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 e) {// 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
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
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
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