I have a List of object as List<Human> humanList;
which human object consist of 5 variable as firstname, lastname,color,length,sex.
How can i check out to see the variable(firstname) how many times appear/exist in the list?
List<Human> humanList = getHumanList();
for(Human human :humanList){
}
Assuming here firstname is of type String, if it is not - change it as needed.
Create a Map<String,Integer> (let it be map) which will be used as a histogram.
Iterate the list, and for each human:
Integer temp = map.get(human.firstName); //search for the current number of occurances
if (temp == null) map.put(human.firstName, 1); //first occurance of this firstname
else map.put(human.firstName,temp+1); //not first occurance, modify the map.
When you are done map holds each firstName as a key and the number of its occurances as the matching value.
You can use guava multiset count method http://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained.
Multiset<Human> set = HashMultiset.create();
set.count(Human.firstName);
You need to traverse the list counting occurences O(N).
A better option if you need this often would be to increment a counter as you add/remove from the list and use that instead of traversing
