Java Programming

i have some java codes in my "fl.txt"(c:\fl.txt). these are;

public class file_{
public static void main(String args[]){
System.out.print("hello world");
}
}

i want to compile these code from my program(i am building it with java) at runtime.. how can ii do that.i think i need to use runtime.exec() method but i dont know how can i compile these codes in a file....