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
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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
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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();
}
}
3. 得到当前方法的名字
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String methodName = Thread.currentThread().getStackTrace()[``1``].getMethodName();
4. 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 );
5. 使用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();
}
}
6. 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());
7. 使用NIO进行快速的文件拷贝
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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();
}
}
}
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();
}
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();
...
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();
}
}
}
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"``);
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;
}
}
另一种实现
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public enum SimpleSingleton {
INSTANCE;
public void doSomething() {
}
}
//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。
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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));
}
...
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);
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();
}
}
16. 解析/读取XML 文件
XML文件
<?``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"``);
}
}
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"``));
}
}
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);
}
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 {
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();
}
}
}
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](https://my.oschina.net/u/2303379) oldArray the old array, to be reallocated.
* [@param](https://my.oschina.net/u/2303379) newSize the new array size.
* [@return](https://my.oschina.net/u/556800) 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]);
}
