20个非常有用的Java功能代码

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我是靠谱客的博主落后荷花，最近开发中收集的这篇文章主要介绍20个非常有用的Java功能代码，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

1. 把Strings转换成int和把int转换成String

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

2. 向Java文件中添加文本

BufferedWriter out = null;
try
{
out = new BufferedWriter(new FileWriter(”
filename”, true));
out.write(”
aString”);
} catch (IOException e) {
// error processing code
} finally
{
if (out != null) {
out.close();
}
}
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

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

7. 使用NIO快速复制Java文件

public static void fileCopy( File in, File out )
throws IOException
{
FileChannel inChannel = new
FileInputStream( in ).getChannel();
FileChannel outChannel = new
FileOutputStream( out ).getChannel();
try
{
//
inChannel.transferTo
(0, inChannel.size(), outChannel);
// original
-- apparently has trouble copying large files on Windows
// magic
number for Windows, 64Mb - 32Kb)
int maxCount
= (64 * 1024 * 1024) - (32 * 1024);
long size =
inChannel.size();
long
position = 0;
while (
position < size )
{
&
nbsp; position += inChannel.transferTo( position, maxCount, outChannel );
}
}
finally
{
if (
inChannel != null )
{
&
nbsp; inChannel.close();
}
if ( outChannel != null )
{
&
nbsp;
outChannel.close();
}
}
}
public static void fileCopy( File in, File out )
throws
IOException
{
FileChannel inChannel = new FileInputStream( in ).getChannel
();
FileChannel outChannel = new
FileOutputStream( out ).getChannel();
try
{
//
inChannel.transferTo
(0, inChannel.size(), outChannel);
// original
-- apparently has trouble copying large files on Windows
// magic
number for Windows, 64Mb - 32Kb)
int maxCount
= (64 * 1024 * 1024) - (32 *
1024);
long
size = inChannel.size
();
long
position = 0;
while ( position < size )
{
position += inChannel.transferTo( position, maxCount, outChannel
);
}
}
finally
{
if (
inChannel != null )
{
inChannel.close
();
}
if (
outChannel != null )
{
outChannel.close
();
}
}
}

8. 在Java中创建缩略图

private void createThumbnail(String filename, int thumbWidth,
int thumbHeight, int
quality, String outFilename)
throws InterruptedException,
FileNotFoundException, IOException
{
// load image from
filename
Image image =
Toolkit.getDefaultToolkit().getImage(filename);
MediaTracker mediaTracker = new
MediaTracker(new Container());
mediaTracker.addImage(image,
0);
mediaTracker.waitForID(0);
// use this to test for errors at
this point: System.out.println
(mediaTracker.isErrorAny());
// determine thumbnail size from
WIDTH and HEIGHT
double
thumbRatio = (double)thumbWidth / (double)thumbHeight;
int imageWidth = image.getWidth
(null);
int imageHeight
= image.getHeight(null);
double imageRatio = (double)imageWidth / (double)imageHeight;
if (thumbRatio < imageRatio)
{
thumbHeight
= (int)(thumbWidth / imageRatio);
} else {
thumbWidth =
(int)(thumbHeight * imageRatio);
}
// draw original image to thumbnail
image object and
//
scale it to the new size on-the-fly
BufferedImage thumbImage = new
BufferedImage(thumbWidth, thumbHeight,
BufferedImage.TYPE_INT_RGB);
Graphics2D graphics2D =
thumbImage.createGraphics();
graphics2D.setRenderingHint
(RenderingHints.KEY_INTERPOLATION,
RenderingHints.VALUE_INTERPOLATION_BILINEAR);
graphics2D.drawImage(image, 0, 0,
thumbWidth, thumbHeight, null);
// save thumbnail image to
outFilename
BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream
(outFilename));
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
JPEGEncodeParam param =
encoder.getDefaultJPEGEncodeParam(thumbImage);
quality = Math.max(0, Math.min
(quality, 100));
param.setQuality((float)quality / 100.0f, false);
encoder.setJPEGEncodeParam
(param);
encoder.encode
(thumbImage);
out.close
();
}
private void createThumbnail(String filename, int thumbWidth, int
thumbHeight, int
quality, String outFilename)
throws InterruptedException,
FileNotFoundException, IOException
{
// load image from
filename
Image image = Toolkit.getDefaultToolkit().getImage
(filename);
MediaTracker mediaTracker = new MediaTracker(new
Container());
mediaTracker.addImage(image,
0);
mediaTracker.waitForID(0);
// use this to test
for errors at this point: System.out.println
(mediaTracker.isErrorAny());
// determine thumbnail size from WIDTH and
HEIGHT
double thumbRatio = (double)thumbWidth / (double)
thumbHeight;
int imageWidth = image.getWidth
(null);
int imageHeight = image.getHeight
(null);
double imageRatio = (double)imageWidth / (double)
imageHeight;
if (thumbRatio < imageRatio)
{
thumbHeight = (int)(thumbWidth /
imageRatio);
} else {
thumbWidth = (int)
(thumbHeight * imageRatio);
}
// draw original image to thumbnail image object
and
// scale it to the new size on-the-
fly
BufferedImage thumbImage = new BufferedImage(thumbWidth,
thumbHeight, BufferedImage.TYPE_INT_RGB);
Graphics2D graphics2D =
thumbImage.createGraphics();
graphics2D.setRenderingHint
(RenderingHints.KEY_INTERPOLATION,
RenderingHints.VALUE_INTERPOLATION_BILINEAR);
graphics2D.drawImag
e(image, 0, 0, thumbWidth, thumbHeight, null);
// save thumbnail image to
outFilename
BufferedOutputStream out = new BufferedOutputStream(new
FileOutputStream(outFilename));
JPEGImageEncoder encoder =
JPEGCodec.createJPEGEncoder(out);
JPEGEncodeParam param =
encoder.getDefaultJPEGEncodeParam
(thumbImage);
quality = Math.max(0, Math.min(quality,
100));
param.setQuality((float)quality / 100.0f,
false);
encoder.setJPEGEncodeParam
(param);
encoder.encode(thumbImage);
out.close
();
}

9. 在Java中创建JSON数据

Read this article for more details.
Download JAR file json
-rpc-1.0.jar (75 kb)
import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mumbai");
json.put("country",
"India");
...
String output = json.toString
();
...
import org.json.JSONObject;
...
...
JSONObject json = new
JSONObject();
json.put("city", "Mumbai");
json.put("country",
"India");
...
String output = json.toString();
...

10. 在Java中使用iText JAR打开PDF

Read this article for more details.
import java.io.File;
import
java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.Date;
import
com.lowagie.text.Document;
import
com.lowagie.text.Paragraph;
import
com.lowagie.text.pdf.PdfWriter;
public class GeneratePDF
{
public static void main(String[]
args) {
try
{
OutputStream
file = new FileOutputStream(new File("C:\Test.pdf"));
Document
document = new Document();
PdfWriter.getInstance(document, file);
document.open();
document.add(new Paragraph("Hello Kiran"));
document.add(new Paragraph(new Date().toString()));
document.close();
file.close
();
} catch
(Exception e) {
e.printStackTrace();
}
}
}
import java.io.File;
import java.io.FileOutputStream;
import
java.io.OutputStream;
import java.util.Date;
import com.lowagie.text.Document;
import
com.lowagie.text.Paragraph;
import com.lowagie.text.pdf.PdfWriter;
public class GeneratePDF {
public static void main(String[] args)
{
try
{
OutputStream file = new FileOutputStream(new File("C:\Test.pdf"));
Document
document = new Document
();
PdfWriter.getInstance(document,
file);
document.open
();
document.add(new Paragraph("Hello
Kiran"));
document.add(new Paragraph(new Date().toString()));
document.close
();
file.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}

11. 在Java上的HTTP代理设置

System.getProperties().put("http.proxyHost",
"someProxyURL");
System.getProperties().put("http.proxyPort",
"someProxyPort");
System.getProperties().put("http.proxyUser",
"someUserName");
System.getProperties().put("http.proxyPassword",
"somePassword");
System.getProperties().put("http.proxyHost",
"someProxyURL");
System.getProperties().put("http.proxyPort",
"someProxyPort");
System.getProperties().put("http.proxyUser",
"someUserName");
System.getProperties().put("http.proxyPassword",
"somePassword");
12. Java Singleton 例子
Read this article for more
details.
Update: Thanks Markus for the comment. I have updated the code and
changed it to
more robust implementation.
public class SimpleSingleton {
private static SimpleSingleton singleInstance =
new SimpleSingleton
();
//Marking default constructor
private
//to avoid direct
instantiation.
private SimpleSingleton()
{
}
//Get instance for class SimpleSingleton
public static SimpleSingleton getInstance() {
return
singleInstance;
}
}
public class SimpleSingleton {
private static SimpleSingleton
singleInstance =
new SimpleSingleton();
//Marking default constructor private
//to avoid direct
instantiation.
private SimpleSingleton() {
}
//Get instance for class SimpleSingleton
public static
SimpleSingleton getInstance() {
return singleInstance;
}
}
One more implementation of
Singleton class. Thanks to Ralph and Lukasz Zielinski
for pointing this out.
public enum SimpleSingleton {
INSTANCE;
public void doSomething()
{
}
}
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
public enum SimpleSingleton {
INSTANCE;
public void
doSomething() {
}
}
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

13. 在Java上做屏幕截图

Read this article for more details.
import
java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import
javax.imageio.ImageIO;
import java.io.File;
...
public void captureScreen(String fileName)
throws Exception {
Dimension screenSize =
Toolkit.getDefaultToolkit().getScreenSize();
Rectangle
screenRectangle = new Rectangle(screenSize);
Robot
robot = new Robot();
BufferedImage image =
robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", new File(fileName));
}
...
import java.awt.Dimension;
import java.awt.Rectangle;
import
java.awt.Robot;
import java.awt.Toolkit;
import
java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import
java.io.File;
...
public void captureScreen(String fileName) throws Exception {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize
();
Rectangle screenRectangle = new Rectangle
(screenSize);
Robot robot = new Robot();
BufferedImage image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", new File(fileName));
}
...

14. 在Java中的文件，目录列表

>File dir = new File("directoryName");
String[] children = dir.list();
if (children ==
null) {
// Either dir does not exist
or is not a directory
} else {
for (int i=0; i < children.length; i++)
{
// Get
filename of file or directory
String filename =
children[i];
}
}
// It is also possible to filter the
list of returned files.
// This example does not return any
files that start with `.'.
FilenameFilter filter = new
FilenameFilter() {
public boolean
accept(File dir, String name) {
return !name.startsWith
(".");
}
};
children = dir.list(filter);
// The list of files can also be retrieved as File objects
File[] files = dir.listFiles();
//
This filter only returns directories
FileFilter fileFilter =
new FileFilter() {
public boolean
accept(File file) {
return file.isDirectory
();
}
};
files = dir.listFiles(fileFilter);
File dir = new File("directoryName");
String[]
children = dir.list();
if (children == null)
{
// Either dir does not exist or is
not a directory
} else
{
for (int i=0; i <
children.length; i++)
{
// Get
filename of file or
directory
String filename = children[i];
}
}
// It is also possible to filter the list of returned
files.
// This example does not return any files that start
with `.'.
FilenameFilter filter = new FilenameFilter()
{
public boolean accept(File dir,
String name)
{
return !
name.startsWith(".");
}
};
children = dir.list(filter);
// The list of files can also be retrieved as File
objects
File[] files = dir.listFiles();
// This filter only returns directories
FileFilter fileFilter = new FileFilter()
{
public boolean accept(File file)
{
return
file.isDirectory();
}
};
files = dir.listFiles
(fileFilter);

15. 在Java中创建ZIP和JAR文件

import java.util.zip.*;
import
java.io.*;
public class ZipIt {
public static void main(String args[]) throws IOException
{
if (args.length <
2) {
System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
System.exit
(-1);
}
File zipFile = new File(args
[0]);
if
(zipFile.exists()) {
System.err.println("Zip file already exists, please try another");
System.exit
(-2);
}
FileOutputStream fos = new
FileOutputStream(zipFile);
ZipOutputStream zos = new
ZipOutputStream(fos);
int bytesRead;
byte[]
buffer = new byte[1024];
CRC32 crc = new CRC32();
for (int i=1, n=args.length; i < n; i++) {
String name
= args[i];
File file =
new File(name);
if (!
file.exists()) {
&
nbsp;
System.err.println("Skipping: " + name);
&
nbsp;
continue;
}
BufferedInputStream bis = new BufferedInputStream(
&
nbsp;
new FileInputStream(file));
crc.reset
();
while
((bytesRead = bis.read(buffer)) != -1) {
&
nbsp;
crc.update(buffer, 0, bytesRead);
}
bis.close();
// Reset to
beginning of input stream
bis = new
BufferedInputStream(
&
nbsp;
new FileInputStream(file));
ZipEntry
entry = new ZipEntry(name);
entry.setMethod(ZipEntry.STORED);
entry.setCompressedSize(file.length());
entry.setSize(file.length());
entry.setCrc(crc.getValue());
zos.putNextEntry(entry);
while
((bytesRead = bis.read(buffer)) != -1) {
&
nbsp;
zos.write(buffer, 0, bytesRead);
}
bis.close();
}
zos.close
();
}
}
import java.util.zip.*;
import java.io.*;
public class ZipIt {
public static void main(String args
[]) throws IOException {
if
(args.length < 2)
{
System.err.println("usage: java ZipIt Zip.zip file1 file2
file3");
System.exit(-1);
}
File zipFile = new File(args
[0]);
if (zipFile.exists())
{
System.err.println("Zip file already exists, please try
another");
System.exit(-2);
}
FileOutputStream fos = new
FileOutputStream(zipFile);
ZipOutputStream zos = new ZipOutputStream
(fos);
int
bytesRead;
byte[] buffer = new byte
[1024];
CRC32 crc = new CRC32
();
for (int i=1, n=args.length; i
< n; i++)
{
String name
= args[i];
File file = new File
(name);
if
(!file.exists())
{
System.err.println("Skipping: " +
name);
&
nbsp;
continue;
}
BufferedInputStream bis = new BufferedInputStream
(
new FileInputStream
(file));
crc.reset();
while ((bytesRead = bis.read(buffer)) != -1)
{
crc.update(buffer, 0,
bytesRead);
}
bis.close
();
// Reset
to beginning of input
stream
bis =
new BufferedInputStream
(
new FileInputStream
(file));
ZipEntry entry = new ZipEntry
(name);
entry.setMethod
(ZipEntry.STORED);
&
nbsp; entry.setCompressedSize(file.length
());
entry.setSize(file.length
());
entry.setCrc(crc.getValue
());
zos.putNextEntry
(entry);
while ((bytesRead = bis.read(buffer)) != -1)
{
zos.write(buffer, 0,
bytesRead);
}
bis.close
();
}
zos.close();
}
}
16. Parsing / Reading XML file in Java
Sample XML file.
John
B
12
Mary
A
11
Simon
A
18
John
B
12
Mary
A
<
AGE>11
Simon
A
18
Java code to parse above XML.
package net.viralpatel.java.xmlparser;
import java.io.File;
import
javax.xml.parsers.DocumentBuilder;
import
javax.xml.parsers.DocumentBuilderFactory;
import
org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import
org.w3c.dom.NodeList;
public class XMLParser
{
public void getAllUserNames(String
fileName) {
try
{
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
File file =
new File(fileName);
if
(file.exists()) {
&
nbsp;
Document doc = db.parse(file);
&
nbsp;
Element docEle = doc.getDocumentElement();
&
nbsp;
// Print root element of the document
&
nbsp;
System.out.println("Root element of the document: "
&
nbsp;
+ docEle.getNodeName
());
&
nbsp;
NodeList studentList = docEle.getElementsByTagName
("student");
&
nbsp;
// Print total student elements in document
&
nbsp;
System.out
&
nbsp;
.println("Total
students: " + studentList.getLength());
&
nbsp;
if (studentList != null && studentList.getLength() > 0)
{
&
nbsp;
for (int i = 0; i < studentList.getLength();
i++) {
&
nbsp;
Node node =
studentList.item(i);
&
nbsp;
if (node.getNodeType()
== Node.ELEMENT_NODE) {
&
nbsp;
System.out
&
nbsp;
.println
("=====================");
&
nbsp;
Element e = (Element) node;
&
nbsp;
NodeList nodeList = e.getElementsByTagName("name");
&
nbsp;
System.out.println("Name: "
&
nbsp;
+ nodeList.item(0).getChildNodes
().item(0)
&
nbsp;
;
.getNodeValue());
&
nbsp;
nodeList = e.getElementsByTagName("grade");
&
nbsp;
System.out.println("Grade: "
&
nbsp;
+ nodeList.item(0).getChildNodes
().item(0)
&
nbsp;
;
.getNodeValue());
&
nbsp;
nodeList = e.getElementsByTagName("age");
&
nbsp;
System.out.println("Age: "
&
nbsp;
+ nodeList.item(0).getChildNodes
().item(0)
&
nbsp;
;
.getNodeValue());
&
nbsp;
}
&
nbsp;
}
&
nbsp;
} else {
&
nbsp;
System.exit(1);
&
nbsp;
}
}
} catch (Exception e)
{
System.out.println(e);
}
}
public
static void main(String[] args) {
XMLParser parser = new XMLParser
();
parser.getAllUserNames("c:\test.xml");
}
}
package net.viralpatel.java.xmlparser;
import java.io.File;
import javax.xml.parsers.DocumentBuilder;
import
javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import
org.w3c.dom.Node;
import org.w3c.dom.NodeList;
public class XMLParser {
public void getAllUserNames(String fileName) {
try
{
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
DocumentBuilder db =
dbf.newDocumentBuilder();
File file = new File
(fileName);
if (file.exists())
{
Document doc = db.parse
(file);
Element docEle = doc.getDocumentElement();
// Print root element of the
document
System.out.println("Root element of the
document: "
+ docEle.getNodeName());
NodeList studentList = docEle.getElementsByTagName
("student");
// Print total student elements in
document
System.out
&nb
sp;.println("Total students: " +
studentList.getLength());
if (studentList != null &&
studentList.getLength()
> 0) {
for (int i = 0; i <
studentList.getLength
(); i++) {
Node node = studentList.item(i);
if (node.getNodeType() ==
Node.ELEMENT_NODE) {
System.out
.println
("=====================");
Element e = (Element)
node;
NodeList nodeList =
e.getElementsByTagName
("name");
System.out.println("Name:
"
+
nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
nodeList =
e.getElementsByTagName
("grade");
System.out.println("Grade:
"
+
nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
nodeList =
e.getElementsByTagName
("age");
System.out.println("Age:
"
+
nodeList.item(0).getChildNodes().item(0)
.getNodeValue());
}
}
} else
{
System.exit(1);
}
}
} catch (Exception e)
{
System.out.println(e);
}
}
public static void main(String[] args) {
XMLParser parser = new XMLParser
();
parser.getAllUserNames("c:\test.xml");
}
}
17.
Convert Array to Map in Java
import java.util.Map;
import
org.apache.commons.lang.ArrayUtils;
public class Main
{
public static void main(String[] args)
{
String[][] countries = { { "United States",
"New York" }, { "United Kingdom",
"London" },
{
"Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" }
};
Map countryCapitals =
ArrayUtils.toMap(countries);
System.out.println("Capital of Japan is " + countryCapitals.get
("Japan"));
System.out.println("Capital of
France is " + countryCapitals.get("France"));
}
}
import java.util.Map;
import org.apache.commons.lang.ArrayUtils;
public class Main {
public static void main(String[] args) {
String[][]
countries = { { "United States", "New York" }, { "United Kingdom",
"London" },
{ "Netherland",
"Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" }
};
Map countryCapitals = ArrayUtils.toMap(countries);
System.out.println("Capital of Japan is " +
countryCapitals.get("Japan"));
System.out.println("Capital of
France is " + countryCapitals.get("France"));
}
}
18. Send Email
using Java
import javax.mail.*;
import
javax.mail.internet.*;
import java.util.*;
public void postMail( String recipients[ ], String subject, String message ,
String
from) throws MessagingException
{
boolean debug = false;
//Set the host smtp address
Properties props = new Properties();
props.put("mail.smtp.host",
"smtp.example.com");
// create some
properties and get the default Session
Session
session = Session.getDefaultInstance(props, null);
session.setDebug(debug);
// create a message
Message msg = new MimeMessage(session);
// set the from and to address
InternetAddress addressFrom = new InternetAddress
(from);
msg.setFrom(addressFrom);
InternetAddress[] addressTo = new
InternetAddress[recipients.length];
for (int i =
0; i < recipients.length; i++)
{
addressTo[i] = new InternetAddress
(recipients[i]);
}
msg.setRecipients(Message.RecipientType.TO,
addressTo);
// Optional : You can
also set your custom headers in the Email if you Want
msg.addHeader("MyHeaderName", "myHeaderValue");
// Setting the Subject and Content
Type
msg.setSubject(subject);
msg.setContent(message, "text/plain");
Transport.send(msg);
}
import javax.mail.*;
import javax.mail.internet.*;
import
java.util.*;
public void postMail( String recipients[ ], String subject, String message ,
String
from) throws MessagingException
{
boolean debug =
false;
//Set the host smtp address
Properties props = new Properties();
props.put
("mail.smtp.host", "smtp.example.com");
// create some properties and get the default
Session
Session session = Session.getDefaultInstance(props,
null);
session.setDebug(debug);
// create a message
Message msg = new
MimeMessage(session);
// set the from and to address
InternetAddress addressFrom = new InternetAddress(from);
msg.setFrom(addressFrom);
InternetAddress[] addressTo = new InternetAddress
[recipients.length];
for (int i = 0; i <
recipients.length; i++)
{
addressTo[i] = new InternetAddress
(recipients[i]);
}
msg.setRecipients
(Message.RecipientType.TO, addressTo);
// Optional : You can also set your custom headers in the
Email if you Want
msg.addHeader("MyHeaderName",
"myHeaderValue");
// Setting the Subject and Content Type
msg.setSubject(subject);
msg.setContent(message,
"text/plain");
Transport.send(msg);
}
19. Send HTTP
request & fetching data using Java
import java.io.BufferedReader;
import
java.io.InputStreamReader;
import java.net.URL;
public class Main {
public static
void main(String[] args)
{
try {
URL my_url =
new URL("http://www.viralpatel.net/blogs/");&nbs
p;
BufferedReader br = new BufferedReader(new InputStreamReader
(my_url.openStream()));
String
strTemp = "";
while(null
!= (strTemp = br.readLine())){
System.out.println(strTemp);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
import java.io.BufferedReader;
import
java.io.InputStreamReader;
import java.net.URL;
public class Main {
public static void main(String[] args)
{
try {
URL my_url = new URL("http://www.viralpatel.net/blogs/");
BufferedReader br = new BufferedReader(new
InputStreamReader(my_url.openStream()));
String strTemp =
"";
while(null != (strTemp = br.readLine()))
{
System.out.println(strTemp);
}
} catch (Exception ex) {
ex.printStackTrace
();
}
}
}
20. Resize an Array in Java
/**
* Reallocates an array with a new size, and copies the
contents
* of the old array to the new array.
* @param
oldArray
the old array, to be reallocated.
* @param
newSize
the new array size.
*
@return
A new array with the
same contents.
*/
private static Object resizeArray (Object
oldArray, int newSize) {
int oldSize =
java.lang.reflect.Array.getLength(oldArray);
Class
elementType = oldArray.getClass().getComponentType();
Object newArray = java.lang.reflect.Array.newInstance(
elementType,newSize);
int preserveLength = Math.min
(oldSize,newSize);
if (preserveLength > 0)
System.arraycopy
(oldArray,0,newArray,0,preserveLength);
return
newArray;
}
// Test routine for
resizeArray().
public static void main (String[] args)
{
int[] a = {1,2,3};
a =
(int[])resizeArray(a,5);
a[3] = 4;
a[4] = 5;
for (int i=0; i
System.out.println (a[i]);
}

16. 在Java中解析/读取XML文件

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<?xml version="1.0"?>
<students>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>Mary</name>
<grade>A</grade>
<age>11</age>
</student>
<student>
<name>Simon</name>
<grade>A</grade>
<age>18</age>
</student>
</students>
<?xml version="1.0"?>
<students>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>Mary</name>
<grade>A</grade>
<age>11</age>
</student>
<student>
<name>Simon</name>
<grade>A</grade>
<age>18</age>
</student>
</students>
Java code to parse above XML.
view plaincopy to clipboardprint?
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]);
}