20个非常有用的Java程序片断

下面是20个非常有用的Java程序片段，希望能对你有用。

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 ex) { //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
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://coolshell.cn/"); 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
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]); }