Count the occurrences of items in ArrayList

Question

I have a java.util.ArrayList<Item> and an Item object.

Now, I want to obtain the number of times the Item is stored in the arraylist.

I know that I can do arrayList.contains() check but it returns true, irrespective of whether it contains one or more Items.

Q1. How can I find the number of time the Item is stored in the list?

Q2. Also, If the list contains more than one Item, then how can I determine the index of other Items because arrayList.indexOf(item) returns the index of only first Item every time?

Answer 1

You can use Collections class:

public static int frequency(Collection<?> c, Object o)

Returns the number of elements in the specified collection equal to the specified object. More formally, returns the number of elements e in the collection such that (o == null ? e == null : o.equals(e)).

If you need to count occurencies of a long list many times I suggest you to use an HashMap to store the counters and update them while you insert new items to the list. This would avoid calculating any kind of counters.. but of course you won't have indices.

HashMap<Item, Integer> counters = new HashMap<Item, Integer>(5000);
ArrayList<Item> items = new ArrayList<Item>(5000);

void insert(Item newEl)
{
   if (counters.contains(newEl))
     counters.put(newEl, counters.get(newEl)+1);
   else
     counters.put(newEl, 1);

   items.add(newEl);
 }

A final hint: you can use other collections framework (like Apache Collections) and use a Bag datastructure that is described as

Defines a collection that counts the number of times an object appears in the collection.

So exactly what you need..

Answer 2

This is easy to do by hand.

public int countNumberEqual(ArrayList<Item> itemList, Item itemToCheck) {
    int count = 0;
    for (Item i : itemList) {
        if (i.equals(itemToCheck)) {
          count++;
        }
    }
    return count;
}

Keep in mind that if you don't override equals in your Item class, this method will use object identity (as this is the implementation of Object.equals()).

Edit: Regarding your second question (please try to limit posts to one question apiece), you can do this by hand as well.

public List<Integer> indices(ArrayList<Item> items, Item itemToCheck) {
    ArrayList<Integer> ret = new ArrayList<Integer>();
    for (int i = 0; i < items.size(); i++) {
        if (items.get(i).equals(itemToCheck)) {
            ret.add(i);
        }
    }
    return ret;
}