下面是20個(gè)非常有用的Java程序片段，希望能對大家有所幫助。

1. 字符串有整型的相互轉(zhuǎn)換

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

2. 向文件末尾添加內(nèi)容

   1.BufferedWriter out = null;  
   2.try {  
   3    out = new BufferedWriter(new FileWriter(”filename”, true));  
   4    out.write(”aString”);  
   5} catch (IOException e) {  
   6   // error processing code  
   7} finally {  
   8   if (out != null) {  
   9        out.close();  
   10   }  
   11}

3. 得到當(dāng)前方法的名字

   1.String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4. 轉(zhuǎn)字符串到日期

   1.java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);

   或者是：

   1.SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );  
   2.Date date = format.parse( myString );

5. 使用JDBC鏈接Oracle

   1.public class OracleJdbcTest  
   2.{  
   3    String driverClass = "oracle.jdbc.driver.OracleDriver";  
   4
   5   Connection con;  
   6
   7   public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException  
   8 {  
   9     Properties props = new Properties();  
   10     props.load(fs);  
   11     String url = props.getProperty("db.url");  
   12     String userName = props.getProperty("db.user");  
   13     String password = props.getProperty("db.password");  
   14     Class.forName(driverClass);  
   15
   16      con=DriverManager.getConnection(url, userName, password);  
   17  }  
   18
   19   public void fetch() throws SQLException, IOException  
   20   {  
   21       PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");  
   22       ResultSet rs = ps.executeQuery();  
   23
   24       while (rs.next())  
   25       {  
   26           // do the thing you do  
   27       }  
   28       rs.close();  
   29       ps.close();  
   30    }  
   31 
   32    public static void main(String[] args)  
   33   {  
   34       OracleJdbcTest test = new OracleJdbcTest();  
   35       test.init();  
   36       test.fetch();  
   37   }  
   38}

6. 把 Java util.Date 轉(zhuǎn)成 sql.Date

   1.java.util.Date utilDate = new java.util.Date();  
   2.java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. 使用NIO進(jìn)行快速的文件拷貝

   1.public static void fileCopy( File in, File out )  
   2.           throws IOException  
   3.   {  
   4.       FileChannel inChannel = new FileInputStream( in ).getChannel();  
   5.      FileChannel outChannel = new FileOutputStream( out ).getChannel();  
   6.      try
   7.      {  
   8.//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows  
   9.
   10.           // magic number for Windows, 64Mb - 32Kb)  
   11.          int maxCount = (64 * 1024 * 1024) - (32 * 1024);  
   12.          long size = inChannel.size();  
   13.          long position = 0;  
   14.         while ( position < size )  
   15.         {  
   16.            position += inChannel.transferTo( position, maxCount, outChannel );  
   17.         }  
   18.     }  
   19.     finally
   20.     {  
   21.          if ( inChannel != null )  
   22.          {  
   23.             inChannel.close();  
   24.          }  
   25.          if ( outChannel != null )  
   26.          {  
   27.              outChannel.close();  
   28.          }  
   29.       }  
   30.   }

8. 創(chuàng)建圖片的縮略圖

   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. 創(chuàng)建 JSON 格式的數(shù)據(jù)

   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 代理設(shè)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. 單實(shí)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;  
    }  
    }

    另一種實(shí)現(xiàn)

    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. 列出文件和目錄

    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. 創(chuàng)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/讀取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代碼

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 轉(zhuǎn)換成 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.發(fā)送郵件

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.發(fā)送代數(shù)據(jù)的HTTP 請求

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("//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.改變數(shù)組的大小

/** 
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]);  
}

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