20个非常实用的Java程序代码片段

  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

  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

  <?xml version="1.0"?>

  

  

  John

  B

  12

  

  

  Mary

  A

  11

  

  

  Simon

  A

  18

  

  

  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: est.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

  System.out.println (a[i]);

  }

  希望本文所述对大家学习java程序设计有所帮助。

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