JAVA_HOME:C:\Program Files\Java\jdk1.6.0_14;
PATH:%JAVA_HOME%\bin;
CLASSPATH:.;%JAVA_HOME%\lib;%JAVA_HOME%\lib\tools.jar;

Update the PATH variable (Optional) You can run the JDK without setting the PATH variable, or you can optionally set it as a convenience.
Should I set the PATH variable?
Set the PATH variable if you want to be able to conveniently run the JDK executables (`javac.exe`, `java.exe`, `javadoc.exe`, etc.) from any directory without having to type the full path of the command. If you don't set the PATH variable, you need to specify the full path to the executable every time you run it, such as:
`    C:> "\Program Files\Java\jdk1.6.0_<version>\bin\javac" MyClass.java`
It's useful to set the PATH permanently so it will persist after rebooting.
How do I set the PATH permanently?
To set the PATH permanently, add the full path of the jdk1.6.0_<version>\bin directory to the PATH variable. Typically this full path looks something like C:\Program Files\Java\jdk1.6.0_<version>\bin. Set the PATH as follows on Microsoft Windows:
1. Click Start > Control Panel > System on Windows XP or Start > Settings > Control Panel > System on Windows 2000.
2. Click Advanced > Environment Variables.
3. Add the location of bin folder of JDK installation for PATH in User Variables and System Variables. A typical value for PATH is:
`C:\Program Files\Java\jdk1.6.0_<version>\bin`
• PATH environment variable is a series of directories separated by semi-colons (;) and is not case sensitive. Microsoft Windows looks for programs in the PATH directories in order, from left to right.
• You should only have one bin directory for a JDK in the path at a time. Those following the first instance are ignored. If one is already present, update it to jdk1.6.0_<version>\bin.
• If you are not sure where to add the path, add it to the right end of the PATH in the User Variables.
• The new path takes effect in each new command window you open after setting the PATH variable.

java  XXXX 类 显示找不到main 类 。这极有可能 是因为在用户环境变量中没有设置CLASSPATH的原因。