Files and folders

Everything a program keeps in its variables is gone when it ends. Data that has to last, such as contacts, high scores or a year of measurements, is kept in files.

• A file is a piece of data stored under a name: a photo, a song, a document, a program. This module reads text files, which hold plain text in lines. Any text editor can open and change them, and your .java files are text files too.
• The end of a file's name, such as .txt, .csv or .java, is its extension. It tells what kind of data the file holds, and it's part of the name: scores and scores.txt are two different names. Windows and macOS can hide extensions when they list files, but the extension is still there.
• A folder (also called a directory) holds files and other folders. A file's path names the folders on the way to it: school/biology/cells.txt is the file cells.txt in the folder biology, which is in the folder school. Windows writes paths with \ instead of /.

Where does the program look?

When a program opens a file by its name alone, such as plants.txt, Java looks for it in the program's working folder (or working directory): the folder the program runs in.

• On your own computer, when you run a project from an editor such as NetBeans, IntelliJ IDEA or VS Code, the working folder is usually the project's main folder, the one that holds the src folder. A file your program reads goes there, next to src, not inside it.
• On Java Arena there's no project folder of your own. Instead, each challenge that reads files provides them: the task shows each file with its name and contents, and the check puts the files in your program's working folder before it runs, just like files next to a program on a computer. Run with my input gets the same files. Hidden tests give files with other contents, so the program has to really read them.

A first look

This program reads the file plants.txt and prints a line for each plant in it:

basil
mint
rosemary

import java.nio.file.Paths;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(Paths.get("plants.txt"))) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                System.out.println("Water the " + line);
            }
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

Water the basil
Water the mint
Water the rosemary

The next steps go through this program part by part. For now, find the file's name in Paths.get("plants.txt"): that's where the program says which file to read. Put another name there, and the same program reads another file.