How to count the number of occurrences of an element in a List

Question

I have an ArrayList, a Collection class of Java, as follows:

ArrayList animals = new ArrayList();
animals.add(\"bat\         


        

Answer 1

Put the elements of the arraylist in the hashMap to count the frequency.

Answer 2

List<String> lst = new ArrayList<String>();

lst.add("Ram");
lst.add("Ram");
lst.add("Shiv");
lst.add("Boss");

Map<String, Integer> mp = new HashMap<String, Integer>();

for (String string : lst) {

    if(mp.keySet().contains(string))
    {
        mp.put(string, mp.get(string)+1);

    }else
    {
        mp.put(string, 1);
    }
}

System.out.println("=mp="+mp);

Output:

=mp= {Ram=2, Boss=1, Shiv=1}

Answer 3

To get the occurrences of the object from the list directly:

int noOfOccurs = Collections.frequency(animals, "bat");

To get the occurrence of the Object collection inside list, override the equals method in the Object class as:

@Override
public boolean equals(Object o){
    Animals e;
    if(!(o instanceof Animals)){
        return false;
    }else{
        e=(Animals)o;
        if(this.type==e.type()){
            return true;
        }
    }
    return false;
}

Animals(int type){
    this.type = type;
}

Call the Collections.frequency as:

int noOfOccurs = Collections.frequency(animals, new Animals(1));

Answer 4

​If you use Eclipse Collections, you can use a Bag. A MutableBag can be returned from any implementation of RichIterable by calling toBag().

MutableList<String> animals = Lists.mutable.with("bat", "owl", "bat", "bat");
MutableBag<String> bag = animals.toBag();
Assert.assertEquals(3, bag.occurrencesOf("bat"));
Assert.assertEquals(1, bag.occurrencesOf("owl"));

The HashBag implementation in Eclipse Collections is backed by a MutableObjectIntMap.

Note: I am a committer for Eclipse Collections.

Answer 5

I didn't want to make this case more difficult and made it with two iterators I have a HashMap with LastName -> FirstName. And my method should delete items with dulicate FirstName.

public static void removeTheFirstNameDuplicates(HashMap<String, String> map)
{

    Iterator<Map.Entry<String, String>> iter = map.entrySet().iterator();
    Iterator<Map.Entry<String, String>> iter2 = map.entrySet().iterator();
    while(iter.hasNext())
    {
        Map.Entry<String, String> pair = iter.next();
        String name = pair.getValue();
        int i = 0;

        while(iter2.hasNext())
        {

            Map.Entry<String, String> nextPair = iter2.next();
            if (nextPair.getValue().equals(name))
                i++;
        }

        if (i > 1)
            iter.remove();

    }

}

Answer 6

Actually, Collections class has a static method called : frequency(Collection c, Object o) which returns the number of occurrences of the element you are searching for, by the way, this will work perfectly for you:

ArrayList<String> animals = new ArrayList<String>();
animals.add("bat");
animals.add("owl");
animals.add("bat");
animals.add("bat");
System.out.println("Freq of bat: "+Collections.frequency(animals, "bat"));