非常有用的Java程序片段_JAVA_编程开发_程序员俱乐部

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

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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

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. }??

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

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

或者是：

1. SimpleDateFormat?format?=?new?SimpleDateFormat(?"dd.MM.yyyy"?);???
2. Date?date?=?format.parse(?myString?);??

5. 使用JDBC链接Oracle

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

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

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

?

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ZIP和JAR文件

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. }??

10. 解析/读取XML 文件

XML文件

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. ackage?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. }??

11.?把 Array?转换成 Map?

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. }??

12. 发送邮件

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. }??

13. 发送代数据的HTTP 请求

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. }?

14. 改变数组的大小

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. }?