20个开发人员非常有用的Java功能代码

String a = String.valueOf(2); 
//integer to numeric string   
int i = Integer.parseInt(a); //numeric 
string to an int 
String a = String.valueOf(2);   
//integer to numeric string
int i = Integer.parseInt(a); //numeric string to an
int

    2. 向Java文件中添加文本

Updated: Thanks Simone for pointing to exception. I have 

changed the code.  
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();   


}   
}  

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. 获取Java现在正调用的方法名

  
String methodName =
Thread.currentThread().getStackTrace()[1].getMethodName();  
String methodName = Thread.currentThread().getStackTrace()[1].getMethodName
();

    4. 在Java中将String型转换成Date型

java.util.Date = 
java.text.DateFormat.getDateInstance().parse(date String);  
java.util.Date = java.text.DateFormat.getDateInstance().parse(date 
String);or  
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" 
);   
Date date = format.parse( myString );  
SimpleDateFormat format = new SimpleDateFormat( "dd.MM.yyyy" );
Date date
= format.parse( myString );

    5. 通过Java JDBC链接Oracle数据库

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

();   
}   
}  

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.将Java中的util.Date转换成sql.Date

    这一片段显示如何将一个java util Date转换成sql Date用于数据库

java.util.Date utilDate = new 
java.util.Date();   
java.sql.Date sqlDate = new java.sql.Date
(utilDate.getTime());  
java.util.Date utilDate = new java.util.Date();
java.sql.Date sqlDate = 
new java.sql.Date(utilDate.getTime());

    7. 使用NIO快速复制Java文件

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 )   


{   


&

nbsp; position += inChannel.transferTo( position, maxCount, outChannel );  


}

   
}   


finally  


{   


if ( 

inChannel != null )   


{   


&

nbsp; inChannel.close();   


}

   
if ( outChannel != null )   


{   


&

nbsp;  outChannel.close();   


}

   
}   


}  

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. 在Java中创建缩略图

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

();   
}  

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.drawImag

e(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. 在Java中创建JSON数据

Read this article for more details.
Download JAR file 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

();   
...  

import org.json.JSONObject;
...
...
JSONObject json = new 

JSONObject();
json.put("city", "Mumbai");
json.put("country", 

"India");
...
String output = json.toString();
...

    10. 在Java中使用iText JAR打开PDF

Read this article for more details.


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();   
}

   
}   
}  

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();
}


}
}