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Asked: 2026-05-25T06:27:27+00:00

What is wrong with private Map<List<K>, V> multiMap= new HashMap<ArrayList<K>,V>(); The complier says that

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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?

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1 Answer

  1. Editorial Team

    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).

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