Count how many times a key value appears in a map?(JAVA)

Question

I am trying to count how many times a key value appears in a map. What i would like to do is count and return the number of times a duplicate name appears and the gender in the JSON array and return it. So with the data below it should return:

Female: 2 Male: 2

with 2 being the amount of times a duplicate name appears.

So far i have read my JSON data from a file into java objects through the use of ObjectMappers and parsers. But not sure how to go about counting the duplicate names and putting it into a variable of a Gender. I have tried using a statement to match a string (map.get(i).get("fullName") == Williams) but it didnt work. Anyone got any ideas or know of a better way to do this. Thanks

My code:

        ObjectMapper mapper = new ObjectMapper();
        JSONParser parser = new JSONParser();

        JSONObject jsonobj = (JSONObject) parser.parse(new FileReader("data.json"));    
        JSONArray jsons = (JSONArray) jsonobj.get("Person");
        //Map<Integer, Map<String, String>> jsonmaps = new HashMap<Integer, Map<String, String>>();

        int i = 0;
        List<Map<String, String>> map = new ArrayList();
            for (Object j : jsons) {
                map.add(mapper.readValue(j.toString(), new TypeReference<Map<String, String>>() {}));
                i++;
            }
        // for testing purposes
        //System.out.println(map.get(1).get("fullName"));
        //System.out.println(map.get(1).get("age"));
        //System.out.println(map.get(1).get("Gender"));
        //System.out.println(map.get(1).get("adult"));
        //System.out.println(map.get(1).get("dob"));
}

JSON file:

{
      "fullName": "Bob Wilkins",
      "age": 25,
      "gender": "male"
},
{
      "fullName": "Hannah Williams",
      "age": 49,
      "gender": "Female",
      "isAdult": true,
      "dob": 29/12/1968
},
{
      "fullName": "Charlie Smith",
      "age": 10,
      "gender": "Male",
      "isAdult": false,
      "dob": 21/05/2008
},
{
      "fullName": "Hannah Williams",
      "age": 31,
      "gender": "Female",
      "isAdult": true,
      "dob": 13/02/1987
},
{
      "fullName": "Charlie Smith",
      "age": 25,
      "gender": "Male",
      "isAdult": true,
      "dob": 08/08/1994
},
..... 

Answer 1

The following does a grouping on gender , then groups each collection by fullName . The result is a Map<String, Long> , where values have been limited to those greater than 1, then added.

Map<String, Long> counts = 
        map.stream()
        .collect(Collectors.groupingBy(m -> m.get("gender")))
        .entrySet()
        .stream()
        .map(entry -> new SimpleEntry<>(entry.getKey(),
                entry.getValue().stream()
                        .collect(Collectors.groupingBy(m -> m.get("fullName"), 
                                                       Collectors.counting()))
                        .values().stream()
                                 .mapToLong(l -> l)
                                 .filter(value -> value > 1).sum()))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

When maps is the test data in the question, the result is:

{Male=2, Female=2, male=0}

So this can be run on your map list.