20个非常有用的Java程序片段

来源：码农网时间：2015-03-17 10:23:28 阅读数：1057

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[导读] 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 stri

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(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)

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("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

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.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

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(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

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

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

import javax.mail.*; 
import javax.mail.internet.*; 
import java.util.*; 
 
public void postMail( String recipients[ ], String subject, String message , String from) throwsMessagingException 
{ 
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

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(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.
*/
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]); 
} 