Reading a File Line by Line into a Collection

A file is stored as a stream of characters, not a labeled array of lines you can jump into. A Scanner reads that stream from the start, one token or line at a time, and has no way to skip to line 7 without passing through lines 1 through 6 first. That is why file reading is sequential: you process what comes next, then advance.

The loop condition does the real work. A while loop built on hasNext() (or hasNextLine(), hasNextInt()) asks the Scanner 'is there more input of this kind waiting?' before every pass. As long as the answer is true, the loop body reads one item with next() or nextInt() and adds it to an ArrayList, say. When the Scanner reaches the end of the file, hasNext() returns false, the loop stops, and the collection holds exactly one element per item read — a four-line file of numbers produces a list of size 4.

Mixing nextInt() and nextLine() trips people up because nextInt() only grabs the numeric token and leaves the rest of that line, including the leftover newline, sitting unread; a following nextLine() then grabs that leftover scrap instead of the next real line. This topic sits right before searching and sorting those loaded collections — you cannot binary search or sort an ArrayList you never finished reading, so getting the read loop exactly right matters for everything that follows.