实用Java代码技巧-优快云博客

网络上看到的，觉得不错，转载了。。

本文将为大家介绍20人员非常有用的Java功能代码。这20段代码，可以成为大家在今后的开发过程中，Java编程手册的重要部分。

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

这无非就是各个类型之间的转换。没啥好说的。

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();   


}   
}

字节流，字符流，管道问题，如何包装的，我需要去看这些书了。

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
();

Thread类里面有个静态方法，获得线程的堆栈，获取堆栈里面的最上面一个的名字。

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 );

这个例子没看清楚，估计不是很准确，但是意思大概清楚，无非是类型转换，前面两句我没用过，SimpleDateFormat format = new SimpleDateFormat( "dd-MM-yyyy" );
Date date = format.parse( myString );就可以了，注意 format里面只有parse，没有parseInt或者parseString之分，他有些重载的方法，可以传入各个值。

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();
}
}

不说了，面试的时候一般会用到，实际应用基本不会用到啦。这些工具类个人有个人的写法。别人写了你应该觉得别扭吧，呵呵

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());

就一个方法的问题，学习了！

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
// 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]);   
}

java经典题目