20个开发人员非常有用的Java功能代码

本文将为大家介绍20个对开发人员非常有用的Java功能代码。这20段代码，可以成为大家在今后的开发过程中，Java编程手册的重要部分。

 

1. 把Strings转换成int和把int转换成String

String a = String.valueOf(2);//integer to numeric string int i = Integer.parseInt(a);//numeric string to an intString a = String.valueOf(2);//integer to numeric stringint i = Integer.parseInt(a);//numeric string to an int

2. 向Java文件中添加文本

Updated: Thanks Simone for pointing to exception. I have changed the code.  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();}}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. 获取Java现在正调用的方法名

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

4. 在Java中将String型转换成Date型

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

    OR

SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );Date date = format.parse( myString );

5. 通过Java JDBC链接Oracle数据库

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();}}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.将Java中的util.Date转换成sql.Date

这一片段显示如何将一个java util Date转换成sql Date用于数据库

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

7. 使用NIO快速复制Java文件

public static void fileCopy( File in, File out ) throws IOException {FileChannel inChannel = new FileInputStream( in ).getChannel();FileChannel outChannel = new FileOutputStream( out ).getChannel();try {// original -- apparently has trouble copying large files on Windows// magic number for Windows, 64Mb - 32Kb)inChannel.transferTo(0, inChannel.size(), outChannel);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. 在Java中创建缩略图

private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename) throws InterruptedException, FileNotFoundException, IOException {// load image from filenameImage 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 HEIGHTdouble 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-flyBufferedImage 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. 在Java中创建JSON数据

Read this article for more details. Download JAR file 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. 在Java中使用iText JAR打开PDF

Read this article for more details.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. 在Java上的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. Java Singleton 例子

Read this article for more details.Update: Thanks Markus for the comment. I have updated the code and changed it to more robust implementation.public class SimpleSingleton {private static SimpleSingleton singleInstance =  new SimpleSingleton();//Marking default constructor private//to avoid direct instantiation.private SimpleSingleton() {}//Get instance for class SimpleSingletonpublic static SimpleSingleton getInstance() {return singleInstance;}}One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.public enum SimpleSingleton {INSTANCE;public void doSomething() {}}//Call the method from Singleton:SimpleSingleton.INSTANCE.doSomething();public enum SimpleSingleton {INSTANCE;public void doSomething() {}}

13. 在Java上做屏幕截图

Read this article for more details.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. 在Java中的文件，目录列表

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 directoryString 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 objectsFile[] files = dir.listFiles();// This filter only returns directoriesFileFilter fileFilter = new FileFilter() {public boolean accept(File file) {return file.isDirectory();}};files = dir.listFiles(fileFilter);

15. 在Java中创建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 streambis = 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. 在Java中解析/读取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 code to parse above XML.

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 documentSystem.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) {ystem.out.println(e);}}public static void main(String[] args) {XMLParser parser = new XMLParser();parser.getAllUserNames("c:\\test.xml");}}

17. 在Java中将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. 在Java中发送电子邮件

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 addressProperties props = new Properties();props.put("mail.smtp.host", "smtp.example.com");// create some properties and get the default SessionSession session = Session.getDefaultInstance(props, null);session.setDebug(debug);// create a messageMessage msg = new MimeMessage(session);// set the from and to addressInternetAddress 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 Wantmsg.addHeader("MyHeaderName", "myHeaderValue");// Setting the Subject and Content Typemsg.setSubject(subject);msg.setContent(message, "text/plain");Transport.send(msg);}

19. 使用Java发送HTTP请求和提取数据

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://www.viralpatel.net/blogs/");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. 在Java中调整数组

/** *  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]);}