I have piece of code I use to generate charts:
HashMap<String, List<Ticket>> openedTicketsPerTeam = getOpenedTicketsPerTeam();
How should I write signature of generic method to count length of list, every Map element? My idea was:
Map<String, Integer> getNumOfValuesPerKey(HashMap<String, List<? extends Object>> map) {...}
but after invocation there's conversion error:
HashMap<String, Integer> numOfTicketsPerTeam = getNumOfValuesPerKey(openedTicketsPerTeam );
Something like this (could be used for any key K and any list of Vs):
public static <K, V> Map<K, Integer> getNumOfValuesPerKey(Map<K, List<V>> map) {
Map<K, Integer> count = new HashMap<K, Integer>();
for (Entry<K, List<V>> entry : map.entrySet())
count.put(entry.getKey(), entry.getValue().size());
return count;
}
public static void main(String[] args) {
HashMap<String, List<Integer>> map = new HashMap<String, List<Integer>>();
map.put("Hello", Arrays.asList(1,2,3));
map.put("World", Arrays.asList(4,5));
System.out.println(getNumOfValuesPerKey(map));
}
Output:
{Hello=3, World=2}
Try this:
Map<String, Integer> numOfTicketsPerTeam = getNumOfValuesPerKey(openedTicketsPerTeam );
Your method (correctly) returns Map<String, Integer>, but you are assigning it to a HashMap<String, Integer> - there are multiple types of Map, the compiler has no way of knowing your implementation returns a HsshMap.
Note that you can, and should, simplify your method signature to:
Map<String, Integer> getNumOfValuesPerKey(Map<String, List<?>> map) {...}
Changes are to make the parameter a Map (not a HashMap) and List<?> (not List<? extends Object>, which is the same thing but more cruft)
No need to parameterize over the value type actually. The problem you had with Map<K, List<?>> is that non-wildcard generic parameters must match exactly. So in the second type parameter of Map, List<?> does not match List<Ticket>. The correct thing to do is to make that a wildcard also (with the added benefit of being able to work with implementing classes e.g. ArrayList as the parameter):
public static <K> Map<K, Integer> getNumOfValuesPerKey(Map<K, ? extends List<?>> map) {
Map<K, Integer> count = new HashMap<K, Integer>();
for (Map.Entry<K, ? extends List<?>> entry : map.entrySet())
count.put(entry.getKey(), entry.getValue().size());
return count;
}
