值得收藏的20个非常有用的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
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)
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
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。
System.getProperties().put("httpxyHost", "someProxyURL");
System.getProperties().put("httpxyPort", "someProxyPort");
System.getProperties().put("httpxyUser", "someUserName");
System.getProperties().put("httpxyPassword", "somePassword");
12. 单实例Singleton 示例
请先阅读这篇文章 了解更多信息
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
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。
import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awtage.BufferedImage;
import javaxageioageIO;
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
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
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(ZipEntryD);
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
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:\ est.xml");
}
}
17. 把 Array 转换成 Map
import java.util.Map;
import org.apachemons.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
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", "smtp.example");
// 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
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("coolshell/");
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
/**
* 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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