Writing Output Of Two Different Java Class Files To One Txt File While Program Runs

So I am trying to write the output of two different java class files to one txt file while the program runs.  The file name is determined before the program is ran, through the command prompt as arguments.  How can I get the second class file to edit the same txt file without running into compile errors.
 
For right now I'm just going to send everything that the second file outputs to a message String variable, so that the Main class outputs to the the text file.  I still want to learn how to write to the same text file directly from the second class file.
 
import java.io.*;
public class Test{
    public static void main(String[] args) throws IOException{
        int x;
        //create a new file internally called doc.  But externally labelled the user input
        File doc = new File(args[0]);
        if (doc.exists()){

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