Java: What is a good data structure for storing a coordinate map for an infinite game world?

Question

I am used to coding in PHP but I am not really proficient with Java and this has been a problem for some time now. I expect it to be a fairly easy solution, however I cannot find any good example code any way I search it, so here goes:

I am programming a game that takes place in a 2d random generated infinite world on a tile based map (nitpicking: I know it will not be truly infinite. I just expect the world to be quite large). The usual approach of map[x][y] multidimensional array started out as a basic idea, but since Java does not provide a way for non-integer (i.e. negative) array key shenanigans like PHP does, I cannot properly have a (-x,+x,-y,+y) coordinate system with array keys.

I need to be able to find the objects on a tile at a specific x,y coordinate as well as finding "adjacent tiles" of a certain tile. (Trivial if I can getObjectAt(x,y), I can get(x+1,y) and so on)

I have read about quad trees and R-trees and the like. The concept is exciting, however I haven't seen any good, simple example implementation in Java. And besides I am not really sure if that is what I need exactly.

Any advice is welcome

Thank you

Answer 1

1) Instead of an array you could use a Map<Integer, Map<Integer, Tile>> or Map<Point, Tile>, which would of course allow negative indexes

2) If you know the dimensions of your world from the start you could just modify your getter to allow the API to accept negatives and [linearly] transform them into positives. So for example if your world is 100x1000 tiles and you want (-5,-100), you would have WorldMap.getTile(-5,-100) which would translate to return tileArray[x+mapWidth/2][y+mapHeight/2]; which is (45,400)

Answer 2

I came to this thread with the same problem, but my solution was to use Map/HashMaps, but these are one dimensional.

To overcome this, instead of using a map within a map (which would be messy and very inefficient) I used a generic Pair class (not something that you'll find in the stock java library) although you could replace this with a Position class (virtually the same code, but not generic, instead integers or floats).

So when defining the map: Map<Pair, Tile> tiles = new HashMap<Pair, Tile>;

For placing tile objects onto the map I used tiles.put(new Pair(x, y), new GrassTile()); and for retrieving the object tiles.get(new Pair(x, y));.

[x/y would be any coordinate you wish to place (this allows negative coordinates without any mess!), "new GrassTile()" is just an example of placing a tile of a certain type during map creation. Obviously - as previously stated - the Pair class is replacable.]

Why not ArrayLists you may ask? Because array lists are much more linear than mapping, and in my opinion are more difficult to add and retrieve tiles, especially on 2 Dimensions.

Update:

For anyone wondering why there isn't a Pair() class in Java, here's an explanation.

Answer 3

Trees, Quad Trees, Binary trees, red and black trees - and all other kinds of trees are USELESS for you (unless you are planning to have a map with a huge forest).

Specialized data structures have their specific uses. Unless you can come up with a good reason why your game needs a spatial index, don't build one. If your typical scenario is "iterate over the visible area, find out what tile is visible at each of the squares", then you need a structure that gives you a quick, random, access to a value stored under a specific key. Such structure is a HashMap (what PHP uses is a kind of a LinkedHashMap, but you were probably not using the "linked" part).

You need to follow xephox's advice (and give him the credit), and that is:

• make a class that describes a location (Pair, Location, Point, whatever);
• make all the fields (probably x and y) final. It is important that a location itself cannot change (it will make your life MUCH easier);
• generate equals and hashcode methods (every IDE will help you with that. Remember that the implementations MUST use both x and y - a wizard in your IDE will help you);
• your map will be: Map map = new HashMap();

The best thing: if you keep using the Map interface, you will not be locked out, and you will be able to make a lot of improvements. Like wrapping the HashMap into an object that creates parts of the map using some algorithmic techniques.