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非常有用的Java程序片段

2012-06-26 15:03 316 查看
下面是20个非常有用的Java程序片段，希望能对你有用。1. 字符串有整型的相互转换
1
2
String a = String.valueOf(
2
);
//integer to numeric string
3
int
 
i = Integer.parseInt(a);
//numericstring to an int
2. 向文件末尾添加内容
01
02
BufferedWriter out =
null
;
03
try
 
{
04
out =
new
 
BufferedWriter(
new
FileWriter(”filename”,
true
));
05
out.write(”aString”);
06
}
catch
(IOException e) {
07
// error processing code
08
}
finally
{
09
if
(out !=
null
) {
10
out.close();
11
}
12
}
3. 得到当前方法的名字
1
String methodName = Thread.currentThread().getStackTrace()[
1
].getMethodName();
4. 转字符串到日期
1
2
java.util.Date =java.text.DateFormat.getDateInstance().parse(date String);
或者是：
1
2
SimpleDateFormat format =
new
SimpleDateFormat(
"dd.MM.yyyy"
);
3
Date date = format.parse( myString );
5. 使用JDBC链接Oracle
01
public
 
class
 
OracleJdbcTest
02
{
03
String driverClass =
"oracle.jdbc.driver.OracleDriver"
;
04
05
Connection con;
06
07
public
void
 
init(FileInputStream fs)
throws
 
ClassNotFoundException,SQLException,FileNotFoundException,IOException
08
{
09
Properties props =
new
 
Properties();
10
props.load(fs);
11
String url = props.getProperty(
"db.url"
);
12
String userName = props.getProperty(
"db.user"
);
13
String password = props.getProperty(
"db.password"
);
14
Class.forName(driverClass);
15
16
con=DriverManager.getConnection(url,userName,password);
17
}
18
19
public
void
 
fetch()
throws
SQLException,IOException
20
{
21
PreparedStatementps = con.prepareStatement(
"selectSYSDATE from dual"
);
22
ResultSet rs = ps.executeQuery();
23
24
while
(rs.next())
25
{
26
// do thething you do
27
}
28
rs.close();
29
ps.close();
30
}
31
32
public
static
 
void
 
main(String[]args)
33
{
34
OracleJdbcTest test =
new
 
OracleJdbcTest();
35
test.init();
36
test.fetch();
37
}
38
}
6. 把 Java util.Date 转成 sql.Date
1
java.util.Date utilDate =
new
java.util.Date();
2
java.sql.Date sqlDate =
new
java.sql.Date(utilDate.getTime());
7. 使用NIO进行快速的文件拷贝
01
public
 
static
 
void
 
fileCopy( Filein,File out )
02
throws
IOException
03
{
04
FileChannelinChannel =
new
 
FileInputStream( in ).getChannel();
05
FileChannel outChannel =
new
 
FileOutputStream( out ).getChannel();
06
try
07
{
08
//inChannel.transferTo(0,inChannel.size(),outChannel);// original -- apparently hastrouble copying large files on Windows
09
10
// magic number for Windows,64Mb - 32Kb)
11
int
maxCount = (
64
 
*
1024
 
*
1024
) - (
32
*
1024
);
12
long
size =inChannel.size();
13
long
position =
0
;
14
while
( position< size )
15
{
16
position += inChannel.transferTo( position,maxCount,outChannel );
17
}
18
}
19
finally
20
{
21
if
( inChannel !=
null
)
22
{
23
inChannel.close();
24
}
25
if
( outChannel !=
null
)
26
{
27
outChannel.close();
28
}
29
}
30
}
8. 创建图片的缩略图
01
private
 
void
 
createThumbnail(String filename,
int
thumbWidth,
int
thumbHeight,
int
quality,String outFilename)
02
throws
InterruptedException,FileNotFoundException,IOException
03
{
04
// load imagefrom filename
05
Image image = Toolkit.getDefaultToolkit().getImage(filename);
06
MediaTracker mediaTracker =
new
 
MediaTracker(
new
Container());
07
mediaTracker.addImage(image,
0
);
08
mediaTracker.waitForID(
0
);
09
// use this to test for errors at this point:System.out.println(mediaTracker.isErrorAny());
10
11
// determine thumbnail size from WIDTH and HEIGHT
12
double
thumbRatio = (
double
)thumbWidth /(
double
)thumbHeight;
13
int
imageWidth =image.getWidth(
null
);
14
int
imageHeight =image.getHeight(
null
);
15
double
imageRatio = (
double
)imageWidth /(
double
)imageHeight;
16
if
(thumbRatio < imageRatio){
17
thumbHeight =(
int
)(thumbWidth/ imageRatio);
18
}
else
 
{
19
thumbWidth = (
int
)(thumbHeight *imageRatio);
20
}
21
22
// draw original image to thumbnail image object and
23
// scale it to the new size on-the-fly
24
BufferedImage thumbImage =
new
 
BufferedImage(thumbWidth,thumbHeight,BufferedImage.TYPE_INT_RGB);
25
Graphics2D graphics2D = thumbImage.createGraphics();
26
graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION,RenderingHints.VALUE_INTERPOLATION_BILINEAR);
27
graphics2D.drawImage(image,
0
,
0
,thumbWidth,thumbHeight,
null
);
28
29
// save thumbnail image to outFilename
30
BufferedOutputStream out =
new
 
BufferedOutputStream(
new
FileOutputStream(outFilename));
31
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
32
JPEGEncodeParam param =encoder.getDefaultJPEGEncodeParam(thumbImage);
33
quality = Math.max(
0
,Math.min(quality,
100
));
34
param.setQuality((
float
)quality /
100
.0f,
false
);
35
encoder.setJPEGEncodeParam(param);
36
encoder.encode(thumbImage);
37
out.close();
38
}
9. 创建 JSON 格式的数据请先阅读这篇文章 了解一些细节，并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)
1
import
 
org.json.JSONObject;
2
...
3
...
4
JSONObject json =
new
JSONObject();
5
json.put(
"city"
,
"Mumbai"
);
6
json.put(
"country"
,
"India"
);
7
...
8
String output =json.toString();
9
...
10. 使用iText JAR生成PDF阅读这篇文章 了解更多细节
01
02
import
 
java.io.File;
03
import
 
java.io.FileOutputStream;
04
import
 
java.io.OutputStream;
05
import
 
java.util.Date;
06
07
import
 
com.lowagie.text.Document;
08
import
 
com.lowagie.text.Paragraph;
09
import
 
com.lowagie.text.pdf.PdfWriter;
10
11
public
 
class
 
GeneratePDF{
12
13
public
static
 
void
 
main(String[]args) {
14
try
{
15
OutputStream file =
new
 
FileOutputStream(
new
File(
"C:\\Test.pdf"
));
16
17
Document document =
new
 
Document();
18
PdfWriter.getInstance(document,file);
19
document.open();
20
document.add(
new
Paragraph(
"HelloKiran"
));
21
document.add(
new
Paragraph(
new
 
Date().toString()));
22
23
document.close();
24
file.close();
25
26
}
catch
 
(Exception e) {
27
28
e.printStackTrace();
29
}
30
}
31
}
11. HTTP 代理设置阅读这篇文章 了解更多细节。
1
2
System.getProperties().put(
"http.proxyHost"
,
"someProxyURL"
);
3
System.getProperties().put(
"http.proxyPort"
,
"someProxyPort"
);
4
System.getProperties().put(
"http.proxyUser"
,
"someUserName"
);
5
System.getProperties().put(
"http.proxyPassword"
,
"somePassword"
);
12. 单实例Singleton 示例请先阅读这篇文章 了解更多信息
01
02
public
 
class
 
SimpleSingleton{
03
private
static
 
SimpleSingletonsingleInstance =
new
 
SimpleSingleton();
04
05
//Marking default constructor private
06
//to avoid direct instantiation.
07
private
SimpleSingleton() {
08
}
09
10
//Get instance for class SimpleSingleton
11
public
static
 
SimpleSingleton getInstance() {
12
13
return
singleInstance;
14
}
15
}
另一种实现
1
public
 
enum
 
SimpleSingleton {
2
INSTANCE;
3
public
void
 
doSomething() {
4
}
5
}
6
7
//Call the method fromSingleton:
8
SimpleSingleton.INSTANCE.doSomething();
13. 抓屏程序阅读这篇文章 获得更多信息。
01
import
 
java.awt.Dimension;
02
import
 
java.awt.Rectangle;
03
import
 
java.awt.Robot;
04
import
 
java.awt.Toolkit;
05
import
 
java.awt.image.BufferedImage;
06
import
 
javax.imageio.ImageIO;
07
import
 
java.io.File;
08
09
...
10
11
public
 
void
 
captureScreen(StringfileName)
throws
Exception {
12
13
DimensionscreenSize = Toolkit.getDefaultToolkit().getScreenSize();
14
RectanglescreenRectangle =
new
 
Rectangle(screenSize);
15
Robotrobot =
new
 
Robot();
16
BufferedImageimage = robot.createScreenCapture(screenRectangle);
17
ImageIO.write(image,
"png"
,
new
File(fileName));
18
19
}
20
...
14. 列出文件和目录
01
File dir =
new
File(
"directoryName"
);
02
String[]children = dir.list();
03
if
(children ==
null
) {
04
// Either dir does not exist or is not a directory
05
}
else
 
{
06
for
(
int
 
i=
0
; i < children.length; i++){
07
// Getfilename of file or directory
08
String filename = children[i];
09
}
10
}
11
12
// It is also possible to filter the list of returned files.
13
// This example does not return any files that start with`.'.
14
FilenameFilter filter =
new
 
FilenameFilter(){
15
public
boolean
 
accept(Filedir,String name) {
16
return
!name.startsWith(
"."
);
17
}
18
};
19
children= dir.list(filter);
20
21
// The list of files can also be retrieved as File objects
22
File[]files = dir.listFiles();
23
24
// This filter only returns directories
25
FileFilterfileFilter =
new
 
FileFilter() {
26
public
boolean
 
accept(File file) {
27
return
file.isDirectory();
28
}
29
};
30
files= dir.listFiles(fileFilter);
15. 创建ZIP和JAR文件
01
02
import
 
java.util.zip.*;
03
import
 
java.io.*;
04
05
public
 
class
 
ZipIt {
06
public
static
 
void
 
main(Stringargs[]) 
throws
 
IOException {
07
if
(args.length <
2
) {
08
System.err.println(
"usage:java ZipIt Zip.zip file1 file2 file3"
);
09
System.exit(-
1
);
10
}
11
File zipFile =
new
 
File(args[
0
]);
12
if
(zipFile.exists()) {
13
System.err.println(
"Zipfile already exists,please try another"
);
14
System.exit(-
2
);
15
}
16
FileOutputStreamfos =
new
 
FileOutputStream(zipFile);
17
ZipOutputStream zos =
new
 
ZipOutputStream(fos);
18
int
bytesRead;
19
byte
[] buffer =
new
 
byte
[
1024
];
20
CRC32 crc =
new
 
CRC32();
21
for
(
int
 
i=
1
,n=args.length; i < n; i++){
22
String name =args[i];
23
File file =
new
 
File(name);
24
if
(!file.exists()){
25
System.err.println(
"Skipping: "
+name);
26
continue
;
27
}
28
BufferedInputStream bis =
new
 
BufferedInputStream(
29
new
FileInputStream(file));
30
crc.reset();
31
while
((bytesRead =bis.read(buffer)) != -
1
) {
32
crc.update(buffer,
0
,bytesRead);
33
}
34
bis.close();
35
// Reset to beginning of input stream
36
bis =
new
 
BufferedInputStream(
37
new
FileInputStream(file));
38
ZipEntry entry =
new
 
ZipEntry(name);
39
entry.setMethod(ZipEntry.STORED);
40
entry.setCompressedSize(file.length());
41
entry.setSize(file.length());
42
entry.setCrc(crc.getValue());
43
zos.putNextEntry(entry);
44
while
((bytesRead =bis.read(buffer)) != -
1
) {
45
zos.write(buffer,
0
,bytesRead);
46
}
47
bis.close();
48
}
49
zos.close();
50
}
51
}
16. 解析/读取XML 文件XML文件
01
<?
xml
version
=
"1.0"
?>
02
<
students
>
03
<
student
>
04
<
name
>John</
name
>
05
<
grade
>B</
grade
>
06
<
age
>12</
age
>
07
</
student
>
08
<
student
>
09
<
name
>Mary</
name
>
10
<
grade
>A</
grade
>
11
<
age
>11</
age
>
12
</
student
>
13
<
student
>
14
<
name
>Simon</
name
>
15
<
grade
>A</
grade
>
16
<
age
>18</
age
>
17
</
student
>
18
</
students
>
Java代码
01
02
package
 
net.viralpatel.java.xmlparser;
03
04
import
 
java.io.File;
05
import
 
javax.xml.parsers.DocumentBuilder;
06
import
 
javax.xml.parsers.DocumentBuilderFactory;
07
08
import
 
org.w3c.dom.Document;
09
import
 
org.w3c.dom.Element;
10
import
 
org.w3c.dom.Node;
11
import
 
org.w3c.dom.NodeList;
12
13
public
 
class
 
XMLParser {
14
15
public
void
 
getAllUserNames(String fileName) {
16
try
{
17
DocumentBuilderFactory dbf =DocumentBuilderFactory.newInstance();
18
DocumentBuilder db = dbf.newDocumentBuilder();
19
File file =
new
 
File(fileName);
20
if
(file.exists()){
21
Documentdoc = db.parse(file);
22
Element docEle = doc.getDocumentElement();
23
24
// Print root element of the document
25
System.out.println(
"Rootelement of the document: "
26
+ docEle.getNodeName());
27
28
NodeList studentList = docEle.getElementsByTagName(
"student"
);
29
30
// Print total student elements in document
31
System.out
32
.println(
"Totalstudents: "
+studentList.getLength());
33
34
if
(studentList !=
null
&&studentList.getLength() > 
0
) {
35
for
(
int
 
i =
0
; i <studentList.getLength(); i++) {
36
37
Node node = studentList.item(i);
38
39
if
(node.getNodeType()== Node.ELEMENT_NODE) {
40
41
System.out
42
.println(
"====================="
);
43
44
Element e = (Element) node;
45
NodeList nodeList = e.getElementsByTagName(
"name"
);
46
System.out.println(
"Name:"
47
+ nodeList.item(
0
).getChildNodes().item(
0
)
48
.getNodeValue());
49
50
nodeList = e.getElementsByTagName(
"grade"
);
51
System.out.println(
"Grade:"
52
+ nodeList.item(
0
).getChildNodes().item(
0
)
53
.getNodeValue());
54
55
nodeList = e.getElementsByTagName(
"age"
);
56
System.out.println(
"Age:"
57
+ nodeList.item(
0
).getChildNodes().item(
0
)
58
.getNodeValue());
59
}
60
}
61
}
else
 
{
62
System.exit(
1
);
63
}
64
}
65
}
catch
 
(Exception e) {
66
System.out.println(e);
67
}
68
}
69
public
static
 
void
 
main(String[]args) {
70
71
XMLParser parser =
new
 
XMLParser();
72
parser.getAllUserNames(
"c:\\test.xml"
);
73
}
74
}
17. 把 Array 转换成 Map 
01
02
import
 
java.util.Map;
03
import
 
org.apache.commons.lang.ArrayUtils;
04
05
public
 
class
 
Main {
06
07
public
static
 
void
 
main(String[]args) {
08
String[][] countries ={ {
"United States"
,
"New York"
 
},{ 
"UnitedKingdom"
,
"London"
 
},
09
{
"Netherland"
,
"Amsterdam"
 
},{ 
"Japan"
,
"Tokyo"
},{ 
"France"
,
"Paris"
 
}};
10
11
Map countryCapitals = ArrayUtils.toMap(countries);
12
13
System.out.println(
"Capital ofJapan is "
+ countryCapitals.get(
"Japan"
));
14
System.out.println(
"Capital ofFrance is "
+ countryCapitals.get(
"France"
));
15
}
16
}
18. 发送邮件
01
import
 
javax.mail.*;
02
import
 
javax.mail.internet.*;
03
import
 
java.util.*;
04
05
public
 
void
 
postMail(String recipients[ ],String subject,String message ,String from)
throws
 
MessagingException
06
{
07
boolean
debug =
false
;
08
09
 
//Set the host smtp address
10
 
Properties props =
new
 
Properties();
11
 
props.put(
"mail.smtp.host"
,
"smtp.example.com"
);
12
13
// create some properties and get the default Session
14
Session session = Session.getDefaultInstance(props,
null
);
15
session.setDebug(debug);
16
17
// create a message
18
Message msg =
new
 
MimeMessage(session);
19
20
// set the from and to address
21
InternetAddress addressFrom =
new
 
InternetAddress(from);
22
msg.setFrom(addressFrom);
23
24
InternetAddress[] addressTo =
new
 
InternetAddress[recipients.length];
25
for
(
int
 
i =
0
; i <recipients.length; i++)
26
{
27
addressTo[i] =
new
 
InternetAddress(recipients[i]);
28
}
29
msg.setRecipients(Message.RecipientType.TO,addressTo);
30
31
// Optional : You can also set your custom headers in theEmail if you Want
32
msg.addHeader(
"MyHeaderName"
,
"myHeaderValue"
);
33
34
// Setting the Subject and Content Type
35
msg.setSubject(subject);
36
msg.setContent(message,
"text/plain"
);
37
Transport.send(msg);
38
}
19. 发送代数据的HTTP 请求
01
02
import
 
java.io.BufferedReader;
03
import
 
java.io.InputStreamReader;
04
import
 
java.net.URL;
05
06
public
 
class
 
Main {
07
public
static
 
void
 
main(String[]args){
08
try
{
09
URL my_url =
new
 
URL(
"http://coolshell.cn/"
);
10
BufferedReader br =
new
 
BufferedReader(
new
InputStreamReader(my_url.openStream()));
11
String strTemp =
""
;
12
while
(
null
!= (strTemp =br.readLine())){
13
System.out.println(strTemp);
14
}
15
}
catch
 
(Exceptionex) {
16
ex.printStackTrace();
17
}
18
}
19
}
20. 改变数组的大小
01
02
/**
03
*Reallocates an arraywith a new size,and copies the contents
04
*of the old array tothe new array.
05
*@param oldArraytheold array,to be reallocated.
06
*@param newSizethenew array size.
07
*@returnA newarray with the same contents.
08
*/
09
private
 
static
 
ObjectresizeArray (Object oldArray,
int
 
newSize) {
10
int
oldSize =java.lang.reflect.Array.getLength(oldArray);
11
ClasselementType = oldArray.getClass().getComponentType();
12
ObjectnewArray = java.lang.reflect.Array.newInstance(
13
 
elementType,newSize);
14
int
preserveLength =Math.min(oldSize,newSize);
15
if
(preserveLength >
0
)
16
System.arraycopy (oldArray,
0
,newArray,
0
,preserveLength);
17
return
newArray;
18
}
19
20
// Test routine forresizeArray().
21
public
 
static
 
void
 
main (String[] args) {
22
int
[] a = {
1
,
2
,
3
};
23
a= (
int
[])resizeArray(a,
5
);
24
a[
3
] =
4
;
25
a[
4
] =
5
;
26
for
(
int
 
i=
0
; i<a.length; i++)
27
System.out.println (a[i]);
28
}

                                            

                                        

                        
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