【转】20非常有用的Java程序片段

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

请先阅读这篇文章 了解一些细节，
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

阅读这篇文章 了解更多细节

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

阅读这篇 文章 了解更多细节

System.getProperties().put("http.proxyHost", "someProxyURL");   
System.getProperties().put("http.proxyPort", "someProxyPort");   
System.getProperties().put("http.proxyUser", "someUserName");   
System.getProperties().put("http.proxyPassword", "somePassword"); 

2. 单实例Singleton 示例

请先阅读这篇文章 了解更多信息

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

阅读这篇文章 获得更多信息。

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：

ackage 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]);   
} 

原文链接：http://coolshell.cn/articles/889.html

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