[Java]20非常有用的Java程序片段

来源：互联网发布：java比较日期是否相等编辑：程序博客网时间：2024/06/10 15:55

1. 字符串有整型的相互转换

2 String a = String.valueOf(2); //integer to numeric string

3 int i = Integer.parseInt(a); //numeric string to an int

2. 向文件末尾添加内容

02 BufferedWriter out = null;

03 try {

04 out = new BufferedWriter(new FileWriter(”filename”, true));

05 out.write(”aString”);

06 } catch (IOException e) {

07 // error processing code

08 } finally {

09 if (out != null) {

10 out.close();

11 }

12 }

3. 得到当前方法的名字

1 String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

4. 转字符串到日期

2 java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);

或者是：

2 SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );

3 Date date = format.parse( myString );

5. 使用JDBC链接Oracle

01 public class OracleJdbcTest

02 {

03 String driverClass = "oracle.jdbc.driver.OracleDriver";

05 Connection con;

07 public void init(FileInputStream fs) throws ClassNotFoundException, SQLException, FileNotFoundException, IOException

08 {

09 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);

16 con=DriverManager.getConnection(url, userName, password);

17 }

19 public void fetch() throws SQLException, IOException

20 {

21 PreparedStatement ps = con.prepareStatement("select SYSDATE from dual");

22 ResultSet rs = ps.executeQuery();

24 while (rs.next())

25 {

26 // do the thing you do

27 }

28 rs.close();

29 ps.close();

30 }

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

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

01 public static void fileCopy( File in, File out )

02 throws IOException

03 {

04 FileChannel inChannel = new FileInputStream( in ).getChannel();

05 FileChannel outChannel = new FileOutputStream( out ).getChannel();

06 try

07 {

08 // inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows

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

01 private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)

02 throws InterruptedException, FileNotFoundException, IOException

03 {

04 // load image from filename

05 Image image = Toolkit.getDefaultToolkit().getImage(filename);

06 MediaTracker mediaTracker = new MediaTracker(new Container());

07 mediaTracker.addImage(image, 0);

08 mediaTracker.waitForID(0);

09 // use this to test for errors at this point: System.out.println(mediaTracker.isErrorAny());

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 }

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);

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)

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

02 import java.io.File;

03 import java.io.FileOutputStream;

04 import java.io.OutputStream;

05 import java.util.Date;

07 import com.lowagie.text.Document;

08 import com.lowagie.text.Paragraph;

09 import com.lowagie.text.pdf.PdfWriter;

11 public class GeneratePDF {

13 public static void main(String[] args) {

14 try {

15 OutputStream file = new FileOutputStream(new File("C://Test.pdf"));

17 Document document = new Document();

18 PdfWriter.getInstance(document, file);

19 document.open();

20 document.add(new Paragraph("Hello Kiran"));

21 document.add(new Paragraph(new Date().toString()));

23 document.close();

24 file.close();

26 } catch (Exception e) {

28 e.printStackTrace();

29 }

30 }

31 }

11. HTTP 代理设置

阅读这篇文章了解更多细节。

2 System.getProperties().put("http.proxyHost", "someProxyURL");

3 System.getProperties().put("http.proxyPort", "someProxyPort");

4 System.getProperties().put("http.proxyUser", "someUserName");

5 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

02 public class SimpleSingleton {

03 private static SimpleSingleton singleInstance = new SimpleSingleton();

05 //Marking default constructor private

06 //to avoid direct instantiation.

07 private SimpleSingleton() {

08 }

10 //Get instance for class SimpleSingleton

11 public static SimpleSingleton getInstance() {

13 return singleInstance;

14 }

15 }

另一种实现

1 public enum SimpleSingleton {

2 INSTANCE;

3 public void doSomething() {

4 }

5 }

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。

01 import java.awt.Dimension;

02 import java.awt.Rectangle;

03 import java.awt.Robot;

04 import java.awt.Toolkit;

05 import java.awt.image.BufferedImage;

06 import javax.imageio.ImageIO;

07 import java.io.File;

09 ...

11 public void captureScreen(String fileName) throws Exception {

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));

19 }

20 ...

14. 列出文件和目录

01 File dir = new File("directoryName");

02 String[] children = dir.list();

03 if (children == null) {

04 // Either dir does not exist or is not a directory

05 } else {

06 for (int i=0; i < children.length; i++) {

07 // Get filename of file or directory

08 String filename = children[i];

09 }

10 }

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);

21 // The list of files can also be retrieved as File objects

22 File[] files = dir.listFiles();

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

02 import java.util.zip.*;

03 import java.io.*;

05 public class ZipIt {

06 public static void main(String args[]) throws IOException {

07 if (args.length < 2) {

08 System.err.println("usage: java ZipIt Zip.zip file1 file2 file3");

09 System.exit(-1);

10 }

11 File zipFile = new File(args[0]);

12 if (zipFile.exists()) {

13 System.err.println("Zip file already exists, please try another");

14 System.exit(-2);

15 }

16 FileOutputStream fos = new FileOutputStream(zipFile);

17 ZipOutputStream zos = new ZipOutputStream(fos);

18 int bytesRead;

19 byte[] buffer = new byte[1024];

20 CRC32 crc = new CRC32();

21 for (int i=1, n=args.length; i < n; i++) {

22 String name = args[i];

23 File file = new File(name);

24 if (!file.exists()) {

25 System.err.println("Skipping: " + name);

26 continue;

27 }

28 BufferedInputStream bis = new BufferedInputStream(

29 new FileInputStream(file));

30 crc.reset();

31 while ((bytesRead = bis.read(buffer)) != -1) {

32 crc.update(buffer, 0, bytesRead);

33 }

34 bis.close();

35 // Reset to beginning of input stream

36 bis = new BufferedInputStream(

37 new FileInputStream(file));

38 ZipEntry entry = new ZipEntry(name);

39 entry.setMethod(ZipEntry.STORED);

40 entry.setCompressedSize(file.length());

41 entry.setSize(file.length());

42 entry.setCrc(crc.getValue());

43 zos.putNextEntry(entry);

44 while ((bytesRead = bis.read(buffer)) != -1) {

45 zos.write(buffer, 0, bytesRead);

46 }

47 bis.close();

48 }

49 zos.close();

50 }

51 }

16. 解析/读取XML 文件

XML文件

01 <?xml version="1.0"?>

02 <students>

03 <student>

04 <name>John</name>

05 <grade>B</grade>

06 <age>12</age>

07 </student>

08 <student>

09 <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

02 package net.viralpatel.java.xmlparser;

04 import java.io.File;

05 import javax.xml.parsers.DocumentBuilder;

06 import javax.xml.parsers.DocumentBuilderFactory;

08 import org.w3c.dom.Document;

09 import org.w3c.dom.Element;

10 import org.w3c.dom.Node;

11 import org.w3c.dom.NodeList;

13 public class XMLParser {

15 public void getAllUserNames(String fileName) {

16 try {

17 DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();

18 DocumentBuilder db = dbf.newDocumentBuilder();

19 File file = new File(fileName);

20 if (file.exists()) {

21 Document doc = db.parse(file);

22 Element docEle = doc.getDocumentElement();

24 // Print root element of the document

25 System.out.println("Root element of the document: "

26 + docEle.getNodeName());

28 NodeList studentList = docEle.getElementsByTagName("student");

30 // Print total student elements in document

31 System.out

32 .println("Total students: " + studentList.getLength());

34 if (studentList != null && studentList.getLength() > 0) {

35 for (int i = 0; i < studentList.getLength(); i++) {

37 Node node = studentList.item(i);

39 if (node.getNodeType() == Node.ELEMENT_NODE) {

41 System.out

42 .println("=====================");

44 Element e = (Element) node;

45 NodeList nodeList = e.getElementsByTagName("name");

46 System.out.println("Name: "

47 + nodeList.item(0).getChildNodes().item(0)

48 .getNodeValue());

50 nodeList = e.getElementsByTagName("grade");

51 System.out.println("Grade: "

52 + nodeList.item(0).getChildNodes().item(0)

53 .getNodeValue());

55 nodeList = e.getElementsByTagName("age");

56 System.out.println("Age: "

57 + nodeList.item(0).getChildNodes().item(0)

58 .getNodeValue());

59 }

60 }

61 } else {

62 System.exit(1);

63 }

64 }

65 } catch (Exception e) {

66 System.out.println(e);

67 }

68 }

69 public static void main(String[] args) {

71 XMLParser parser = new XMLParser();

72 parser.getAllUserNames("c://test.xml");

73 }

74 }

17. 把 Array 转换成 Map

02 import java.util.Map;

03 import org.apache.commons.lang.ArrayUtils;

05 public class Main {

07 public static void main(String[] args) {

08 String[][] countries = { { "United States", "New York" }, { "United Kingdom", "London" },

09 { "Netherland", "Amsterdam" }, { "Japan", "Tokyo" }, { "France", "Paris" } };

11 Map countryCapitals = ArrayUtils.toMap(countries);

13 System.out.println("Capital of Japan is " + countryCapitals.get("Japan"));

14 System.out.println("Capital of France is " + countryCapitals.get("France"));

15 }

16 }

18. 发送邮件

01 import javax.mail.*;

02 import javax.mail.internet.*;

03 import java.util.*;

05 public void postMail( String recipients[ ], String subject, String message , String from) throws MessagingException

06 {

07 boolean debug = false;

09 //Set the host smtp address

10 Properties props = new Properties();

11 props.put("mail.smtp.host", "smtp.example.com");

13 // create some properties and get the default Session

14 Session session = Session.getDefaultInstance(props, null);

15 session.setDebug(debug);

17 // create a message

18 Message msg = new MimeMessage(session);

20 // set the from and to address

21 InternetAddress addressFrom = new InternetAddress(from);

22 msg.setFrom(addressFrom);

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);

31 // Optional : You can also set your custom headers in the Email if you Want

32 msg.addHeader("MyHeaderName", "myHeaderValue");

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

02 import java.io.BufferedReader;

03 import java.io.InputStreamReader;

04 import java.net.URL;

06 public class Main {

07 public static void main(String[] args) {

08 try {

09 URL my_url = new URL("http://coolshell.cn/");

10 BufferedReader br = new BufferedReader(new InputStreamReader(my_url.openStream()));

11 String strTemp = "";

12 while(null != (strTemp = br.readLine())){

13 System.out.println(strTemp);

14 }

15 } catch (Exception ex) {

16 ex.printStackTrace();

17 }

18 }

19 }

20. 改变数组的大小

02 /**

03 * Reallocates an array with a new size, and copies the contents

04 * of the old array to the new array.

05 * @param oldArray the old array, to be reallocated.

06 * @param newSize the new array size.

07 * @return A new array with the same contents.

08 */

09 private static Object resizeArray (Object oldArray, int newSize) {

10 int oldSize = java.lang.reflect.Array.getLength(oldArray);

11 Class elementType = oldArray.getClass().getComponentType();

12 Object newArray = java.lang.reflect.Array.newInstance(

13 elementType,newSize);

14 int preserveLength = Math.min(oldSize,newSize);

15 if (preserveLength > 0)

16 System.arraycopy (oldArray,0,newArray,0,preserveLength);

17 return newArray;

18 }

20 // Test routine for resizeArray().

21 public static void main (String[] args) {

22 int[] a = {1,2,3};

23 a = (int[])resizeArray(a,5);

24 a[3] = 4;

25 a[4] = 5;

26 for (int i=0; i<a.length; i++)

27 System.out.println (a[i]);

28 }

(全文完)