: " 

371  + docEle.getNodeName()); 

372   

373  NodeList studentList = docEle.getElementsByTagName("student"); 

374   

375  // Print total student elements in document  

376  System.out  

377  .println("Total students: " + studentList.getLength()); 

378   

379  if (studentList != null && studentList.getLength() > 0) {  

380  for (int i = 0; i < studentList.getLength(); i++) { 

381   

382  Node node = studentList.item(i); 

383   

384  if (node.getNodeType() == Node.ELEMENT_NODE) { 

385   

386  System.out  

387  .println("====================="); 

388   

389  Element e = (Element) node;  

390  NodeList nodeList = e.getElementsByTagName("name");  

391  System.out.println("Name: " 

392  + nodeList.item(0).getChildNodes().item(0)  

393  .getNodeValue()); 

394   

395  nodeList = e.getElementsByTagName("grade");  

396  System.out.println("Grade: " 

397  + nodeList.item(0).getChildNodes().item(0)  

398  .getNodeValue()); 

399   

400  nodeList = e.getElementsByTagName("age");  

401  System.out.println("Age: " 

402  + nodeList.item(0).getChildNodes().item(0)  

403  .getNodeValue());  

404  }  

405  }  

406  } else {  

407  System.exit(1);  

408  }  

409  }  

410  } catch (Exception e) {  

411  System.out.println(e);  

412  }  

413  }  

414  public static void main(String[] args) { 

415   

416  XMLParser parser = new XMLParser();  

417  parser.getAllUserNames("c:\\test.xml");  

418  }  

419 }  

420 //17. 把 Array 转换成 Map  

421 import java.util.Map;  

422 import org.apache.commons.lang.ArrayUtils; 

423   

424 public class Main { 

425   

426  public static void main(String[] args) {  

427  String[ ][ ] countries = { { "United States", "New York" }, { "United Kingdom", "London" },  

428  { "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } }; 

429   

430  Map countryCapitals = ArrayUtils.toMap(countries); 

431   

432  System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));  

433  System.out.println("Capital of France is " + countryCapitals.get("France"));  

434  }  

435 } 

436   

437 //18. 发送邮件  

438 import javax.mail.*;  

439 import javax.mail.internet.*;  

440 import java.util.*; 

441   

442 public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException  

443 {  

444  boolean debug = false; 

445   

446  //Set the host smtp address  

447  Properties props = new Properties();  

448  props.put("mail.smtp.host", "smtp.example.com"); 

449   

450  // create some properties and get the default Session  

451  Session session = Session.getDefaultInstance(props, null);  

452  session.setDebug(debug); 

453   

454  // create a message  

455  Message msg = new MimeMessage(session); 

456   

457  // set the from and to address  

458  InternetAddress addressFrom = new InternetAddress(from);  

459  msg.setFrom(addressFrom); 

460   

461  InternetAddress[] addressTo = new InternetAddress[recipients.length];  

462  for (int i = 0; i < recipients.length; i++)  

463  {  

464  addressTo[i] = new InternetAddress(recipients[i]);  

465  }  

466  msg.setRecipients(Message.RecipientType.TO, addressTo); 

467   

468  // Optional : You can also set your custom headers in the Email if you Want  

469  msg.addHeader("MyHeaderName", "myHeaderValue"); 

470   

471  // Setting the Subject and Content Type  

472  msg.setSubject(subject);  

473  msg.setContent(message, "text/plain");  

474  Transport.send(msg);  

475 } 

476   

477 //19. 发送代数据的HTTP 请求  

478 import java.io.BufferedReader;  

479 import java.io.InputStreamReader;  

480 import java.net.URL; 

481   

482 public class Main {  

483  public static void main(String[] args) {  

484  try {  

485  URL my_url = new URL("http://cocre.com/");  

486  BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));  

487  String strTemp = "";  

488  while(null != (strTemp = br.readLine())){  

489  System.out.println(strTemp);  

490  }  

491  } catch (Exception ex) {  

492  ex.printStackTrace();  

493  }  

494  }  

495 } 

496   

497 //20. 改变数组的大小  

498 /**  

499 * Reallocates an array with a new size, and copies the contents  

500 * of the old array to the new array.  

501 * @param oldArray the old array, to be reallocated.  

502 * @param newSize the new array size.  

503 * @return A new array with the same contents.  

504 */ 

505 private static Object resizeArray (Object oldArray, int newSize) {  

506  int oldSize = java.lang.reflect.Array.getLength(oldArray);  

507  Class elementType = oldArray.getClass().getComponentType();  

508  Object newArray = java.lang.reflect.Array.newInstance(  

509  elementType,newSize);  

510  int preserveLength = Math.min(oldSize,newSize);  

511  if (preserveLength > 0)  

512  System.arraycopy (oldArray,0,newArray,0,preserveLength);  

513  return newArray;  

514 } 

515   

516 // Test routine for resizeArray().  

517 public static void main (String[] args) {  

518  int[] a = {1,2,3};  

519  a = (int[])resizeArray(a,5);  

520  a[3] = 4;  

521  a[4] = 5;  

522  for (int i=0; i<a.length; i++)   

523  System.out.println (a[i]);  

524 }