Java开发常用代码总结和分享

1. 字符串有整型的相互转换

1
2
String a = String.valueOf(2); //integer to numeric string
int i = Integer.parseInt(a); //numeric string to an int 　　

1. 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();
}
}

1. 得到当前方法的名字
   String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
2. 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 );

3. 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();

4. 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());
5. 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();
   

6. 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);
   //代码效果参考：http://0791zd.com/bx/art_631.html

9.创建 JSON 格式的数据
import org.json.JSONObject;
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
String output = json.toString();

1. 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();
2. 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");
3. 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;
4. 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;
   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));
5. 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);
6. 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(
   ZipEntry entry = new ZipEntry(name);
   entry.setMethod(ZipEntry.STORED);
   entry.setCompressedSize(file.length());
   entry.setSize(file.length());
   entry.setCrc(crc.getValue());
   zos.putNextEntry(entry);

   zos.write(buffer, 0, bytesRead);
   

   zos.close();
7. 解析/读取XML 文件
   XML文件
   <?xml version="1.0"?>
   
   
   John
   B
   12
   
   Mary
   A
   11
   Simon
   18
   
   Java代码
   package net.viralpatel.java.xmlparser;
   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) {
   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 = e.getElementsByTagName("age");
         System.out.println("Age: "
       }
     }
   } else {
     System.exit(1);
   }
   

   System.out.println(e);
   XMLParser parser = new XMLParser();
   parser.getAllUserNames("c:\test.xml");
8. 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"));
9. 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);

10. 发送代数据的HTTP 请求
    import java.io.BufferedReader;
    import java.io.InputStreamReader;
    import java.net.URL;
    //代码效果参考：http://0791zd.com/bx/art_3475.html

    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();

11. 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<a.length; i++)
  System.out.println (a[i]);

1. 遍历出页面传过来的所有参数和值
   Enumeration<?> en = request.getParameterNames();

   while(en.hasMoreElements()){
       String el = en.nextElement().toString();
     System.out.println("||||"+el+"="+request.getParameter(el));
   

2. JSON解析为List
   • JSON解析
   • @param jsonString
   • @return
   • @throws JSONException
     */
     public List jsonTest(String jsonString) throws JSONException{
     // String jsonString="{\"answers\":[{\"loginname\":\"zhangfan\",\"password\":\"userpass\"},{\"password\":\"userpass\",\"email\":\"822393@qq.com\"}]}";
     JSONObject json= new JSONObject(jsonString);
     JSONArray jsonArray=json.getJSONArray("Arr");
     List answerList = new ArrayList();
     for(int i=0;i<jsonArray.length();i++){

      JSONObject relue=(JSONObject) jsonArray.get(i);
      Long id=(Long) relue.get("id");
      String val =(String) relue.get("val");
      DataVo dataVo = new DataVo();
      dataVo.setId(id);
      dataVo.setVal(val);
      answerList.add(dataVo);
     

     return answerList;