How to access the "name" of things in my array list that is an array list of my interface type?

Question

The title is wordy and possibly confusing but I am not sure how to make it better...I want to be able to access values in my array list and print them out.

I have an interface called ThingBagInterface. This ThingBagInterface only has one method, and looks like this:

interface ThingBagInterface {
    public String getType();
}

I now have a class called ThingBag, it's a bag that hold a bunch of different stuff, such as Creatures, Buildings etc.

In my ThingBag class, I have initialized all of my Creatures like this:

public void initCreatures(){
     waterSnake = new Creature("Water Snake", Terrain.SWAMP, false, false, false, false, 1, 0);
    etc...
}

and then I have a function populateBag() that looks like this:

public void populateBag(){
    initCreatures();

    bag.add(bears);
}

My array list definition is in ThingBag and looks like this:

ArrayList<ThingBagInterface> bag = new ArrayList<ThingBagInterface>();

My Creature constructor looks like this:

    public Creature(String n, Terrain startTerrain, boolean flying, boolean magic, boolean charge, boolean ranged, int combat, int o){
        name = n;
        flyingCreature = flying;
        magicCreature = magic;
        canCharge = charge;
        rangedCombat = ranged;
        combatValue = combat;
        owned = o;
    }

I want to print out the name of the bear.

So in main I am doing this:

ThingBag tb = new ThingBag();
tb.populateBag();
for(int i= 0; i<tb.bag.size(); i++){
    System.out.println(i+". "+tb.bag.get(i));
}

Why can I not access the name in my bag? If I wasn't using an interface I would be able to say:

System.out.println(i+". "+tb.bag.get(i).name)

But I can't now. Any ideas on how I can access that value? I can only access memory addresses now...

Sotirios Delimanolis · Accepted Answer · 2014-01-28 02:27:10Z

2

Your bag variable is declared as

ArrayList<ThingBagInterface> bag ...

This, conceptually, means that it contains at least ThingBagInterface objects. It can contain any type of ThingBagInterface, but it has to at least be a ThingBagInterface.

What this also means is that the compiler can only guarantee that it contains ThingBagInterface and therefore you can only interact with its elements as ThingBagInterface instances.

name is not a field that exists on the ThingBagInterface type, it exists on Creature.

You can either cast the returned value of bag.get(i) or declare a getName() method of ThingBagInterface, implement it in the sub types, and invoke it in your loop.

answered Jan 28, 2014 at 2:27

Sotirios Delimanolis

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4 Comments

Sarah Over a year ago

Thank you, this is my first time trying to use an interface so I am going to make a lot of dumb mistakes until I actually figure it out. Thank you again.

user2357112 Over a year ago

Note that if you're going to cast the elements to Creature, there's not much reason to have a ThingBagInterface at all. ArrayList<Creature> would work better.

Sotirios Delimanolis Over a year ago

@Sarah Take note of what user2357112 just said. Do you have many ThingBagInterface? Do they all have a name? Consider making a base class for all of them.

Sarah Over a year ago

They do all have a name. I'm unsure if I want an interface or inheritance really. I did have an array list of Creatures, but I needed to change it because my Array List needs to be holding several different types of things. They do all have a name, so maybe that is where I should go, but name is all they have in common.

user2357112 · Accepted Answer · 2014-01-28 02:27:37Z

2

You'll need to design the ThingBagInterface interface more sensibly. Is having a name a requirement? If so, the interface needs a way to access the object's name. (This needs to be a method, as interfaces can't specify fields.) While we're at it, picking a more informative name than ThingBagInterface would be a good idea.

answered Jan 28, 2014 at 2:27

user2357112

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2 Comments

Sarah Over a year ago

it's an interface for a bag that holds "things" which is the game piece names for the board game I'm trying to implement, so it's pretty informative, but I do get what you are trying to say, so thank you

user2357112 Over a year ago

@Sarah: The name doesn't give any information about how it relates to ThingBag. It sounds like ThingBag is supposed to implement it, rather than the things contained in a bag. Imagine if the interface for passengers in a car was called CarInterface.

user_loser · Accepted Answer · 2014-01-28 02:33:49Z

1

Maybe do a cast like so System.out.println(i+". "+(((Creature)tb.bag.)get(i));

This is because the interface does not have a name attribute. Remember fields in interfaces are always implicitly public static and final.

I hope this helps.

answered Jan 28, 2014 at 2:33

user_loser

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