20个常用的Java程序块

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 create Thumbnail(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(newContainer());
    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

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(newFile("C:\\Test.pdf"));

            Document document = new Document();
            PdfWriter.getInstance(document, file);
            document.open();
            document.add(new Paragraph("Hello Kiran"));
            document.add(new Paragraph(newDate().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",newFile(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.new Instance();
 　　　　DocumentBuilder db = dbf.new DocumentBuilder();
 　　　　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(inti =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 {
　　booleandebug =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(inti =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://cocre.com/");
 　　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(inti=0; i<a.length; i++)
 　　System.out.println (a[i]);
}