抄自这里。
1. 字符串有整型的相互转换
String a = String.valueOf(2);
int i = Integer.parseInt(a);
2. 向文件末尾添加内容
BufferedWriter out = null;
try {
out = new BufferedWriter(new FileWriter("filename", true));
out.write("aString");
} catch(IOException e) {
if(out != null) {
out.close();
}
}
3. 得到当前方法的名字
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
4. 转字符串到日期
java.util.Date = java.text.DateFormat.getDateInstance().parse(data String);
// 或者是：
SimpleDateFormat format = new SimpleDateFormat("dd.MM.yyyy");
Date date = format.parse(myString);
5. 使用JDBC连接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 what you want
}
rs.close();
ps.close();
}
public static void main(String[] args) {
OracleJdbcTest test = new OracleJdbcTest();
test.init();
test.fetch();
}
}
6. 把Java util Date转成sql.Date
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
7. 使用NIO进行快速的文件拷贝
public static void fastCopy(File in, File out) throws IOException{
FileChannel inChannel = new FileInputStream(in).getChannel();
FileChannel outChannel = new FileOutputStream(out).getChannel();
try {
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. 创建图片的缩略图
private void createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename) throws InterruptedException, FileNotFoundException, IOException {
Image iamge = Toolkit.getDefaultToolkit().getImage(filename);
MediaTracker mediaTracker = new MediaTracker(new Container());
mediaTracker.addImage(image, 0);
mediaTracker.waitForID(0);
double thumbRatio = (double)thumbWidth / (double)thumbleHeight;
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);
}
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);
BufferedOutputStream out = new BufferedOuptStream(new FileOutputStream(outFilename));
JPEGImageEncoder encoder = JPEGCoderc.createJPEGEncoderParam(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. 创建JSON格式的数据
请先阅读这篇文章了解一些细节，并下面这个JAR文件：json-rpc-1.0jar(75kb)
import org.json.JSONObject;
...
...
JSONObject json = new JSONObject();
json.put("city", "Mubai");
json.put("country", "India");
...
String output = json.toString();
...
10. 使用iText JAR生成PDF
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.io.util.Date;
import com.lowagie.text.Document;
import com.lowagie.text.Paragraph;
import com.lowagie.text.pdf.PdfWriter;
pubic 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.printStackTrance();
}
}
}
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. 单实例Singleton实例
public class SimpleSingleton {
private static SimpleSingleton singleInstace = new SimpleSingleton();
private SimpleSingleton() {
}
public static SimpleSingleton getInstance() {
return singleInstance;
}
}
另一种实现
public enum SingleSingleton {
INSTANCE;
public void doSomething() {
}
}
13. 抓屏程序
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. 列出文件和目录
File dir = new File("directoryName");
String[] children = dir.list();
if(children == null) {
} else {
for(int i = 0; i < children.length; i++) {
String filename = children[i];
}
}
FilenameFilter filter = new FilenameFilter() {
public boolean accept(File dir, String name) {
return !name.startsWith(".");
}
};
children = dir.list(filter);
File[] files = dir.listFiles();
FIleFilter fileFilter = new FileFilter() {
public boolean accept(File file) {
return file.isDirectory();
}
};
files = dir.listFiles(fileFilter);
15. 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.exits()) {
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 = 0, n = args.length; i < n; i++) {
String name = args[i];
File file = new File(name);
if(!file.exists()) {
System.err.println("Skipping: " + name);
}
BufferedInputStream bis = new BufferedInputStream(new FileInputStream(file));
crc.reset();
while((bytesRead = bis.read(buffer)) != -1) {
crc.update(buffer, 0, bytesRead);
}
bis.close();
bis = new BufferedInputStream(new FileInputStream(file));
ZipEntry entry = new ZipEntry(name);
entry.setMethod(ZipEntry.STORED);
entry.setCompressedSize(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/读取XML文件
XML文件
<?xml version="1.0"?>
<students>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
</students>
Java代码
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();
System.out.println("Root element of the document:" + docEle.getNodeName());
NodeList studentList = docEle.getElementsByTagName("student");
Sysout.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.getElementsNyTagName("name");
System.out.println("Name: " + nodeList.item(0).getChildNodes().item(0).getNodeValue());
NodeList nodeList = e.getElementsNyTagName("grade");
System.out.println("Grade: " + nodeList.item(0).getChildNodes().item(0).getNodeValue());
NodeList nodeList = e.getElementsNyTagName("age");
System.out.println("Age: " + nodeList.item(0).getChildNodes().item(0).getNodeValue());
}
} else {
System.exit(1);
}
}
}
} catch (Exception e) {
Sysout.out.println(e);
}
}
public static void main(String[] args) {
XMLParser parser = new XMLParser();
parser.getAllUserNames("c:\\test.xml");
}
}
17. 把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. 发送邮件
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;
Properties props = new Properties();
props.put("mail.smtp.host", "smtp.example.com");
Session session = Session.getDefaultInstance(props, null);
session.setDebug(debug);
Message msg = new MimeMessage(session);
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);
msg.addHeader("MyHeaderName", "myHeaderName");
msg.setSubject(subject);
msg.setContent(message, "text/plain");
Transport.send(msg);
}
19. 发送带数据的HTTP请求
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
public class Main {
public staitc 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. 改变数组大小
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;
}
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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