20个有用的java代码分别是什么，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。
1. Strings转int和int转String
String a = String.valueOf(2); //integer to numeric stringint i = Integer.parseInt(a); //numeric string to an int
2.在文件内追加文字
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.获取当前的方法名
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
4.String转换为Date
java.util.Date = java.text.DateFormat.getDateInstance().parse(date String);//orSimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" ); Date date = format.parse( myString );
5.JDBC连接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();
}
}6.把util.Date转换为sql.Date
java.util.Date utilDate = new java.util.Date(); java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
7.Java Fast File Copy using NIO
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.创建一个图片的缩略图
private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)throws InterruptedException, FileNotFoundException, IOException
{// load image from filenameImage 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 HEIGHTdouble 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-flyBufferedImage 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 outFilenameBufferedOutputStream 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.生成JSON数据
// 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.生成PDF
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.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.单例
public class SimpleSingleton {private static SimpleSingleton singleInstance = new SimpleSingleton(); //Marking default constructor private//to avoid direct instantiation.private SimpleSingleton() {
} //Get instance for class SimpleSingletonpublic 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();13.截屏
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.文件监控
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.生成.jar,.zip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 streambis = 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.操作XML
<?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>
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 documentSystem.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.Array转化为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.发邮件
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 SessionSession session = Session.getDefaultInstance(props, null);
session.setDebug(debug); // create a messageMessage msg = new MimeMessage(session); // set the from and to addressInternetAddress 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 Wantmsg.addHeader("MyHeaderName", "myHeaderValue"); // Setting the Subject and Content Type msg.setSubject(subject);
msg.setContent(message, "text/plain");
Transport.send(msg);
}19.远程抓取数据
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.重新定义数组大小
/*** 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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