Java开发人员常用的20个java功能

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

Updated: Thanks Simone for pointing to exception. I have 

changed the code.  
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();   


}   
}  



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();  

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.


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.