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
- <pre class="java" name="code">
- String a = String.valueOf(2);
- //integer to numeric string
- int i = Integer.parseInt(a);
- //numeric string to an int
- String a = String.valueOf(2);
- //integer to numeric string
- int i = Integer.parseInt(a);
- //numeric string to an int</pre></pre>
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 );
- 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 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. 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 SimpleSingleton
- public 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 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. 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 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. 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 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) {
- 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 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. 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]);
- }
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