20个java常用片段

1. 字符串有整型的相互转换
Java代码
1.String a = String.valueOf(2);   //integer to numeric string
2.int i = Integer.parseInt(a); //numeric string to an int

2. 向文件末尾添加内容
Java代码
1.BufferedWriter out = null;
2.try {
3.    out = new BufferedWriter(new FileWriter(”filename”, true));
4.    out.write(”aString”);
5.} catch (IOException e) {
6.    // error processing code
7.} finally {
8.    if (out != null) {
9.        out.close();
10.    }
11.}

Java代码
1. 3. 得到当前方法的名字
2.String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4. 转字符串到日期
Java代码
1.java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);

或者是：
Java代码
1.SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
2.Date date = format.parse( myString );

5. 使用JDBC链接Oracle
Java代码
1.public class OracleJdbcTest
2.{
3.    String driverClass = "oracle.jdbc.driver.OracleDriver";
4.
5.    Connection con;
6.
7.    public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException
8.    {
9.        Properties props = new Properties();
10.        props.load(fs);
11.        String url = props.getProperty("db.url");
12.        String userName = props.getProperty("db.user");
13.        String password = props.getProperty("db.password");
14.        Class.forName(driverClass);
15.
16.        con=DriverManager.getConnection(url, userName, password);
17.    }
18.
19.    public void fetch() throws SQLException, IOException
20.    {
21.        PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");
22.        ResultSet rs = ps.executeQuery();
23.
24.        while (rs.next())
25.        {
26.            // do the thing you do
27.        }
28.        rs.close();
29.        ps.close();
30.    }
31.
32.    public static void main(String[] args)
33.    {
34.        OracleJdbcTest test = new OracleJdbcTest();
35.        test.init();
36.        test.fetch();
37.    }
38.}

6. 把 Java util.Date 转成 sql.Date
Java代码
1.java.util.Date utilDate = new java.util.Date();
2.java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. 使用NIO进行快速的文件拷贝
Java代码
1.public static void fileCopy( File in, File out )
2.            throws IOException
3.    {
4.        FileChannel inChannel = new FileInputStream( in ).getChannel();
5.        FileChannel outChannel = new FileOutputStream( out ).getChannel();
6.        try
7.        {
8.//          inChannel.transferTo(0, inChannel.size(), outChannel);      // original -- apparently has trouble copying large files on Windows
9.
10.            // magic number for Windows, 64Mb - 32Kb)
11.            int maxCount = (64 * 1024 * 1024) - (32 * 1024);
12.            long size = inChannel.size();
13.            long position = 0;
14.            while ( position < size )
15.            {
16.               position += inChannel.transferTo( position, maxCount, outChannel );
17.            }
18.        }
19.        finally
20.        {
21.            if ( inChannel != null )
22.            {
23.               inChannel.close();
24.            }
25.            if ( outChannel != null )
26.            {
27.                outChannel.close();
28.            }
29.        }
30.    }

8. 创建图片的缩略图
Java代码
1.private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)
2.        throws InterruptedException, FileNotFoundException, IOException
3.    {
4.        // load image from filename
5.        Image image = Toolkit.getDefaultToolkit().getImage(filename);
6.        MediaTracker mediaTracker = new MediaTracker(new Container());
7.        mediaTracker.addImage(image, 0);
8.        mediaTracker.waitForID(0);
9.        // use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny());
10.
11.        // determine thumbnail size from WIDTH and HEIGHT
12.        double thumbRatio = (double)thumbWidth / (double)thumbHeight;
13.        int imageWidth = image.getWidth(null);
14.        int imageHeight = image.getHeight(null);
15.        double imageRatio = (double)imageWidth / (double)imageHeight;
16.        if (thumbRatio < imageRatio) {
17.            thumbHeight = (int)(thumbWidth / imageRatio);
18.        } else {
19.            thumbWidth = (int)(thumbHeight * imageRatio);
20.        }
21.
22.        // draw original image to thumbnail image object and
23.        // scale it to the new size on-the-fly
24.        BufferedImage thumbImage = new BufferedImage(thumbWidth, thumbHeight, BufferedImage.TYPE_INT_RGB);
25.        Graphics2D graphics2D = thumbImage.createGraphics();
26.        graphics2D.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
27.        graphics2D.drawImage(image, 0, 0, thumbWidth, thumbHeight, null);
28.
29.        // save thumbnail image to outFilename
30.        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(outFilename));
31.        JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(out);
32.        JPEGEncodeParam param = encoder.getDefaultJPEGEncodeParam(thumbImage);
33.        quality = Math.max(0, Math.min(quality, 100));
34.        param.setQuality((float)quality / 100.0f, false);
35.        encoder.setJPEGEncodeParam(param);
36.        encoder.encode(thumbImage);
37.        out.close();
38.    }

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)
Java代码
1.import org.json.JSONObject;
2....
3....
4.JSONObject json = new JSONObject();
5.json.put("city", "Mumbai");
6.json.put("country", "India");
7....
8.String output = json.toString();
9....

10. 使用iText JAR生成PDF

阅读这篇文章了解更多细节
Java代码
1.import java.io.File;
2.import java.io.FileOutputStream;
3.import java.io.OutputStream;
4.import java.util.Date;
5.
6.import com.lowagie.text.Document;
7.import com.lowagie.text.Paragraph;
8.import com.lowagie.text.pdf.PdfWriter;
9.
10.public class GeneratePDF {
11.
12.    public static void main(String[] args) {
13.        try {
14.            OutputStream file = new FileOutputStream(new File("C:\\Test.pdf"));
15.
16.            Document document = new Document();
17.            PdfWriter.getInstance(document, file);
18.            document.open();
19.            document.add(new Paragraph("Hello Kiran"));
20.            document.add(new Paragraph(new Date().toString()));
21.
22.            document.close();
23.            file.close();
24.
25.        } catch (Exception e) {
26.
27.            e.printStackTrace();
28.        }
29.    }
30.}

11. HTTP 代理设置

阅读这篇文章了解更多细节。
Java代码
1.System.getProperties().put("http.proxyHost", "someProxyURL");
2.System.getProperties().put("http.proxyPort", "someProxyPort");
3.System.getProperties().put("http.proxyUser", "someUserName");
4.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
Java代码
1.public class SimpleSingleton {
2.    private static SimpleSingleton singleInstance =  new SimpleSingleton();
3.
4.    //Marking default constructor private
5.    //to avoid direct instantiation.
6.    private SimpleSingleton() {
7.    }
8.
9.    //Get instance for class SimpleSingleton
10.    public static SimpleSingleton getInstance() {
11.
12.        return singleInstance;
13.    }
14.}

另一种实现
Java代码
1.public enum SimpleSingleton {
2.    INSTANCE;
3.    public void doSomething() {
4.    }
5.}
6.
7.//Call the method from Singleton:
8.SimpleSingleton.INSTANCE.doSomething();

13. 抓屏程序

阅读这篇文章获得更多信息。
Java代码
1.import java.awt.Dimension;
2.import java.awt.Rectangle;
3.import java.awt.Robot;
4.import java.awt.Toolkit;
5.import java.awt.image.BufferedImage;
6.import javax.imageio.ImageIO;
7.import java.io.File;
8.
9....
10.
11.public void captureScreen(String fileName) throws Exception {
12.
13.   Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
14.   Rectangle screenRectangle = new Rectangle(screenSize);
15.   Robot robot = new Robot();
16.   BufferedImage image = robot.createScreenCapture(screenRectangle);
17.   ImageIO.write(image, "png", new File(fileName));
18.
19.}
20....

14. 列出文件和目录
Java代码
1.File dir = new File("directoryName");
2.  String[] children = dir.list();
3.  if (children == null) {
4.      // Either dir does not exist or is not a directory
5.  } else {
6.      for (int i=0; i < children.length; i++) {
7.          // Get filename of file or directory
8.          String filename = children[i];
9.      }
10.  }
11.
12.  // It is also possible to filter the list of returned files.
13.  // This example does not return any files that start with `.'.
14.  FilenameFilter filter = new FilenameFilter() {
15.      public boolean accept(File dir, String name) {
16.          return !name.startsWith(".");
17.      }
18.  };
19.  children = dir.list(filter);
20.
21.  // The list of files can also be retrieved as File objects
22.  File[] files = dir.listFiles();
23.
24.  // This filter only returns directories
25.  FileFilter fileFilter = new FileFilter() {
26.      public boolean accept(File file) {
27.          return file.isDirectory();
28.      }
29.  };
30.  files = dir.listFiles(fileFilter);

15. 创建ZIP和JAR文件
Java代码
1.import java.util.zip.*;
2.import java.io.*;
3.
4.public class ZipIt {
5.    public static void main(String args[]) throws IOException {
6.        if (args.length < 2) {
7.            System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");
8.            System.exit(-1);
9.        }
10.        File zipFile = new File(args[0]);
11.        if (zipFile.exists()) {
12.            System.err.println("Zip file already exists, please try another");
13.            System.exit(-2);
14.        }
15.        FileOutputStream fos = new FileOutputStream(zipFile);
16.        ZipOutputStream zos = new ZipOutputStream(fos);
17.        int bytesRead;
18.        byte[] buffer = new byte[1024];
19.        CRC32 crc = new CRC32();
20.        for (int i=1, n=args.length; i < n; i++) {
21.            String name = args[i];
22.            File file = new File(name);
23.            if (!file.exists()) {
24.                System.err.println("Skipping: " + name);
25.                continue;
26.            }
27.            BufferedInputStream bis = new BufferedInputStream(
28.                new FileInputStream(file));
29.            crc.reset();
30.            while ((bytesRead = bis.read(buffer)) != -1) {
31.                crc.update(buffer, 0, bytesRead);
32.            }
33.            bis.close();
34.            // Reset to beginning of input stream
35.            bis = new BufferedInputStream(
36.                new FileInputStream(file));
37.            ZipEntry entry = new ZipEntry(name);
38.            entry.setMethod(ZipEntry.STORED);
39.            entry.setCompressedSize(file.length());
40.            entry.setSize(file.length());
41.            entry.setCrc(crc.getValue());
42.            zos.putNextEntry(entry);
43.            while ((bytesRead = bis.read(buffer)) != -1) {
44.                zos.write(buffer, 0, bytesRead);
45.            }
46.            bis.close();
47.        }
48.        zos.close();
49.    }
50.}

16. 解析/读取XML 文件

XML文件
代码
1.<?xml version="1.0"?>
2.<students>
3.    <student>
4.        <name>John</name>
5.        <grade>B</grade>
6.        <age>12</age>
7.    </student>
8.    <student>
9.        <name>Mary</name>
10.        <grade>A</grade>
11.        <age>11</age>
12.    </student>
13.    <student>
14.        <name>Simon</name>
15.        <grade>A</grade>
16.        <age>18</age>
17.    </student>
18.</students>

Java代码
1.package net.viralpatel.java.xmlparser;
2.
3.import java.io.File;
4.import javax.xml.parsers.DocumentBuilder;
5.import javax.xml.parsers.DocumentBuilderFactory;
6.
7.import org.w3c.dom.Document;
8.import org.w3c.dom.Element;
9.import org.w3c.dom.Node;
10.import org.w3c.dom.NodeList;
11.
12.public class XMLParser {
13.
14.    public void getAllUserNames(String fileName) {
15.        try {
16.            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
17.            DocumentBuilder db = dbf.newDocumentBuilder();
18.            File file = new File(fileName);
19.            if (file.exists()) {
20.                Document doc = db.parse(file);
21.                Element docEle = doc.getDocumentElement();
22.
23.                // Print root element of the document
24.                System.out.println("Root element of the document: "
25.                        + docEle.getNodeName());
26.
27.                NodeList studentList = docEle.getElementsByTagName("student");
28.
29.                // Print total student elements in document
30.                System.out
31.                        .println("Total students: " + studentList.getLength());
32.
33.                if (studentList != null && studentList.getLength() > 0) {
34.                    for (int i = 0; i < studentList.getLength(); i++) {
35.
36.                        Node node = studentList.item(i);
37.
38.                        if (node.getNodeType() == Node.ELEMENT_NODE) {
39.
40.                            System.out
41.                                    .println("=====================");
42.
43.                            Element e = (Element) node;
44.                            NodeList nodeList = e.getElementsByTagName("name");
45.                            System.out.println("Name: "
46.                                    + nodeList.item(0).getChildNodes().item(0)
47.                                            .getNodeValue());
48.
49.                            nodeList = e.getElementsByTagName("grade");
50.                            System.out.println("Grade: "
51.                                    + nodeList.item(0).getChildNodes().item(0)
52.                                            .getNodeValue());
53.
54.                            nodeList = e.getElementsByTagName("age");
55.                            System.out.println("Age: "
56.                                    + nodeList.item(0).getChildNodes().item(0)
57.                                            .getNodeValue());
58.                        }
59.                    }
60.                } else {
61.                    System.exit(1);
62.                }
63.            }
64.        } catch (Exception e) {
65.            System.out.println(e);
66.        }
67.    }
68.    public static void main(String[] args) {
69.
70.        XMLParser parser = new XMLParser();
71.        parser.getAllUserNames("c:\\test.xml");
72.    }
73.}

17. 把 Array 转换成 Map
Java代码
1.import java.util.Map;
2.import org.apache.commons.lang.ArrayUtils;
3.
4.public class Main {
5.
6.  public static void main(String[] args) {
7.    String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" },
8.        { "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } };
9.
10.    Map countryCapitals = ArrayUtils.toMap(countries);
11.
12.    System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));
13.    System.out.println("Capital of France is " + countryCapitals.get("France"));
14.  }
15.}

18. 发送邮件
Java代码
1.import javax.mail.*;
2.import javax.mail.internet.*;
3.import java.util.*;
4.
5.public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException
6.{
7.    boolean debug = false;
8.
9.     //Set the host smtp address
10.     Properties props = new Properties();
11.     props.put("mail.smtp.host", "smtp.example.com");
12.
13.    // create some properties and get the default Session
14.    Session session = Session.getDefaultInstance(props, null);
15.    session.setDebug(debug);
16.
17.    // create a message
18.    Message msg = new MimeMessage(session);
19.
20.    // set the from and to address
21.    InternetAddress addressFrom = new InternetAddress(from);
22.    msg.setFrom(addressFrom);
23.
24.    InternetAddress[] addressTo = new InternetAddress[recipients.length];
25.    for (int i = 0; i < recipients.length; i++)
26.    {
27.        addressTo[i] = new InternetAddress(recipients[i]);
28.    }
29.    msg.setRecipients(Message.RecipientType.TO, addressTo);
30.
31.    // Optional : You can also set your custom headers in the Email if you Want
32.    msg.addHeader("MyHeaderName", "myHeaderValue");
33.
34.    // Setting the Subject and Content Type
35.    msg.setSubject(subject);
36.    msg.setContent(message, "text/plain");
37.    Transport.send(msg);
38.}

19. 发送代数据的HTTP 请求
Java代码
1.import java.io.BufferedReader;
2.import java.io.InputStreamReader;
3.import java.net.URL;
4.
5.public class Main {
6.    public static void main(String[] args)  {
7.        try {
8.            URL my_url = new URL("http://coolshell.cn/");
9.            BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));
10.            String strTemp = "";
11.            while(null != (strTemp = br.readLine())){
12.            System.out.println(strTemp);
13.        }
14.        } catch (Exception ex) {
15.            ex.printStackTrace();
16.        }
17.    }
18.}

20. 改变数组的大小
Java代码
1./**
2.* Reallocates an array with a new size, and copies the contents
3.* of the old array to the new array.
4.* @param oldArray  the old array, to be reallocated.
5.* @param newSize   the new array size.
6.* @return          A new array with the same contents.
7.*/
8.private static Object resizeArray (Object oldArray, int newSize) {
9.   int oldSize = java.lang.reflect.Array.getLength(oldArray);
10.   Class elementType = oldArray.getClass().getComponentType();
11.   Object newArray = java.lang.reflect.Array.newInstance(
12.         elementType,newSize);
13.   int preserveLength = Math.min(oldSize,newSize);
14.   if (preserveLength > 0)
15.      System.arraycopy (oldArray,0,newArray,0,preserveLength);
16.   return newArray;
17.}
18.
19.// Test routine for resizeArray().
20.public static void main (String[] args) {
21.   int[] a = {1,2,3};
22.   a = (int[])resizeArray(a,5);
23.   a[3] = 4;
24.   a[4] = 5;
25.   for (int i=0; i<a.length; i++)
26.      System.out.println (a[i]);
27.}

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