java泛型方法调用转换错误

Question

我有一段用于生成图表的代码:

HashMap<String, List<Ticket>> openedTicketsPerTeam = getOpenedTicketsPerTeam();

我应该如何编写通用方法的签名来计算每个Map元素的列表长度？ 我的想法是:

Map<String, Integer> getNumOfValuesPerKey(HashMap<String, List<? extends Object>> map) {...}

但调用后出现转换错误:

HashMap<String, Integer> numOfTicketsPerTeam = getNumOfValuesPerKey(openedTicketsPerTeam );

Answer 1

类似这样的东西(可用于任何键K和任何V列表):

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;
}

---

示例:

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));
}

输出:

{Hello=3, World=2}