How to track and increment the duplicates count using hashmap

Question

My program is a very simple one. All I need to do is track the duplicates and print the unique and the duplicate elements separately. I am using a hashmap for this. (dup is an arraylist containing all the elements)

    Map<Employee, Integer> newMap = new HashMap();
    int count = 0;
    for (Employee element : dup) {

        System.out.println("oooo" + element);
        if (newMap.put((Employee) element, count) != null) {
            newMap.put((Employee) element, newMap.get(element) + 1);
        }
    }
    System.out.println("oooo" + newMap);

The o/p generated is:

    Employee No :9 Employee Name :Swasti Employee MailId :swasti@gmail.com=0, Employee No :2 Employee Name :Shanthi Employee MailId :shanthi@gmail.com=0

The records with duplicates are incremented and the o/p is :

    Employee Name :Shreya Employee MailId :shreya@gmail.com=1, Employee Name :Sujatha Employee MailId :suravich@gmail.com=1

The problem here is that the count is not incrementing more than one for records with more number of duplicates. I don't understand why. Any sort of help on this would be appreciated.

P.S: I tried doing this with an arraylist and a hashset and did get an output but using a map seemed like a better way.

fge · Answer 1 · 2014-03-18T06:30:55.537

1

This is because with this line:

if (newMap.put((Employee) element, count) != null) {

you put 0 again each time (since count is always 0 in your code). And on the next line, your newMap.get(element) will therefore be 0...

You need to grab the old value and check that it is not null:

Integer oldValue = newMap.put((Employee) element, 0);
if (oldValue != null)
    newMap.put((Employee) element, oldValue + 1);

Note that your dup should be a List<Employee>, not a raw List. This would avoid all the casts you have to make currently.

edited Mar 18 '14 at 06:30

answered Mar 18 '14 at 06:25

fge

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The count also needs to start at 1. : ) – Radiodef Mar 18 '14 at 06:32
@Radiodef hmm, unclear as per the OP's question but I guess you are right – fge Mar 18 '14 at 06:36
Actually I think you're right, if the count is for number of duplicates it would start at 0. – Radiodef Mar 18 '14 at 06:37
@fge : This worked perfectly. Thank you. As you mentioned, the problem was with the `count=0` at all times. – suravich Mar 18 '14 at 07:00
@fge : Also, `dup` is of `List` type. I kept doing all sorts of edits and ended up casting it. Thanks for mentioning that. I've changed it. – suravich Mar 18 '14 at 07:05

score 0 · Answer 2 · answered Mar 18 '14 at 06:25

0

        if (newMap.containsKey(element)) {
            newMap.put(element, newMap.get(element) + 1);
        }

Also since you using generics you need to typecast.

answered Mar 18 '14 at 06:25

Aniket Thakur

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score 0 · Answer 3 · answered Mar 18 '14 at 06:25

0

if (newMap.get(element) != null) {
    newMap.put(element, newMap.get(element) + 1);
} else {
    newMap.put(element, 1);
}

answered Mar 18 '14 at 06:25

DmitryKanunnikoff

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I think the answer is quite sufficient. I have given to the author of the decision, and wanted to continue, he thought himself. – DmitryKanunnikoff Mar 18 '14 at 06:48

How to track and increment the duplicates count using hashmap

3 Answers3