Question

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 );

Answer 1

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;
}

---

Example:

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}