Count the occurrences of items in ArrayList

Question

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

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

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 counters = new HashMap(5000);
ArrayList items = new ArrayList(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..