Comparing two text files in random order with Java

Question

I am trying to compare two text files that are randomized and print out the lines that match in both of the files. File 1:

Student1
Student2
Student3
Student4

File 2:

Student6
Student1
Student2

I want the output as

Student1
Student2

My code is below.

public static void main(String[] args) throws IOException {

     String first = "file1.txt";
     String second = "file2.txt";
     BufferedReader fBr = new BufferedReader(new FileReader(first));
     BufferedReader sBr = new BufferedReader(new FileReader(second));   




     PrintWriter writer = new PrintWriter("test.txt", "UTF-8");  
     while ((first = fBr.readLine()) != null) {
         String partOne1 = fBr.readLine();
         String partTwo1 = sBr.readLine();
         while ((second = sBr.readLine()) != null) {
                System.out.println(first);
                writer.println(first);  
                break;                   

         }
     }


     writer.close();
     fBr.close();
     sBr.close(); 

Answer 1

It's quite simple=) Try to store all results from first file and compare with all lines from second. It will be like this:

package com.company;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;

public class Main {

    public static void main(String[] args) throws IOException {

        String first = "file1.txt";
        String second = "file2.txt";
        BufferedReader fBr = new BufferedReader(new FileReader(first));
        BufferedReader sBr = new BufferedReader(new FileReader(second));

        ArrayList<String> strings = new ArrayList<String>();

        while ((first = fBr.readLine()) != null) {
            strings.add(first);
        }
        fBr.close();

        while ((second = sBr.readLine()) != null) {
            if (strings.contains(second)) {
                System.out.println(second);
            }
        }
        sBr.close();
    }
}

It's better to use memory when possible, your 'while' inside different while can work too long time and obfuskate logic.

Answer 2

Another alternative is to put both your files in two arraylists and use the arraylist's retainAll() method to get the common files. And do the operations on it like printing or something else.

public static void main(String[] args) throws IOException {
     String first = "file1.txt";
     String second = "file2.txt";
     BufferedReader fBr = new BufferedReader(new FileReader(first));
     BufferedReader sBr = new BufferedReader(new FileReader(second));   

     List<String> firstFile = new ArrayList<>();
     List<String> secondFile = new ArrayList<>();

     PrintWriter writer = new PrintWriter("test.txt", "UTF-8");  
     while ((first = fBr.readLine()) != null) {
         firstFile.add(first);
     }
     while ((second = sBr.readLine()) != null) {
         secondFile.add(second);                  
     }

     List<String> commonFile = new ArrayList<>(firstFile);
     commonFile.retainAll(secondFile);
     System.out.println(commonFile);

     writer.close();
     fBr.close();
     sBr.close(); 
}