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

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.


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; 

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]); 
} 

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