Some classes are missing when I have installed the JDK

Some classes in the IO package, like File are missing from the IO package. Or at least NetBeans can't find them. For instance, DataInputStream works:

import java.io.DataInputStream; (no error)

import java.io.File; (error: can not find symbol, symbol: class File,

location package java.io)

To solve this problem, I uninstalled the JDK and Netbeans. Then I downloaded the JDK again and installed it, then I installed Netbeans. But the same thing happened again. Why are these classes missing from the IO package? I know they should be there, because File is in the IO package in the API Specification:

http://java.sun.com/j2se/1.5.0/docs/api/index.html

[689 byte] By [Eyvind5a] at [2007-10-2 8:44:04]