1. 字符串有整型的相互转换
String a = String.valueOf(2); //integer to numeric string
int i = Integer.parseInt(a); //numeric string to an int
2. 向文件末尾添加内容
BufferedWriter out = null;
try {
out = new BufferedWriter(new FileWriter("D:\\asxxsst20170817.log", true));// \需要用\转义
out.write("aString"); //在这编写增加的内容
} catch (IOException e) {
// error processing code
} finally {
if (out != null) {
try {
out.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
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
Date date1 = java.text.DateFormat.getDateInstance().parse("2017-08-24");
或者是：
String myString0 = "24.08.2017";
String myString = "2017-08-24 09:47:50";
SimpleDateFormat format0 = new SimpleDateFormat( "dd.MM.yyyy" );
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
try {
Date date0 = format0.parse(myString0);
Date date = format.parse(myString);
System.out.println(date0);
System.out.println(date);
} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
5. 使用JDBC链接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 转成 sql.Date
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());
System.out.println("今天是："+utilDate+"(java.util.Date)");
System.out.println("今天是："+sqlDate+"(java.sql.Date)");
7. 使用NIO进行快速的文件拷贝
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();
}
}
}
//在主函数中调用
File in = new File("D:\\asxxsst20170817.log"); File out = new File("D:\\456.txt"); fileCopy(in, out);
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{
// 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.创建 JSON 格式的数据
并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)/http://t.cn/Rz0bhUA
10. 使用iText JAR生成PDF
try{ OutputStream file = new FileOutputStream(new File("C:\\Test.pdf")); 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
12. 单实例Singleton 示例
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;
}
}
13. 抓屏程序
public class PrintScreen{
public static void main(String[] args) throws Exception {
captureScreen("D:\\抓屏.png");
}
public static 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
public class Lists{
public static void main(String[] args) {
File dir = new File("D:\\zx");
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];
System.out.println(filename);
}
}
// 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. 创建ZIP和JAR文件
public class ZipAndJar {
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/读取XML 文件
XML文件
Java代码
package ParseXML;
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("1.xml");
// parser.getAllUserNames("D:\\1.xml");
}
}
17. 把 Array 转换成 Map
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
public class sendMail{
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 = newInternetAddress(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. 发送代数据的HTTP 请求
public class Main {
public static void main(String[] args) {
try{
// URL my_url = new URL("http://coolshell.cn/");
URL my_url = new URL("http://baidu.com/");
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. 改变数组的大小
public class ChangeArray {
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};
for(int i=0; i<a.length; i++)
System.out.println(a[i]);
a = (int[])resizeArray(a,5);
System.out.println("----------");
a[3] = 4;
a[4] = 5;
for(int i=0; i<a.length; i++)
System.out.println (a[i]);
}
}