Step 4 of 5

Saving and loading

save and load

A program that writes its data when it ends, and reads it back when it starts, remembers the data from one run to the next. The data usually lives in an object, so that object's class gets two methods: save writes everything the object holds into a file, and load reads such a file back into the object. The two must agree on the format: load has to understand whatever save writes.

Here's a to-do list. It keeps its tasks in a list of Strings, and its file has one task per line:

Main.java

public class Main {
    public static void main(String[] args) throws Exception {
        TodoList monday = new TodoList();
        monday.add("Water the ferns");
        monday.add("Pay the phone bill");
        monday.save("todo.txt");

        TodoList tuesday = new TodoList();
        tuesday.load("todo.txt");
        tuesday.add("Return the library books");
        tuesday.print();
    }
}

TodoList.java

import java.io.PrintWriter;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Scanner;

public class TodoList {
    private ArrayList<String> tasks;

    public TodoList() {
        this.tasks = new ArrayList<>();
    }

    public void add(String task) {
        this.tasks.add(task);
    }

    public void print() {
        for (int i = 0; i < this.tasks.size(); i++) {
            System.out.println((i + 1) + ". " + this.tasks.get(i));
        }
    }

    public void save(String fileName) throws Exception {
        try (PrintWriter writer = new PrintWriter(fileName)) {
            for (String task : this.tasks) {
                writer.println(task);
            }
        }
    }

    public void load(String fileName) throws Exception {
        try (Scanner reader = new Scanner(Paths.get(fileName))) {
            while (reader.hasNextLine()) {
                String line = reader.nextLine();
                if (!line.isEmpty()) {
                    this.tasks.add(line);
                }
            }
        }
    }
}

Output

1. Water the ferns
2. Pay the phone bill
3. Return the library books

tuesday is a new object, and its list starts empty. Its first two tasks come from the file, as they would in a program started again the next day.
save writes the whole list every time, with a PrintWriter that replaces what the file held, so the file always matches the list. Appending would put every old task into the file a second time.
load adds each line of the file to the list. It skips empty lines, which a person who edits the file by hand may leave behind.
Both methods pass their exceptions on with throws Exception. The class can't know what a program wants to do when its file can't be read or written, so the code that calls them decides, as module 42 said.

The first run

The very first time the program runs, there's no todo.txt yet, and load's Scanner throws a NoSuchFileException. For a to-do app that isn't a problem: it just starts with an empty list. So the program catches the exception around load, and goes on:

TodoList list = new TodoList();
try {
    list.load("todo.txt");
} catch (Exception e) {
    System.out.println("No saved tasks yet");
}

Load when it starts, save when it ends

Put together with a loop that reads commands (module 25), this gives a to-do app that remembers its tasks. main loads the list, the loop runs the commands, and after quit, main saves the list:

The file todo.txt

Water the ferns
Pay the phone bill

Main.java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        TodoList list = new TodoList();
        try {
            list.load("todo.txt");
        } catch (Exception e) {
            System.out.println("No saved tasks yet");
        }

        Scanner scanner = new Scanner(System.in);
        while (true) {
            String command = scanner.nextLine();
            if (command.equals("quit")) {
                break;
            }
            if (command.equals("add")) {
                list.add(scanner.nextLine());
            } else if (command.equals("list")) {
                list.print();
            } else {
                System.out.println("Unknown command: " + command);
            }
        }

        try {
            list.save("todo.txt");
            System.out.println("Saved");
        } catch (Exception e) {
            System.out.println("Could not save: " + e.getMessage());
        }
    }
}

TodoList.java

import java.io.PrintWriter;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Scanner;

public class TodoList {
    private ArrayList<String> tasks;

    public TodoList() {
        this.tasks = new ArrayList<>();
    }

    public void add(String task) {
        this.tasks.add(task);
    }

    public void print() {
        for (int i = 0; i < this.tasks.size(); i++) {
            System.out.println((i + 1) + ". " + this.tasks.get(i));
        }
    }

    public void save(String fileName) throws Exception {
        try (PrintWriter writer = new PrintWriter(fileName)) {
            for (String task : this.tasks) {
                writer.println(task);
            }
        }
    }

    public void load(String fileName) throws Exception {
        try (Scanner reader = new Scanner(Paths.get(fileName))) {
            while (reader.hasNextLine()) {
                String line = reader.nextLine();
                if (!line.isEmpty()) {
                    this.tasks.add(line);
                }
            }
        }
    }
}

Input

add
Buy birdseed
list
quit

Output

1. Water the ferns
2. Pay the phone bill
3. Buy birdseed
Saved

After the run, todo.txt holds the three tasks, and the next time the program starts, it has them all. In a larger program the command loop would be in a user interface class of its own, as in module 25, and main would still load at the start and save at the end.

Saving and loading

save and load

The first run

Load when it starts, save when it ends

Your task

Challenge 2

Challenge 3