下面是20个非常有用的java程序片段，希望能对你有用。内容比较早，有些函数可能过时了，但是总体思路是不错滴，供参考。
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
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
8、创建图片的缩略图
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) http://viralpatel.net/blogs/download/json/json-rpc-1.0.jar
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
11、http 代理设置
阅读这篇 文章 了解更多细节。
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 示例
请先阅读这篇文章 了解更多信息
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;
}
另一种实现
public enum simplesingleton {
instance;
public void dosomething() {
}
//call the method from singleton: simplesingleton.instance.dosomething();
13、抓屏程序
阅读这篇文章 获得更多信息。
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
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.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(
zipentry entry = new zipentry(name);
entry.setmethod(zipentry.stored);
entry.setcompressedsize(file.length());
entry.setsize(file.length());
entry.setcrc(crc.getvalue());
zos.putnextentry(entry);
zos.write(buffer, 0, bytesread);
zos.close();
16、解析/读取xml 文件
<?xml version="1.0"?>
<students>
<student>
<name>john</name>
<grade>b</grade>
<age>12</age>
</student>
<name>mary</name>
<grade>a</grade>
<age>11</age>
<student>
<name>simon</name>
<age>18</age>
</students>
xml文件
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 = e.getelementsbytagname("age");
system.out.println("age: "
}
}
} 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
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;
//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
import java.io.bufferedreader;
import java.io.inputstreamreader;
import java.net.url;
public static void main(string[] args) {
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
/**
* 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]);
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作者：陈皓
来源：51cto