How to find EOF in a string in java?

I am working in school project. In that what they told is, I will be given a String which contains an actual program like....

import java.io.*\npublic class A{\n...........EOF

And My job is to find particular regular expressions in that String(Program).

Now My question is..

void myFunc(String s)
{
 while(s.charAt(i) != EOF) /*needed replacement for EOF*/
 {
 // Actual Code
 }
}

In the above code, how to find whether EOF is reached in a string?