下面是20个非常有用的Java程序片段，期望能对你有用。
- 字符串有整型的相互转换
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String a = String.valueOf(2); //integer to numeric string
int i = Integer.parseInt(a);//numeric string to an int
- 向文件末尾添加内容
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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();
} }
- 得到当前办法的名字
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String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
- 转字符串到日期
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java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);
或者是：
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SimpleDateFormat format =new SimpleDateFormat("dd.MM.yyyy" );
Date date = format.parse( myString );
- 运用JDBC链接Oracle
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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();
} }
- 把 Java util.Date 转成 sql.Date
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java.util.Date utilDate =new java.util.Date();
java.sql.Date sqlDate =new java.sql.Date(utilDate.getTime());
- 运用NIO进行快速的文件复制
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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();
}
}
}- 创立图片的缩略图
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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();
}- 创立 JSON 格局的数据
请先阅览这篇文章 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();
...
- 运用iText JAR生成PDF
阅览这篇文章 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();
}
} }
- HTTP 署理设置
阅览这篇 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");
- 单实例Singleton 示例
请先阅览这篇文章 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;
} }
另一种完成
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public enum SimpleSingleton {
INSTANCE;
public void doSomething() {
} }
//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();
- 抓屏程序
阅览这篇文章 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));
}
...
- 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);
- 创立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();
} }
- 解析/读取XML 文件
XML文件
John
B
12
Mary
A
11
Simon
A
18
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");
} }
- 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")); }
}
- 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); }
- 发送代数据的HTTP 恳求
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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 {
URL my_url =new URL("http://coolshell.cn/");
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();
}
} }