How to have an ArrayList by composition

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How to have an ArrayList by composition

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#1 by (2 votes)

1

I understand how to do composition but I have never done with lists, although the principle should be the same, I do not understand how to do it.

To start I have a class CentroComercial that has a ArrayList called lojas which will pass by attribute in construtor .

Lojas is a ArrayList that stores objects of type Loja . being this super-class divided into several subclasses. And I have two doubts.

How do I do the

public void setLojas(ArrayList<Loja> lojas) {}

What is the best way to instantiate Arraylist?

private ArrayList<Loja> lojas;

private ArrayList<Loja> lojas = new ArrayList<Loja>();

or some other way?

java arraylist

asked by anonymous 15.04.2018 / 16:47

1 answer

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score 2 · Accepted Answer

If the method you are going to have is

public void setLojas(ArrayList<Loja> lojas) {}

Then just declare the ArrayList as

private ArrayList<Loja> lojas;

However, if you want to ensure that you have a valid loja object in order to avoid possible NullPointerException use:

private ArrayList<Loja> lojas = new ArrayList<Loja>();

This is the preferred way as it allows you to safely identify / indicate when the Shopping Center has no associated stores (you can use lojas.size() ).

The setLojas() method will be like this

public void setLojas(ArrayList<Loja> lojas){
    this.lojas = lojas;
}

If you do not want to externally change the saved ArrayList, create a new instance.

public void setLojas(ArrayList<Loja> lojas){
    lojas = new ArrayList<Loja>(lojas);
}

If you have a get method this should also return a new instance

public ArrayList<Loja> getLojas(){
    return new ArrayList<Loja>(lojas);
}

The object loja should also be immutable.