20非常有用的Java程序片段
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
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
请先阅读这篇文章 了解一些细节，
并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)
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
阅读这篇文章 了解更多细节
??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
阅读这篇?文章 了解更多细节。
??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
请先阅读这篇文章 了解更多信息
??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
阅读这篇文章 获得更多信息。
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));????}??...??
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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://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
??/**?* 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]);??}(全文完)