Very Basic Java

Yes the problem is that you can only have one public class per file and this file should have the same name than the class. You can just remove the public in front of the definition of the first class. A better way to do would be to make it a static method of the main class.

To solve you second problem you can do this:

public class firstprog {


    public static int largest(int a,int b,int c) 
    {
        if(a>b)
        { 
            if(a>c)
                return a;
            else 
                if(b>c)
                    return b;
                else 
                    return c;
        }
        else
        {
            if(b>c)
                return b;
            else 
                return c;
        }
    }   

    public static  void main(String args[]) {

        int a=19;
        int b=2;
        int c=1;

        System.out.println(largest(a,b,c));  
  }
}