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。