Add maps concept

[kahgoh]

Add maps concept

Resolves #2583

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[josealonso]

Typo: number of

[josealonso]

@kahgoh, a very complete explanation and well-written exercise!

[manumafe98]

Good job! @kahgoh this one will be very useful to the students of exercism!

[kahgoh]

@manumafe98 I'm wondering whether I should remove this? I had originally put this here to make it clearer where the Map is defined, but then again classes is set as a prerequisite (so students could probably work out to define a field). I would add a hint if removed.

Another might option is to replace this with something like:

// TODO: Make a Map instance
private final Map<Integer, String> countryByDialingCode = null;

Do you have any thoughts on what we should do here?

[manumafe98]

Yeah I will remove it too, I think if they reach this part of the track they should know were to define it.
And also it give us leverage to work on an extra actionable if for example a student does something like this

    public Map<Integer, String> getCodes() {
        Map<Integer, String> countryByDialingCode = new HashMap<>();
        return Map.copyOf(countryByDialingCode);
    }

To check that they initialize the map as a field and not inside the methods

[kahgoh]

Cool! I'll remove it from the stub and update the hints.

[josealonso]

I agree with @manumafe98. Students have already defined a list in a previous learning exercise.

[josealonso]

Very easy to understand explanation on the hash value. Good work!!