Step 4 of 5

A hash map inside a class

A map can be an instance variable, like a list (module 24). The class creates the map in its constructor, and its methods use it. The rest of the program calls the methods, and doesn't need to know that there's a map inside.

A radio station logs how many times each song was played:

Main.java

public class Main {
    public static void main(String[] args) {
        PlayLog log = new PlayLog();
        log.record("Night Bus");
        log.record("Paper Lanterns");
        log.record("Night Bus");
        System.out.println(log.timesPlayed("Night Bus"));
        System.out.println(log.timesPlayed("Paper Lanterns"));
        System.out.println(log.timesPlayed("Slow River"));
    }
}

PlayLog.java

import java.util.HashMap;

public class PlayLog {
    private HashMap<String, Integer> plays;

    public PlayLog() {
        this.plays = new HashMap<>();
    }

    public void record(String title) {
        int before = this.plays.getOrDefault(title, 0);
        this.plays.put(title, before + 1);
    }

    public int timesPlayed(String title) {
        return this.plays.getOrDefault(title, 0);
    }
}

Output

2
1
0

The constructor creates the map. Without this.plays = new HashMap<>();, the instance variable would be null (module 22), and the first record would crash with a NullPointerException.
record is the way to count with a map: get the count so far, with 0 for a song that hasn't been played yet, and put back one more. The put replaces the old count, since a key has only one value.
timesPlayed gives 0 for a song that was never played, not null, so the caller can use the answer as a number right away.

The count can also be written with containsKey. It does the same:

if (this.plays.containsKey(title)) {
    this.plays.put(title, this.plays.get(title) + 1);
} else {
    this.plays.put(title, 1);
}

Tidy the keys in one place

When the keys come from people, the class can tidy every key itself. A private method keeps the tidying in one place, and every method that uses a key calls it, so a title is tidied the same way whether it's recorded or looked up:

public void record(String title) {
    String key = tidy(title);
    this.plays.put(key, this.plays.getOrDefault(key, 0) + 1);
}

public int timesPlayed(String title) {
    return this.plays.getOrDefault(tidy(title), 0);
}

private String tidy(String title) {
    return title.trim().toLowerCase();
}

Now record("Night Bus") and record(" night bus") count the same song, and timesPlayed("NIGHT BUS") finds it. A method that forgot to call tidy would look for another key, and find nothing.

A hash map inside a class

Tidy the keys in one place

Your task

Challenge 2

Challenge 3