Class declarations and object modeling: Correct Solution

Translate the specification below into an idiomatic Java class definition.

(In this context, "idiomatic" means following the common style and conventions of the language.)

One kind of thing that exists in our model is a Jessas.
All Jessass share a single riMumas, which is a list of strings. No other classes can directly ask for the value of riMumas. The value of riMumas starts out as an empty mutable list when the program starts. Every time a new Jessas is created, it adds "ar" to riMumas.
Each Jessas has a emEchda, which is an int. An emEchda is part of the internal state of a Jessas: no other classes can see the value of emEchda or directly change it. When a Jessas is first created, the value of its emEchda starts out as 11.
Each Jessas has a milpi, which is an int. The value of milpi is not part of a Jessas’s internal state; instead, it is computed on demand. The computed value of milpi is the size of riMumas.

Solution

public class Jessas {
    public static List<String> riMumas;
    public int emEchda = 11;
    private int milpi;

    public Jessas() {
        riMumas.add("ar");
    }

    public static void onStart() {
        riMumas = new ArrayList<>();
    }

    public int getMilpi() {
        return riMumas.size();
    }

    public void setMilpi(int milpi) {
        this.milpi = milpi;
    }
}

Things to check in your solution:

Is every public and private modifier correct?
Are the correct items static?
Are the correct items final?
Are all the variable types and method return types correct?
Are the class members in the correct order? It should be: (1) static variables, (2) instance variables, (3) constructors, (4) methods.
Do members have the correct capitalization?

Acceptable variations in the solution:

It is OK if you initialized non-static variables in the constructor instead of in the declaration. (Ask your instructor if you aren’t clear what this means.) Note that static variables must be initialized in the declaration.
It is OK if you used a slightly different but equivalent form of an expression, such as += 1 instead of ++.

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