Step 3 of 5

Objects as values

The values of a map can be objects of your own class. Then the map finds a whole object by its key. A museum's audio guide works this way: each exhibit has a number on a small sign, and visitors type the number into the guide to hear about the exhibit. The keys are the numbers, and the values are GuideStop objects.

Main.java

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<Integer, GuideStop> stops = new HashMap<>();
        GuideStop harbor = new GuideStop(12, "The Harbor at Dawn", 3);
        GuideStop horse = new GuideStop(27, "Bronze Horse", 5);
        stops.put(harbor.getNumber(), harbor);
        stops.put(horse.getNumber(), horse);

        GuideStop stop = stops.get(27);
        System.out.println(stop.getTitle() + " is in room " + stop.getRoom());
        stop.listen();
        stops.get(27).listen();
        stops.get(12).listen();
        System.out.println(stops.get(27));
        System.out.println(stops.get(12));
    }
}

GuideStop.java

public class GuideStop {
    private int number;
    private String title;
    private int room;
    private int listens;

    public GuideStop(int number, String title, int room) {
        this.number = number;
        this.title = title;
        this.room = room;
        this.listens = 0;
    }

    public int getNumber() {
        return this.number;
    }

    public String getTitle() {
        return this.title;
    }

    public int getRoom() {
        return this.room;
    }

    public void listen() {
        this.listens++;
    }

    public String toString() {
        return this.title + " (room " + this.room + "), listens: " + this.listens;
    }
}

Output

Bronze Horse is in room 5
Bronze Horse (room 5), listens: 2
The Harbor at Dawn (room 3), listens: 1

The key comes from the object itself: stops.put(harbor.getNumber(), harbor). That's common: a product found by its code, a user by their username. The key is then kept twice, in the object and in the map, and the two should match.
A map keeps references to its objects, as a list does (module 22). stops.get(27) gives a reference to the Bronze Horse object in the map, not a copy of it. So stop.listen() changes the object in the map, and there's no need to put it back.
stops.get(27).listen() finds the object and calls its method in one go. It's the same object again, so it now has 2 listens.

A number with no stop

A visitor types 72 instead of 27. The map has no such key, get gives null, and calling a method on null crashes:

Main.java

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<Integer, GuideStop> stops = new HashMap<>();
        GuideStop horse = new GuideStop(27, "Bronze Horse", 5);
        stops.put(horse.getNumber(), horse);
        stops.get(72).listen();
    }
}

GuideStop.java

public class GuideStop {
    private int number;
    private String title;
    private int room;
    private int listens;

    public GuideStop(int number, String title, int room) {
        this.number = number;
        this.title = title;
        this.room = room;
        this.listens = 0;
    }

    public int getNumber() {
        return this.number;
    }

    public void listen() {
        this.listens++;
    }
}

The program crashes with

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "GuideStop.listen()" because the return value of "java.util.HashMap.get(Object)" is null
	at Main.main(Main.java:8)

When a key may be missing, keep what get gives in a variable, and check it before calling anything on it:

GuideStop stop = stops.get(number);
if (stop == null) {
    System.out.println("No stop with the number " + number);
} else {
    stop.listen();
    System.out.println(stop);
}

Objects as values

A number with no stop

Your task

Challenge 2

Challenge 3