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
String a = String.valueOf(2); //integer to numeric string int i = Integer.parseInt(a); //numeric string to an int2. 向文件末尾添加內容
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. 得到當前方法的名字
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();4. 轉字符串到日期
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);或者是：
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); Date date = format.parse( myString );5. 使用JDBC鏈接Oracle
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. 把 Java util.Date 轉成 sql.Date
java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());7. 使用NIO進行快速的文件拷貝
public static void fileCopy( File in, File out )
throws IOException
{
FileChannel inChannel = new FileInputStream( in ).getChannel();
FileChannel outChannel = new FileOutputStream( out ).getChannel(); try { // inChannel.transferTo(0, inChannel.size(), outChannel); // original -- apparently has trouble copying large files on Windows // magic number for Windows, 64Mb - 32Kb) 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. 創建圖片的縮略圖
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. 創建 JSON 格式的數據
請先閱讀這篇文章 了解一些細節，
并下面這個JAR 文件：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. 使用iText JAR生成PDF
閱讀這篇文章 了解更多細節
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; /**
* Java學習交流QQ群：589809992 我們一起學Java！
*/ 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. HTTP 代理設置
閱讀這篇 文章 了解更多細節。
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. 單實例Singleton 示例
請先閱讀這篇文章 了解更多信息
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;
}
}另一種實現
public enum SimpleSingleton {
INSTANCE; public void doSomething() {
}
} //Call the method from Singleton: SimpleSingleton.INSTANCE.doSomething();13. 抓屏程序
閱讀這篇文章 獲得更多信息。
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. 列出文件和目錄
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. 創建ZIP和JAR文件
import java.util.zip.*; import java.io.*; /**
* Java學習交流QQ群：589809992 我們一起學Java！
*/ 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. 解析/讀取XML 文件
XML文件
<?xml version="1.0"?> <students> <student> <name>John</name> <grade>B</grade> <age>12</age> </student> <student> <name>Mary</name> <grade>A</grade> <age>11</age> </student> <student> <name>Simon</name> <grade>A</grade> <age>18</age> </student> </students>Java代碼
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) {
System.out.println(e); }
}
public static void main(String[] args) {
XMLParser parser = new XMLParser(); parser.getAllUserNames("c:\\test.xml"); }
}17. 把 Array 轉換成 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. 發送郵件
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. 發送代數據的HTTP 請求
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://coolshell.cn/");
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. 改變數組的大小
/**
* 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<a.length; i++)
System.out.println (a[i]);
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