private Map, V> multiMap= new HashMap,V>();

Question

What is wrong with

private Map<List<K>, V> multiMap= new HashMap<ArrayList<K>,V>();

The complier says that it Type mismatch: cannot convert from HashMap<ArrayList<K>,V> to Map<List<K>,V>. Do I have to give a specific class of List? Why?

Generics is more pedantic than regular Java casts. You cannot up cast a generic like you can with a regular expression. — Peter Lawrey, Sep 05 '11 at 09:09
Where the answers helpful? If so please accept one of them. If not, you can provide further details to help people answering the question. — Philipp Wendler, Sep 15 '11 at 08:18

score 14 · Accepted Answer · answered Sep 05 '11 at 09:05

You have to write

private Map<List<K>, V> multiMap= new HashMap<List<K>,V>();

The values of the generic parameters have to match exactly on both sides (as long as you don't use wildcards). The reason is that Java Generics do not have contra-/covariance (HashMap<List> is not a supertype of HashMap<ArrayList>, although List of course is a supertype of ArrayList).

score 2 · Answer 2 · edited May 23 '17 at 11:43

2

This is because a Map<ArrayList> is not a Map<List>, even though ArrayList is a List. Although this sounds counterintuitive, there is a good reason for this. Here is a previous answer of mine to a similar question, which explains the reasons behind this in more detail.

edited May 23 '17 at 11:43

Community

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answered Sep 05 '11 at 09:07

Péter Török

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score 2 · Answer 3 · answered Sep 05 '11 at 09:07

2

Try :

private Map<? extends List<K>, V> multiMap= new HashMap<ArrayList<K>,V>();

answered Sep 05 '11 at 09:07

Jean Logeart

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score 2 · Answer 4 · answered Sep 05 '11 at 09:24

BTW: If you want a Multimap, I am pretty sure you want a Map<K, List<V>> and not a Map<List<K>>, V>. Lists make miserable Hash keys, and I can't think of any usage where it would make sense to use the List as key and a single Object as value. You should re-think your design.

private Map, V> multiMap= new HashMap,V>();

4 Answers4