JAVA中常用的代码汇总

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)/http://t.cn/Rz0bhUA

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;

  }

}

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"?>

<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代码

 

<span style="font-family:Arial;font-size:14px;">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<a.length; i++)

   System.out.println (a[i]);

}