How to read two text files and print something that is not in either file.

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Java

file

i have two text files
README.TXT->THIS IS THE PROJECT README FILE.
README1.TXT->THIS IS THE PROJECT README FILE BY KUNAL SHAH.

MY CODE IS:

import java.util.Scanner;
import java.io.*;
import java.util.ArrayList;
public class files
{
    // instance variables - replace the example below with your own
    public static void main (String[] args)throws IOException
    {
        Scanner scan,scan1;
        String str,str1,word,word1;
        scan=new Scanner(new File("README.TXT"));
        scan1=new Scanner(new File("README1.TXT"));
        Scanner line,line1;
        ArrayList<string> al1=new ArrayList<string>();
        ArrayList<string> al=new ArrayList<string>();
        while(scan.hasNext())
        {   
            while(scan1.hasNext())
            {
                str1=scan1.nextLine();
                str=scan.nextLine();
                
                line=new Scanner(str);
                line1=new Scanner(str1);
                
                while(line.hasNext())
                {
                    while(line1.hasNext())
                    {
                        word1=line1.next();
                        al1.add(word1);
                    }
                    word=line.next();
                    al.add(word);
                    
                }
            }
        }
    }
}

//reading two text files is accomplished. however i need to compare contents from two files and output that is not common in two files
for my example it should be by kunal shah.

What I have tried:

i have tried to create arraylist<string> and push all tokes from files into two arrayList<string>. however i am not understanding how to output content that is not common in two files.