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(”filename”, true));
out.write(”aString”);
} catch (IOException e) {
// error processing code
} finally {
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(date String);
或者是：
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date = format.parse( myString );
5. 使用JDBC链接Oracle
publicclass OracleJdbcTest
{
String driverClass = "oracle.jdbc.driver.OracleDriver";
Connection con;
publicvoid 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);
}
publicvoid 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();
}
publicstaticvoid 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
publicstaticvoid 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. 创建图片的缩略图
privatevoid createThumbnail(String filename, int thumbWidth, int thumbHeight, int quality, String outFilename)
throws InterruptedException, FileNotFoundException, IOException
{
// load p_w_picpath from filename
Image p_w_picpath = Toolkit.getDefaultToolkit().getImage(filename);
MediaTracker mediaTracker = new MediaTracker(new Container());
mediaTracker.addImage(p_w_picpath, 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 p_w_picpathWidth = p_w_picpath.getWidth(null);
int p_w_picpathHeight = p_w_picpath.getHeight(null);
double p_w_picpathRatio = (double)p_w_picpathWidth / (double)p_w_picpathHeight;
if (thumbRatio < p_w_picpathRatio) {
thumbHeight = (int)(thumbWidth / p_w_picpathRatio);
} else {
thumbWidth = (int)(thumbHeight * p_w_picpathRatio);
}
// draw original p_w_picpath to thumbnail p_w_picpath 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(p_w_picpath, 0, 0, thumbWidth, thumbHeight, null);
// save thumbnail p_w_picpath 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();
}
13. 抓屏程序
阅读这篇 获得更多信息。
import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.p_w_picpath.BufferedImage;
import javax.p_w_picpathio.ImageIO;
import java.io.File;
...
publicvoid captureScreen(String fileName) throws Exception {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Rectangle screenRectangle = new Rectangle(screenSize);
Robot robot = new Robot();
BufferedImage p_w_picpath = robot.createScreenCapture(screenRectangle);
ImageIO.write(p_w_picpath, "png", new File(fileName));
}
...
14. 列出文件和目录
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() {
publicboolean 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() {
publicboolean 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.*;
publicclass ZipIt {
publicstaticvoid 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 = newbyte[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();
}
}
20. 改变数组的大小
/**
* 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.
*/
privatestatic 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().
publicstaticvoid 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]);
}
原文链接：