20非常有用的Java程序片段 (中)
12. 单实例Singleton 示例
请先阅读这篇文章 了解更多信息
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public class SimpleSingleton {
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private static SimpleSingleton singleInstance = new SimpleSingleton();
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//Marking default constructor private
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//to avoid direct instantiation.
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private SimpleSingleton() {
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}
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//Get instance for class SimpleSingleton
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public static SimpleSingleton getInstance() {
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return singleInstance;
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}
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}
另一种实现
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public enum SimpleSingleton {
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INSTANCE;
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public void doSomething() {
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}
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}
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//Call the method from Singleton:
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SimpleSingleton.INSTANCE.doSomething();
13. 抓屏程序
阅读这篇文章 获得更多信息。
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import java.awt.Dimension;
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import java.awt.Rectangle;
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import java.awt.Robot;
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import java.awt.Toolkit;
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import java.awt.image.BufferedImage;
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import javax.imageio.ImageIO;
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import java.io.File;
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...
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public void captureScreen(String fileName) throws Exception {
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Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
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Rectangle screenRectangle = new Rectangle(screenSize);
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Robot robot = new Robot();
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BufferedImage image = robot.createScreenCapture(screenRectangle);
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ImageIO.write(image, "png" , new File(fileName));
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}
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...
14. 列出文件和目录
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File dir = new File( "directoryName" );
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String[] children = dir.list();
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if (children == null ) {
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// Either dir does not exist or is not a directory
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} else {
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for ( int i= 0 ; i < children.length; i++) {
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// Get filename of file or directory
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String filename = children[i];
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}
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}
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// It is also possible to filter the list of returned files.
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// This example does not return any files that start with `.'.
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FilenameFilter filter = new FilenameFilter() {
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public boolean accept(File dir, String name) {
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return !name.startsWith( "." );
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}
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};
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children = dir.list(filter);
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// The list of files can also be retrieved as File objects
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File[] files = dir.listFiles();
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// This filter only returns directories
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FileFilter fileFilter = new FileFilter() {
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public boolean accept(File file) {
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return file.isDirectory();
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}
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};
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files = dir.listFiles(fileFilter);
15. 创建ZIP和JAR文件
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import java.util.zip.*;
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import java.io.*;
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public class ZipIt {
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public static void main(String args[]) throws IOException {
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if (args.length < 2 ) {
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System.err.println( "usage: java ZipIt Zip.zip file1 file2 file3" );
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System.exit(- 1 );
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}
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File zipFile = new File(args[ 0 ]);
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if (zipFile.exists()) {
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System.err.println( "Zip file already exists, please try another" );
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System.exit(- 2 );
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}
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FileOutputStream fos = new FileOutputStream(zipFile);
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ZipOutputStream zos = new ZipOutputStream(fos);
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int bytesRead;
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byte [] buffer = new byte [ 1024 ];
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CRC32 crc = new CRC32();
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for ( int i= 1 , n=args.length; i < n; i++) {
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String name = args[i];
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File file = new File(name);
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if (!file.exists()) {
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System.err.println( "Skipping: " + name);
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continue ;
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}
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BufferedInputStream bis = new BufferedInputStream(
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new FileInputStream(file));
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crc.reset();
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while ((bytesRead = bis.read(buffer)) != - 1 ) {
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crc.update(buffer, 0 , bytesRead);
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}
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bis.close();
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// Reset to beginning of input stream
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bis = new BufferedInputStream(
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new FileInputStream(file));
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ZipEntry entry = new ZipEntry(name);
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entry.setMethod(ZipEntry.STORED);
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entry.setCompressedSize(file.length());
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entry.setSize(file.length());
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entry.setCrc(crc.getValue());
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zos.putNextEntry(entry);
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while ((bytesRead = bis.read(buffer)) != - 1 ) {
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zos.write(buffer, 0 , bytesRead);
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}
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bis.close();
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}
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zos.close();
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}
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}
16. 解析/读取XML 文件
XML文件
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<? xml version = "1.0" ?>
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< students >
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< student >
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< name >John</ name >
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< grade >B</ grade >
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< age >12</ age >
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</ student >
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< student >
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< name >Mary</ name >
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< grade >A</ grade >
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< age >11</ age >
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</ student >
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< student >
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< name >Simon</ name >
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< grade >A</ grade >
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< age >18</ age >
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</ student >
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</ students >
Java代码
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package net.viralpatel.java.xmlparser;
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import java.io.File;
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import javax.xml.parsers.DocumentBuilder;
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import javax.xml.parsers.DocumentBuilderFactory;
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import org.w3c.dom.Document;
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import org.w3c.dom.Element;
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import org.w3c.dom.Node;
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import org.w3c.dom.NodeList;
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public class XMLParser {
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public void getAllUserNames(String fileName) {
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try {
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DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
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DocumentBuilder db = dbf.newDocumentBuilder();
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File file = new File(fileName);
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if (file.exists()) {
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Document doc = db.parse(file);
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Element docEle = doc.getDocumentElement();
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// Print root element of the document
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System.out.println( "Root element of the document: "
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+ docEle.getNodeName());
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NodeList studentList = docEle.getElementsByTagName( "student" );
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// Print total student elements in document
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System.out
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.println( "Total students: " + studentList.getLength());
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if (studentList != null && studentList.getLength() > 0 ) {
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for ( int i = 0 ; i < studentList.getLength(); i++) {
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Node node = studentList.item(i);
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if (node.getNodeType() == Node.ELEMENT_NODE) {
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System.out
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.println( "=====================" );
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Element e = (Element) node;
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NodeList nodeList = e.getElementsByTagName( "name" );
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System.out.println( "Name: "
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+ nodeList.item( 0 ).getChildNodes().item( 0 )
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.getNodeValue());
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nodeList = e.getElementsByTagName( "grade" );
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System.out.println( "Grade: "
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+ nodeList.item( 0 ).getChildNodes().item( 0 )
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.getNodeValue());
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nodeList = e.getElementsByTagName( "age" );
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System.out.println( "Age: "
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+ nodeList.item( 0 ).getChildNodes().item( 0 )
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.getNodeValue());
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}
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}
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} else {
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System.exit( 1 );
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}
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}
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} catch (Exception e) {
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System.out.println(e);
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}
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}
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public static void main(String[] args) {
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XMLParser parser = new XMLParser();
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parser.getAllUserNames( "c:\\test.xml" );
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}
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}