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
inti = Integer.parseInt(a); //numeric string to an int
2. 向文件末尾添加内容
BufferedWriter out = null;
try{
out = newBufferedWriter(newFileWriter(”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 = newSimpleDateFormat( "dd.MM.yyyy");
Date date = format.parse( myString );
5. 使用JDBC链接Oracle
publicclassOracleJdbcTest
{
String driverClass = "oracle.jdbc.driver.OracleDriver";
Connection con;
publicvoidinit(FileInputStream fs) throwsClassNotFoundException, SQLException, FileNotFoundException, IOException
{
Properties props = newProperties();
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);
}
publicvoidfetch() throwsSQLException, 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();
}
publicstaticvoidmain(String[] args)
{
OracleJdbcTest test = newOracleJdbcTest();
test.init();
test.fetch();
}
}
6. 把 Java util.Date 转成 sql.Date
java.util.Date utilDate = newjava.util.Date();
java.sql.Date sqlDate = newjava.sql.Date(utilDate.getTime());
7. 使用NIO进行快速的文件拷贝
publicstaticvoidfileCopy( File in, File out )
throwsIOException
{
FileChannel inChannel = newFileInputStream( in ).getChannel();
FileChannel outChannel = newFileOutputStream( out ).getChannel();
try
{
// inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows
// magic number for Windows, 64Mb - 32Kb)
intmaxCount = (64* 1024* 1024) - (32* 1024);
longsize = inChannel.size();
longposition = 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
privatevoidcreateThumbnail(String filename, intthumbWidth, intthumbHeight, intquality, String outFilename)
throwsInterruptedException, FileNotFoundException, IOException
{
// load image from filename
Image image = Toolkit.getDefaultToolkit().getImage(filename);
MediaTracker mediaTracker = newMediaTracker(newContainer());
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
doublethumbRatio = (double)thumbWidth / (double)thumbHeight;
intimageWidth = image.getWidth(null);
intimageHeight = image.getHeight(null);
doubleimageRatio = (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 = newBufferedImage(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 = newBufferedOutputStream(newFileOutputStream(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. 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)
importorg.json.JSONObject;
...
...
JSONObject json = newJSONObject();
json.put("city", "Mumbai");
json.put("country", "India");
...
String output = json.toString();
...
10. 使用iText JAR生成PDF
阅读这篇文章 了解更多细节
importjava.io.File;
importjava.io.FileOutputStream;
importjava.io.OutputStream;
importjava.util.Date;
importcom.lowagie.text.Document;
importcom.lowagie.text.Paragraph;
importcom.lowagie.text.pdf.PdfWriter;
publicclassGeneratePDF {
publicstaticvoidmain(String[] args) {
try{
OutputStream file = newFileOutputStream(newFile("C:\\Test.pdf"));
Document document = newDocument();
PdfWriter.getInstance(document, file);
document.open();
document.add(newParagraph("Hello Kiran"));
document.add(newParagraph(newDate().toString()));
document.close();
file.close();
} catch(Exception e) {
e.printStackTrace();
}
}
}
11. HTTP 代理设置
阅读这篇 文章 了解更多细节。
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. 单实例Singleton 示例
请先阅读这篇文章 了解更多信息
publicclassSimpleSingleton {
privatestaticSimpleSingleton singleInstance = newSimpleSingleton();
//Marking default constructor private
//to avoid direct instantiation.
privateSimpleSingleton() {
}
//Get instance for class SimpleSingleton
publicstaticSimpleSingleton getInstance() {
returnsingleInstance;
}
}
另一种实现
publicenumSimpleSingleton {
INSTANCE;
publicvoiddoSomething() {
}
}
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
13. 抓屏程序
阅读这篇文章 获得更多信息。
importjava.awt.Dimension;
importjava.awt.Rectangle;
importjava.awt.Robot;
importjava.awt.Toolkit;
importjava.awt.image.BufferedImage;
importjavax.imageio.ImageIO;
importjava.io.File;
...
publicvoidcaptureScreen(String fileName) throwsException {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Rectangle screenRectangle = newRectangle(screenSize);
Robot robot = newRobot();
BufferedImage image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", newFile(fileName));
}
...
14. 列出文件和目录
File dir = newFile("directoryName");
String[] children = dir.list();
if(children == null) {
// Either dir does not exist or is not a directory
} else{
for(inti=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 = newFilenameFilter() {
publicbooleanaccept(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 = newFileFilter() {
publicbooleanaccept(File file) {
returnfile.isDirectory();
}
};
files = dir.listFiles(fileFilter);
15. 创建ZIP和JAR文件
importjava.util.zip.*;
importjava.io.*;
publicclassZipIt {
publicstaticvoidmain(String args[]) throwsIOException {
if(args.length < 2) {
System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
System.exit(-1);
}
File zipFile = newFile(args[0]);
if(zipFile.exists()) {
System.err.println("Zip file already exists, please try another");
System.exit(-2);
}
FileOutputStream fos = newFileOutputStream(zipFile);
ZipOutputStream zos = newZipOutputStream(fos);
intbytesRead;
byte[] buffer = newbyte[1024];
CRC32 crc = newCRC32();
for(inti=1, n=args.length; i < n; i++) {
String name = args[i];
File file = newFile(name);
if(!file.exists()) {
System.err.println("Skipping: "+ name);
continue;
}
BufferedInputStream bis = newBufferedInputStream(
newFileInputStream(file));
crc.reset();
while((bytesRead = bis.read(buffer)) != -1) {
crc.update(buffer, 0, bytesRead);
}
bis.close();
// Reset to beginning of input stream
bis = newBufferedInputStream(
newFileInputStream(file));
ZipEntry entry = newZipEntry(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();
}
}
16. 解析/读取XML 文件
XML文件
John
B
12
Mary
A
11
Simon
A
18
Java代码
packagenet.viralpatel.java.xmlparser;
importjava.io.File;
importjavax.xml.parsers.DocumentBuilder;
importjavax.xml.parsers.DocumentBuilderFactory;
importorg.w3c.dom.Document;
importorg.w3c.dom.Element;
importorg.w3c.dom.Node;
importorg.w3c.dom.NodeList;
publicclassXMLParser {
publicvoidgetAllUserNames(String fileName) {
try{
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
File file = newFile(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(inti = 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);
}
}
publicstaticvoidmain(String[] args) {
XMLParser parser = newXMLParser();
parser.getAllUserNames("c:\\test.xml");
}
}
17. 把 Array 转换成 Map
importjava.util.Map;
importorg.apache.commons.lang.ArrayUtils;
publicclassMain {
publicstaticvoidmain(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. 发送邮件
importjavax.mail.*;
importjavax.mail.internet.*;
importjava.util.*;
publicvoidpostMail( String recipients[ ], String subject, String message , String from) throwsMessagingException
{
booleandebug = false;
//Set the host smtp address
Properties props = newProperties();
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 = newMimeMessage(session);
// set the from and to address
InternetAddress addressFrom = newInternetAddress(from);
msg.setFrom(addressFrom);
InternetAddress[] addressTo = newInternetAddress[recipients.length];
for(inti = 0; i < recipients.length; i++)
{
addressTo[i] = newInternetAddress(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. 发送代数据的HTTP 请求
importjava.io.BufferedReader;
importjava.io.InputStreamReader;
importjava.net.URL;
publicclassMain {
publicstaticvoidmain(String[] args) {
try{
URL my_url = newURL("http://coolshell.cn/");
BufferedReader br = newBufferedReader(newInputStreamReader(my_url.openStream()));
String strTemp = "";
while(null!= (strTemp = br.readLine())){
System.out.println(strTemp);
}
} catch(Exception ex) {
ex.printStackTrace();
}
}
}
20. 改变数组的大小
/**
* 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.
*/
privatestaticObject resizeArray (Object oldArray, intnewSize) {
intoldSize = java.lang.reflect.Array.getLength(oldArray);
Class elementType = oldArray.getClass().getComponentType();
Object newArray = java.lang.reflect.Array.newInstance(
elementType,newSize);
intpreserveLength = Math.min(oldSize,newSize);
if(preserveLength > 0)
System.arraycopy (oldArray,0,newArray,0,preserveLength);
returnnewArray;
}
// Test routine for resizeArray().
publicstaticvoidmain (String[] args) {
int[] a = {1,2,3};
a = (int[])resizeArray(a,5);
a[3] = 4;
a[4] = 5;
for(inti=0; i