Adding an object to an Arraylist before it's big enough

Newbie & Debugging Questions
#1

So in my game, each room has it’s own arraylist containing a bunch of entities
I’m dealing with this specific room (randomly generated) that has two entities (the exit and the entrance)
Now, it’s VERY important that the objects are in a specific order in the arraylist, because the room-changing-code depends on it
The entrance must be at index 0, and the exit must be at index 1

It’s not normally a problem, but because the room is randomly generated, I can’t control which one (the exit or entrance) is created first
So half of the time, I get an error that I can’t add the exit at index 1 because the size isn’t big enough yet

But here’s the thing, the API ( http://docs.oracle.com/javase/1.4.2/docs/api/java/util/ArrayList.html ) says that when you create an arraylist through the parameter-less constructor (the one I use) it ensures a capacity of ten. So what gives? How can I make sure I can add an object at index 1 before I add one at index 0?

Thanks

#2

Surely if you added your entrance and exit entities to the list before you generated the rest of the room you could solve that problem? Or not…

EDIT:
I didn’t really understand your problem at first but here is what I see:
You generate the room, each entity gets generated randomly and added to the ArrayList and your entrance and exit are not necessarily generated at a time where there are added at index 0 and 1.
Is that right?

#3

The capacity is just how much “slack” you have til it has to reallocate the underlying array. It’s purely an implementation detail you shouldn’t worry about, and it doesn’t mean you can index elements that don’t exist. Nor will it create intervening null elements if you try to assign to an index past the end.

Why aren’t the entrance and exit simply fields on your class?

#4

Yup, that’s it

#5

Prooooobably because I don’t know what a “field” is
:confused:
I’m learning all of this as I go along
I’ll be sure to look up what that is

#6

adding exit:
if arraylist.size() == 0//, entrance wasnt added yet
save exit temporarily somewhere else, and add it AFTER arraylist is size 1 or greater
this all depends on your structure…

#7

To be honest, that’s what I did, but I was kinda curious if there was another way that would be slightly simpler and cleaner (other than storing the location and other properties in ints and making it at the end), especially for bigger rooms with more entities but it’s not that big a deal

But thanks to everyone for all their help, though! (Fields seem useful!)

#8

well yeah, but it really depends on your structure
if you write it so that, naturally the entrance is generated first, problem solved.

#9

I’d recommend a new structure.

Make a class called “Entity” which stores the fields (global values which every entity needs; position, image etc.), and give it all the getters and setters you’ll need for an entity. Then add a field: int type
If it’s an entrance, then it has type=0, and if it’s an exit it has type=1.
Then you can easily distinguish between the two, and make easy checks as to what’s what, and what has already been added to your list.
AND, then you can just iterate through that list, and check which entity is an entrance and which entity is an exit by checking the type-field, so you don’t need to have them on a specific spot in the list.

What I’d REALLY recommend, is that you read up on inheritance, because that can really save you time when having a lot of different things in your game-world. That Entity-class can be simple enough, to let you have other classes extend it (inherit all its methods and fields). That way you could also have a class called InteractableEntity, extending Entity, which holds an extra field String text, which you could use to display some information about the entity, if the player activates it somehow. Or a trap maybe.

#10

[quote]If it’s an entrance, then it has type=0, and if it’s an exit it has type=1.
[/quote]
Oh Jesus Crack Smoking Christ, no. Just no. Use Classes. Write it in Java, not some ad-hoc bastardized dynamic language embedded in Java. Even if you litter your code with instanceof, it’d be better than this kind of nonsense. This “entity system” insanity has got to stop, especially among people who aren’t already experts in OOP to know what they’re trying to replace.

#11

Oh Jesus Crack Smoking Christ, no. Just no. Use Classes. Write it in Java, not some ad-hoc bastardized dynamic language embedded in Java. Even if you litter your code with instanceof, it’d be better than this kind of nonsense. This “entity system” insanity has got to stop, especially among people who aren’t already experts in OOP to know what they’re trying to replace.
[/quote]
Well, OP didn’t know what ‘fields’ were, so I thought I’d give him an easy option for this game, and tell him that he should look into inheritance if he felt up to it. We’re not all on the same level.

What would you suggest? I’m not a fan of doing instanceOf-checks, but what’re you gonna do when you have a list of many objects in a room or inventory? You could have books the player can read, weapons you can equip, potions you can drink. Would you rather juggle several lists containing just one type of item each?

#12

What I would suggest is learning the java language. If you don’t know what “fields” are, the last thing you should be doing is dinking around with entity systems.

The problem of finding applicable actions on an equipment list is somewhat tricky … and is going to be made a whole lot more difficult if one doesn’t understand the most basic things like what fields are.