20个开发人员非常有用的Java功能代码

本文将为大家介绍20个对开发人员非常有用的Java功能代码。这20段代码，可以成为大家在今后的开发过程中，Java编程手册的重要部分。

 

1. 把Strings转换成int和把int转换成String

<pre class="java" name="code">
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</pre></pre>

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);

    OR

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());

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 {
        // original -- apparently has trouble copying large files on Windows
        // magic number for Windows, 64Mb - 32Kb)
        inChannel.transferTo(0, inChannel.size(), outChannel);
        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();
}

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();
...

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();
        }
    }
}

11. 在Java上的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");

Read this article for more details.
Update: Thanks Markus for the comment. I have updated the code and changed it to more robust implementation.

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;
    }
}

One more implementation of Singleton class. Thanks to Ralph and Lukasz Zielinski for pointing this out.

public enum SimpleSingleton {
    INSTANCE;
    public void doSomething() {
    }
}

//Call the method from Singleton:
SimpleSingleton.INSTANCE.doSomething();

public enum SimpleSingleton {
    INSTANCE;
    public void doSomething() {
    }
}

13. 在Java上做屏幕截图

Read this article for more details.

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. 在Java中的文件，目录列表

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() {
    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);

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.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();
    }
}

<?xml version="1.0"?>
<students>
<student>
<name>John</name>
<grade>B</grade>
<age>12</age>
</student>
<student>
<name>Mary</name>
<grade>A</grade>
<age>11</age>
</student>
<student>
<name>Simon</name>
<grade>A</grade>
<age>18</age>
</student>
</students>

    Java code to parse above XML.

package net.viralpatel.java.xmlparser;

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) {
        ystem.out.println(e);
    }
}
    public static void main(String[] args) {
        XMLParser parser = new XMLParser();
        parser.getAllUserNames("c:\\test.xml");
    }
}

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. 在Java中发送电子邮件

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;

    //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 = 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);

    // 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);
}

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;

public class Main {
    public static void main(String[] args) {
        try {
            URL my_url = new URL("http://www.viralpatel.net/blogs/");
            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. 在Java中调整数组

/**
 *  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.
**/
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};
    a = (int[])resizeArray(a,5);
    a[3] = 4;
    a[4] = 5;
    for (int i=0; i<a.length; i++)
        System.out.println (a[i]);
}