Objects from a file

In the previous step, each line's values were used right away and then forgotten. When the program needs them later, each line becomes an object: a class holds one record's values together, and a list holds the objects. Reading the file then takes one loop, and after it the program answers its questions from the list:

Coast Path,14
Pine Loop,6
Ridge Walk,9

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

public class Main {
    public static void main(String[] args) {
        ArrayList<Trail> trails = new ArrayList<>();
        try (Scanner scanner = new Scanner(Paths.get("trails.txt"))) {
            while (scanner.hasNextLine()) {
                String[] parts = scanner.nextLine().split(",");
                trails.add(new Trail(parts[0], Integer.valueOf(parts[1])));
            }
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }

        System.out.println("Trails: " + trails.size());
        System.out.println("Ten kilometers or less:");
        for (Trail trail : trails) {
            if (trail.getLength() <= 10) {
                System.out.println(trail);
            }
        }
    }
}

public class Trail {
    private String name;
    private int length;

    public Trail(String name, int length) {
        this.name = name;
        this.length = length;
    }

    public String getName() {
        return this.name;
    }

    public int getLength() {
        return this.length;
    }

    public String toString() {
        return this.name + " (" + this.length + " km)";
    }
}

Trails: 3
Ten kilometers or less:
Pine Loop (6 km)
Ridge Walk (9 km)

• Each line is split, and its parts go to the constructor in the order of its parameters: new Trail(parts[0], Integer.valueOf(parts[1])). The new object goes straight into the list.
• The list is made before the try, so it's still there after it. If the file can't be read, the list simply stays empty.
• After the loop, the file is done with. The questions (how many, which ones, the longest) are answered from the list, with the tools of module 19.

Reading in a method

A method that reads the file and returns the list keeps main short, and it can read any file of trails:

public static ArrayList<Trail> readTrails(String fileName) {
    ArrayList<Trail> trails = new ArrayList<>();
    try (Scanner scanner = new Scanner(Paths.get(fileName))) {
        while (scanner.hasNextLine()) {
            String[] parts = scanner.nextLine().split(",");
            trails.add(new Trail(parts[0], Integer.valueOf(parts[1])));
        }
    } catch (Exception e) {
        System.out.println("Error: " + e.getMessage());
    }
    return trails;
}

main then starts with ArrayList<Trail> trails = readTrails("trails.txt");.

The end of part 4

This step ends the material of the MOOC's fourth part. Part 4 was the start of object-oriented programming: you wrote classes of your own, created objects from them with constructors, gave them methods such as getters and toString, and kept many objects in a list. This module then brought in data from outside the program: files, read line by line and turned into numbers, records and objects. Classes and files together let a program work with as much data as it's given. The MOOC closes the part with a summary page on its own site, for reading only.