私人地图 <List<K> ，V&gt; multiMap = new HashMap <ArrayList<K> ，V&gt;（）;

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?

Answer 1

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 ).

Answer 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.

Answer 3

试试：

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

Answer 4

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.