可以在Hashmap java中将Collection用作键吗？

Question

I have following scenario (modified one than actual business purpose).

1. I have a program which predicts how much calories a person will loose for the next 13 weeks based on certain attributes.
2. I want to cache this result in the database so that i don't call the prediction again for the same combination.
3. I have class person class Person { int personId; String weekStartDate; } class Person { int personId; String weekStartDate; }
4. I have HashMap<List<Person>, Integer> - The key is 13 weeks data of a person and the value is the prediction
5. I will keep the hashvalue in the database for caching purpose

Is there a better way to handle above scenario? Any design pattern to support such scenarios

Answer 1

Depends: the implementation of hashCode() uses the elements of your list. So adding elements later on changes the result of that operation:

public int hashCode() {
    int hashCode = 1;
    for (E e : this)
        hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
    return hashCode;
}

Maps aren't build for keys that can change their hash values! And of course, it doesn't really make sense to implement that method differently.

So: it can work when your lists are all immutable , meaning that neither the list nor any of its members is modified after the list was used as key. But there is a certain risk: if you forget about that contract later on, and these lists see modifications, then you will run into interesting issues.

Answer 2

This works because the hashcode of the standard List implementations is computed with the hashcodes of the contents. You need to make sure, however, to also implement hashCode and equals in the Person class, otherwise you will get the same problem this guy had . See also my answer on that question.

Answer 3

I would suggest you define a class (say Data ) and use it as a key in your hashmap. Override equals/hashcode accordingly with knowledge of data over weeks.