Okay so I've understood the program enough to write something, but I don't understand why....?

package com.codegym.task.task19.task1916; import java.io.*; import java.util.ArrayList; import java.util.List; /* Tracking changes */ public class Solution { public static List<LineItem> lines = new ArrayList<>(); public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); String file1 = reader.readLine(); String file2 = reader.readLine(); reader.close(); BufferedReader fileReader1 = new BufferedReader(new FileReader(file1)); BufferedReader fileReader2 = new BufferedReader(new FileReader(file2)); //int i = 0; while (true) { String string1 = fileReader1.readLine(); String string2 = fileReader2.readLine(); //Break when both strings are null if (string1 == null && string2 == null) break; //READ THE COMMENT BELOW TO UNDERSTAND MY CODE. IT'S VERY SIMPLE!!! //Each string can either be null or empty, so test for these two conditions. //For SAME, only one test (condition) is needed: if string1 is equal to string2. //For ADDED, two conditions must satisfy: string1 must either be empty or null. //For REMOVED, two conditions must also satisfy: string2 must either be empty or null. //Lastly, let the null conditions come before the empty conditions. /*example: FOR REMOVED ---- string2 (second file) string1 REMOVED (at this point, the computer considers string2 empty). string1 REMOVED (but at this point, the computer considers string2 null). This also applies to ADDED. A string is considered null when it is empty and is the last line in the file. */ if (string1.equals(string2)) lines.add(new LineItem(Type.SAME, string1)); else if (string1 == null && !string2.isEmpty()) //Let the null condition come first: string1 is null -- ADDED lines.add(new LineItem(Type.ADDED, string2)); else if (string1.isEmpty() && !string2.isEmpty()) //String1 is empty but string2 is not -- ADDED lines.add(new LineItem(Type.ADDED, string2)); else if (!string1.isEmpty() && string2 == null) // Let the null condition come first: string2 is null -- REMOVED lines.add(new LineItem(Type.REMOVED, string1)); else if (!string1.isEmpty() && string2.isEmpty()) //string2 is empty -- REMOVED lines.add(new LineItem(Type.REMOVED, string1)); //This is to help view the output. It wasn't required in the task. Uncomment lines 23, 60 and 61 //System.out.println(string1 + " " + string2 + " --- " + lines.get(i).type + " " + lines.get(i).line); //i++; } //end while loop fileReader1.close(); fileReader2.close(); } //close main public static enum Type { ADDED, // New line added REMOVED, // Line deleted SAME // No change } public static class LineItem { public Type type; public String line; public LineItem(Type type, String line) { this.type = type; this.line = line; } } }