20个开发人员非常有用的Java功能代码

本文将为大家介绍20个对开发人员非常有用的Java功能代码。这20段代码，可以成为大家在今后的开发过程中，Java编程手册的重要部分。

 

1. 把Strings转换成int和把int转换成String

String a = String.valueOf(2);
//integer to numeric string 
int i = Integer.parseInt(a);
//numeric string to an int
String a = String.valueOf(2);
//integer to numeric string
int i = Integer.parseInt(a);
//numeric string to an int

2. 向Java文件中添加文本

Updated: Thanks Simone for pointing to exception. I have changed the code.  
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();
	}
}

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. 获取Java现在正调用的方法名

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

4. 在Java中将String型转换成Date型

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

    OR

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

5. 通过Java JDBC链接Oracle数据库

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();
	}
}


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.将Java中的util.Date转换成sql.Date

这一片段显示如何将一个java util Date转换成sql Date用于数据库

java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

7. 使用NIO快速复制Java文件

public static void fileCopy( File in, File out ) throws IOException {
	FileChannel inChannel = new FileInputStream( in ).getChannel();
	FileChannel outChannel = new FileOutputStream( out ).getChannel();

	try {
		// original -- apparently has trouble copying large files on Windows
		// magic number for Windows, 64Mb - 32Kb)
		inChannel.transferTo(0, inChannel.size(), outChannel);
		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. 在Java中创建缩略图

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. 在Java中创建JSON数据

Read this article for more details. Download JAR file 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. 在Java中使用iText JAR打开PDF

Read this article for more details.

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. 在Java上的HTTP代理设置

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. Java Singleton 例子

Read this article for more details.
Update: Thanks Markus for the comment. I have updated the code and changed it to more robust implementation.

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;
	}
}

One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.

public enum SimpleSingleton {
	INSTANCE;
	public void doSomething() {
	}
}

//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

public enum SimpleSingleton {
	INSTANCE;
	public void doSomething() {
	}
}

13. 在Java上做屏幕截图

Read this article for more details.

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. 在Java中的文件，目录列表

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. 在Java中创建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. 在Java中解析/读取XML文件

John
B
12


Mary
A
11


Simon
A
18

    Java code to parse above XML.

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) {
		ystem.out.println(e);
	}
}
	public static void main(String[] args) {
		XMLParser parser = new XMLParser();
		parser.getAllUserNames("c:\\test.xml");
	}
}

17. 在Java中将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. 在Java中发送电子邮件

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. 使用Java发送HTTP请求和提取数据

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://www.viralpatel.net/blogs/");
			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. 在Java中调整数组

/**
 *  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