Very useful java program snippets

下面是非常有用的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");

12. 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 `.&#39;. 
  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

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]); 
}