20个开发人员非常有用的Java功能代码

1. 把Strings转换成int和把int转换成String

String a = String.valueOf(2); //integer to numeric string 
  
int i = Integer.parseInt(a); //numeric string to an int 

String a = String.valueOf(2);   //integer to numeric string

int i = Integer.parseInt(a); //numeric string to an int

 

 

2. 向Java文件中添加文本

		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. 获取Java现在正调用的方法名

  String methodName =

Thread.currentThread().getStackTrace()[1].getMethodName();  

 


4. 在Java中将String型转换成Date型

java.util.Date = 
java.text.DateFormat.getDateInstance().parse(date String);  
java.util.Date = java.text.DateFormat.getDateInstance().parse(date 
String);or  
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" 
);   
Date date = format.parse( myString );  
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date
= format.parse( myString );

 

5. 通过Java JDBC链接Oracle数据库



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

();  
}  
} 



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.将Java中的util.Date转换成sql.Date

这一片段显示如何将一个java util Date转换成sql Date用于数据库

java.util.Date utilDate = new
java.util.Date();  
java.sql.Date sqlDate = new java.sql.Date
(utilDate.getTime()); 
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate =
new java.sql.Date(utilDate.getTime());

7. 使用NIO快速复制Java文件


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 )  


{  


&

nbsp; position += inChannel.transferTo( position, maxCount, outChannel ); 




}

  
}  


finally 


{  


if (

inChannel != null )  


{  


&

nbsp; inChannel.close();  


}

  
if ( outChannel != null )  


{  


&

nbsp;  outChannel.close();  


}

  
}  


} 



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. 在Java中创建缩略图

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

();  
} 



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.drawImag

e(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. 在Java中创建JSON数据

Read this article for more details.
Download JAR file 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

();  
... 



import org.json.JSONObject;
...
...
JSONObject json = new

JSONObject();
json.put("city", "Mumbai");
json.put("country",

"India");
...
String output = json.toString();
...

10. 在Java中使用iText JAR打开PDF


Read this article for more details.



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();  
}

  
}  
} 



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. 在Java上的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"); 


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. Java Singleton 例子
Read this article for more

details.
Update: Thanks Markus for the comment. I have updated the code and

changed it to

more robust implementation.




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;  
}  
} 



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;
}
}
One more implementation of

Singleton class. Thanks to Ralph and Lukasz Zielinski


for pointing this out.




public enum SimpleSingleton {  
INSTANCE;  
public void doSomething()

{  
}  
}  

//Call the method from Singleton:  


SimpleSingleton.INSTANCE.doSomething(); 



public enum SimpleSingleton {
INSTANCE;
public void

doSomething() {
}
}


//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

13. 在Java上做屏幕截图

Read this article for more details.

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));  

}

  
... 



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. 在Java中的文件，目录列表

>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); 


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. 在Java中创建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()) {  


&

nbsp;  System.err.println("Skipping: " + name);  


&

nbsp;  continue;  


}

  
BufferedInputStream bis = new BufferedInputStream(  


&

nbsp;  new FileInputStream(file));  


crc.reset

();  


while

((bytesRead = bis.read(buffer)) != -1) {  


&

nbsp;  crc.update(buffer, 0, bytesRead);  


}

  
bis.close();  


// Reset to

beginning of input stream  


bis = new

BufferedInputStream(  


&

nbsp;  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) {  


&

nbsp;  zos.write(buffer, 0, bytesRead);  


}

  
bis.close();  
}

  
zos.close

();  
}  
} 


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);
&

nbsp;  

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);
&

nbsp; 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. Parsing / Reading XML file in Java
Sample XML file.




 
 


 


John 


B 


12 


 
 


Mary 


A 


11 


 
 


Simon 


A 


18 


 
 






John
B
12


Mary
A
<

AGE>11


Simon
A
18




Java code to parse above XML.




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()) {  


&

nbsp;  Document doc = db.parse(file);  


&

nbsp;  Element docEle = doc.getDocumentElement();  

&

nbsp;  // Print root element of the document  


&

nbsp;  System.out.println("Root element of the document: " 


&

nbsp;          + docEle.getNodeName

());  

&

nbsp;  NodeList studentList = docEle.getElementsByTagName

("student");  

&

nbsp;  // Print total student elements in document  


&

nbsp;  System.out  


&

nbsp;          .println("Total

students: " + studentList.getLength());  

&

nbsp;  if (studentList != null && studentList.getLength() > 0)

{  


&

nbsp;      for (int i = 0; i < studentList.getLength();

i++) {  

&

nbsp;          Node node =

studentList.item(i);  

&

nbsp;          if (node.getNodeType()

== Node.ELEMENT_NODE) {  

&

nbsp;             

System.out  


&

nbsp;            

         .println

("=====================");  

&

nbsp;             

Element e = (Element) node;  


&

nbsp;             

NodeList nodeList = e.getElementsByTagName("name");  


&

nbsp;             

System.out.println("Name: " 


&

nbsp;            

         + nodeList.item(0).getChildNodes

().item(0)  


&

nbsp;            

             

;   .getNodeValue());  

&

nbsp;             

nodeList = e.getElementsByTagName("grade");  


&

nbsp;             

System.out.println("Grade: " 


&

nbsp;            

         + nodeList.item(0).getChildNodes

().item(0)  


&

nbsp;            

             

;   .getNodeValue());  

&

nbsp;             

nodeList = e.getElementsByTagName("age");  


&

nbsp;             

System.out.println("Age: " 


&

nbsp;            

         + nodeList.item(0).getChildNodes

().item(0)  


&

nbsp;            

             

;   .getNodeValue());  


&

nbsp;          }  


&

nbsp;      }  


&

nbsp;  } else {  


&

nbsp;      System.exit(1);  


&

nbsp;  }  


}

  
} catch (Exception e)

{  


System.out.println(e);  
}  
}  
public

static void main(String[] args) {  

XMLParser parser = new XMLParser

();  
parser.getAllUserNames("c:\\test.xml");  
}

  
} 



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
&nb

sp;.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.

Convert Array to Map in Java



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"));  
}  


} 



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. Send Email

using Java



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);  
} 



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. Send HTTP

request & fetching data using Java



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://www.viralpatel.net/blogs/");&nbs

p; 
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();  
}

  
}  
} 



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://www.viralpatel.net/blogs/");
   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. Resize an Array in Java



/** 
* 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      System.out.println (a[i]);  


} 

16. 在Java中解析/读取XML文件

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<?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> 


<?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 code to parse above XML.


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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");  
}  
} 



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. 在Java中将Array转换成Map

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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"));  
}  
} 


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. 在Java中发送电子邮件

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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);  
} 


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. 使用Java发送HTTP请求和提取数据

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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://www.viralpatel.net/blogs/");  
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();  
}  
}  
} 


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://www.viralpatel.net/blogs/");
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. 在Java中调整数组

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/** 
* 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]);  
}