20个非常实用的Java程序代码片段

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

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

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;
 }
}

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

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

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]);
}

希望本文所述对大家学习java程序设计有所帮助。