20个非常有用的Java程序片段

可莉
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       开学之后很久没有更博了,每天上课 游泳 打球去了…… 今天是十一小长假的第4天,闲来无事,在微博上看见转载的java开发的文章,分享给大家。

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

1)利用java反射机制实现

//获取执行某对象的方法

public Object invokeMethod(Object owner, String methodName, Object[] args) throws Exception {   

     Class ownerClass = owner.getClass();   
     Class[] argsClass = new Class[args.length];   
     for (int i = 0, j = args.length; i < j; i++) {   
         argsClass[i] = args[i].getClass();   
     }   
     Method method = ownerClass.getMethod(methodName,argsClass);   
     return method.invoke(owner, args);   
}  

2)

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("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

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:\\\\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) 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

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

/** * 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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