CODEWITHSUNDEEP
javacollections
user1278265
user1278265

Reputation: 41

How to get the latest value of a duplicate key in a Map in java?

Following is my requirement: I have to count the occurrence of a particular item in a List and then print the item along with its occurrence count. For this I have the following code: 

public void countOccurrences(Collection<String> list){
      Map<String, Integer> occurrenceMap = new HashMap<String, Integer>();

      for(String obj: list){
           Integer numOccurrence = occurrenceMap.get(obj);
           if(numOccurrence == null){
                //first count
                occurrenceMap.put(obj, 1);
           } else{
                occurrenceMap.put(obj, numOccurrence++);

           }
      }
      for (Map.Entry<String, Integer> entry : occurrenceMap.entrySet()) {
            System.out.println(entry.getKey()+" : "+entry.getValue());
    }
  }

Now say for example items added to my list are as follows:

list.add("apple");
list.add("apple");
list.add("apple");
list.add("banana");
list.add("banana");
list.add("mango");

The output that I'm expecting is 

apple:3
banana:2
mango:1

However, the output that is coming is

apple:1
banana:1
mango:1

What am I missing here?

Upvotes: 1

Views: 82

Answers (4)

Developer
Developer

Reputation: 544

you can also use following piece of code to achieve your objective. 

public static void countOccurrences(Collection<String> list){
  Map<String, Integer> occurrenceMap = new HashMap<String, Integer>();
  Integer temp;
  for(String obj: list){
       if((temp = occurrenceMap.put(obj, 1))!= null){
            occurrenceMap.put(obj, ++temp);
       }
  }
  for (Map.Entry<String, Integer> entry : occurrenceMap.entrySet()) {
        System.out.println(entry.getKey()+" : "+entry.getValue());
    }
}

Upvotes: 0

Shiladittya Chakraborty
Shiladittya Chakraborty

Reputation: 4418

You can try in this way :

List<String> list = new ArrayList<String>();
list.add("apple");
list.add("apple");
list.add("apple");
list.add("banana");
list.add("banana");
list.add("mango");

Set<String> unique = new HashSet<String>(list);
for (String key : unique) {
 System.out.println(key + ": " + Collections.frequency(list, key));
}

Output: 

banana: 2
apple: 3
mango: 1

Upvotes: 0

Suresh Atta
Suresh Atta

Reputation: 122008

You need to change the logic in your else part

 occurrenceMap.put(obj, ++numOccurrence);

Because the post increment 

numOccurrence ++

Returns the value of numOccurrence and then increments numOccurrence's value by 1. Hence you are not putting the latest one in the map.

Upvotes: 2

Damien O&#39;Reilly
Damien O&#39;Reilly

Reputation: 992

I think you need to change

occurrenceMap.put(obj, numOccurrence++);

to

occurrenceMap.put(obj, numOccurrence + 1); or occurrenceMap.put(obj, ++numOccurrence);

numOccurrence++ will increment numOccurrence after the statement. And it will never be used as numOccurrence is reasigned next time the loop goes around.

Upvotes: 1

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