计算机二级辅导:20个非常有用的Java程序片段(2)
发布时间：2010/3/13 9:29:05 来源：城市学习网 编辑：MOON
14. 列出文件和目录
Java代码
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);
15. 创建ZIP和JAR文件
Java代码
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();
}
}
16. 解析/读取XML 文件
Java代码
XML文件
?xml version="1.0"?
students
student
nameJohn/name
gradeB/grade
age12/age
/student
student
nameMary/name
gradeA/grade
age11/age
/student
student
nameSimon/name
gradeA/grade
age18/age
/student
/students
Java代码
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) {
System.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
Java代码
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
Java代码
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);
}
19. 发送代数据的HTTP 请求
Java代码
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://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. 改变数组的大小
Java代码
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/**
* 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 = 4;
a = 5;
for (int i=0; ia.length; i++)
System.out.println (a[i]);
}
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