20非常有用的Java程序片段
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
String a = String.valueOf(2); //integer to numeric string int i = Integer.parseInt(a); //numeric string to an int
2. 向文件末尾添加内容
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. 得到当前方法的名字
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
4. 转字符串到日期
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
或者是：
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); Date date = format.parse( myString );
5. 使用JDBC链接Oracle
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. 把 Java util.Date 转成 sql.Date
java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime())
7. 使用NIO进行快速的文件拷贝
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. 创建图片的缩略图
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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();
}
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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)
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import
org.json.JSONObject;
...
...
JSONObject json =
new
JSONObject();
json.put(
"city"
,
"Mumbai"
);
json.put(
"country"
,
"India"
);
...
String output = json.toString();
...
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10. 使用iText JAR生成PDF
阅读这篇文章 了解更多细节
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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();
}
}
}
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11. HTTP 代理设置
阅读这篇 文章 了解更多细节。
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System.getProperties().put(
"http.proxyHost"
,
"someProxyURL"
);
System.getProperties().put(
"http.proxyPort"
,
"someProxyPort"
);
System.getProperties().put(
"http.proxyUser"
,
"someUserName"
);
System.getProperties().put(
"http.proxyPassword"
,
"somePassword"
);
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12. 单实例Singleton 示例
请先阅读这篇文章 了解更多信息
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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;
}
}
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另一种实现
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public
enum
SimpleSingleton {
INSTANCE;
public
void
doSomething() {
}
}
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
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13. 抓屏程序
阅读这篇文章 获得更多信息。
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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));
}
...
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14. 列出文件和目录
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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);
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15. 创建ZIP和JAR文件
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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);
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);
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();
}
}
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16. 解析/读取XML 文件
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
>
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Java代码
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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"
);
}
}
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17. 把 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"
));
}
}
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18. 发送邮件
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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);
}
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19. 发送代数据的HTTP 请求
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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
{
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();
}
}
}
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20. 改变数组的大小
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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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