非常有用的Java程序片段

下面是20個非常有用的Java程序片段，希望能對你有用。

1. 字符串有整型的相互轉換

1

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

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

2. 向文件末尾添加內容

01

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. 得到當前方法的名字

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

4. 轉字符串到日期

1

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

或者是：

1

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

3	Date date = format.parse( myString );

5. 使用JDBC鏈接Oracle

01	public class OracleJdbcTest

02

{

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

04

05	Connection con;

06

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);

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 轉成 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進行快速的文件拷貝

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

09

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. 創建圖片的縮略圖

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());

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. 創建 JSON 格式的數據

請先閱讀這篇文章 瞭解一些細節，
並下面這個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

閱讀這篇文章 瞭解更多細節

01

02	import java.io.File;

03	import java.io.FileOutputStream;

04	import java.io.OutputStream;

05	import java.util.Date;

06

07	import com.lowagie.text.Document;

08	import com.lowagie.text.Paragraph;

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

10

11	public class GeneratePDF  {

12

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

14

try {

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

16

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()));

22

23	document.close();

24	file.close();

25

26	} catch (Exception e) {

27

28	e.printStackTrace();

29

}

30

}

31

}

11. HTTP 代理設置

閱讀這篇 文章 瞭解更多細節。

1

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. 單實例Singleton 示例

請先閱讀這篇文章 瞭解更多信息

01

02	public class SimpleSingleton  {

03	private static SimpleSingleton  singleInstance =  new SimpleSingleton();

04

05	//Marking default constructor private

06	//to avoid direct instantiation.

07	private SimpleSingleton() {

08

}

09

10	//Get instance for class SimpleSingleton

11	public static SimpleSingleton getInstance() {

12

13	return singleInstance;

14

}

15

}

另一種實現

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

閱讀這篇文章 獲得更多信息。

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;

08

09

...

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. 列出文件和目錄

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

}

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. 創建ZIP和JAR文件

01

02	import java.util.zip.*;

03	import java.io.*;

04

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/讀取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代碼

01

02	package net.viralpatel.java.xmlparser;

03

04	import java.io.File;

05	import javax.xml.parsers.DocumentBuilder;

06	import javax.xml.parsers.DocumentBuilderFactory;

07

08	import org.w3c.dom.Document;

09	import org.w3c.dom.Element;

10	import org.w3c.dom.Node;

11	import org.w3c.dom.NodeList;

12

13	public class XMLParser {

14

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();

23

24	// Print root element of the document

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

26	+ docEle.getNodeName());

27

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

29

30	// Print total student elements in document

31	System.out

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

33

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

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

36

37	Node node = studentList.item(i);

38

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

40

41	System.out

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

43

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());

49

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

51	System.out.println("Grade:  "

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

53	.getNodeValue());

54

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) {

70

71	XMLParser parser = new XMLParser();

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

73

}

74

}

17. 把 Array 轉換成 Map

01

02	import java.util.Map;

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

04

05	public class Main {

06

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

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

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

10

11	Map countryCapitals = ArrayUtils.toMap(countries);

12

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. 發送郵件

01	import javax.mail.*;

02	import javax.mail.internet.*;

03	import java.util.*;

04

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

06

{

07	boolean debug = false;

08

09	//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. 發送代數據的HTTP 請求

01

02	import java.io.BufferedReader;

03	import java.io.InputStreamReader;

04	import java.net.URL;

05

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. 改變數組的大小

01

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

}

19

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

}