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]);
    }
}