Java HashMap/HashSet polymorphism

Question

I have an inheritance chain in which Superclass have 3 immediate subclasses, Subclass1, Subclass2, Subclass3.

I have a:

HashMap<Integer, HashMap<Integer, HashSet<Superclass>>> map = new HashMap<>();

I want map to contain 3 hashmaps at integer values 1, 2 and 3. These 3 hashmaps will each have values of HashSet that contain only one subclass of Superclass.

For instance map.get(1) should refer to

HashMap<Integer, HashSet<Subclass1>>

But I'm not allowed to add the above HashMap to map, because of a compiler error:

(actual argument HashMap<Integer, HashSet<Subclass1>> cannot be converted to
HashMap<String, HashSet<Superclass>> by method invocation conversion)

Before I answer the generics problem here. Why on earth would you have a HashMap if you intend to index it by numbers 1, 2 and 3? — Kayaman
– Kayaman, Commented Oct 2, 2013 at 12:56
Because I couldn't make an array of arrayLists.. and since conception my arrayLists became HashSets because I didn't want duplicates. So I opted to replace the unavailable array with a Map. Isn't that what mapping is for? associating keys (index) with certain values? — user2651804
– user2651804, Commented Oct 2, 2013 at 13:04
Why didn't you make an array of Maps? Where did you get ArrayLists into this one... It makes no sense to use a Map just to map 3 indexes. — Kayaman
– Kayaman, Commented Oct 2, 2013 at 13:12
It's quite a long time since I opted to make the HashMap, but when I did, I was definitely assured that making an array of ArrayLists was not possible. I assume the same is true for maps? I'll explain what I'm trying to do and you can tell me how you would do it: I have some hundred individual objects. I need a method to return only one of these based on 3 integer values. These hundred objects are put in 6 different categories, each category containing 3 subcategories, each subcategory containing a set amount of objects. — user2651804
– user2651804, Commented Oct 2, 2013 at 13:37
An array of ArrayLists is definitely possible, just like an array of any other class or interface. I'm not sure 3 levels of nested maps is the greatest way to store your items though. — Kayaman
– Kayaman, Commented Oct 2, 2013 at 13:43

Steven Mastandrea · Accepted Answer · 2013-10-02 13:00:31Z

2

If you want to be able to add subclasses into your HashSet at runtime, then you can declare your variable to be:

HashMap<Integer, HashMap<Integer, HashSet<? extends Superclass>>> map ...

answered Oct 2, 2013 at 13:00

Steven Mastandrea

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2 Comments

user2651804 Over a year ago

Well, that's what I stumbled upon in researching the answer.. but take a look at this thread: stackoverflow.com/questions/17008511/…

user2651804 Over a year ago

"new HashSet <? extends Vegetable> (); is not a working construction of course"

Kayaman · Accepted Answer · 2013-10-02 13:58:07Z

0

Since our OP doesn't seem to believe that you can create arrays of objects (mainly ArrayLists), let's go through some basics.

ArrayList<String>[] arrayOfArraylists = new ArrayList[10];
arrayOfArraylists[0] = new ArrayList<String>();

Amazing.

Now for the original problem.

Map<Integer, Map<Integer,HashSet<? extends SuperClass>>> map = new HashMap<>();
map.put(1, new HashMap<Integer, HashSet<? extends SuperClass>>());
map.get(1).put(2, new HashSet<SubClass>());

I don't know if I would want to layer the maps like this, or if I would bother to use the first map just for 3 items.

You could create an indexing class and have only one layer of maps.

Map<Triplet, HashSet<? extends SuperClass>> map = new HashMap<>();
map.put(new Triplet(1, 2, 3), new HashSet<SubClass>());

edited Oct 2, 2013 at 13:58

answered Oct 2, 2013 at 13:00

Kayaman

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