What are the commonly used Java code snippets?

常用的Java代码段有：1、字符串有整型的相互转换；2、向文件末尾添加内容；3、得到当前方法的名字；4、转字符串到日期；5、使用JDBC链接Oracle。

本教程操作环境：windows7系统、java10版，DELL G3电脑。

常用的Java代码段有：

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

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());

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();  
            }  
        }  
    }

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

请先阅读这篇文章 了解一些细节，

并下面这个JAR 文件：json-rpc-1.0.jar (75 kb)

import org.json.JSONObject;  
...  
...  
JSONObject json = new JSONObject();  
json.put("city", "Mumbai");  
json.put("country", "India");  
...  
String output = json.toString();  
...

10. 使用iText JAR生成PDF

阅读这篇文章 了解更多细节

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();  
        }  
    }  
}

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