6680235: (collection) Object count method.

[Leszek Sliwko]

Hello,


I hope I'm on right dev list and I'm posting it as it supposed to be posted.

There is a RFE:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6680235

"A DESCRIPTION OF THE REQUEST :
Java Lists currently do not have a way of getting the count of a
particular  customer  within a List.

JUSTIFICATION :
I ran into this issue when I had a list of objects in which it
contained some duplicates, and I needed to know the number of
occurrences of specific objects within the list. Basically i built a
list of referenced objects from a list of objects and needed the
result of how many times a specific referenced  customer  was in this
built list, for later use.


I needed something like...   myList.elementCount(obj);
where elementCount() returns int number of occurrences or -1 if it
does not exist in the list, and obj is the assumed element in the
list.

I was able to continue by creating a custom iterator, but the above
alternative would have been a quicker solution.


Posted Date : 2008-03-26 12:31:26.0"


I see we can do the following:

1) add a method to java.util.AbstractCollection:

    /**
     * Returns the amount of specified element in this collection. If the
     * collection contains more than <tt>Integer.MAX_VALUE</tt> occurrences
of
     * specified element, returns <tt>Integer.MAX_VALUE</tt>.
     *
     * This implementation iterates over the elements in the collection,
     * checking each element in turn for equality with the specified
element.
     *
     * @param o object to be counted for in this collection.
     * @return the amount of specified element in this collection.
     */
    public int elementCount(Object o) {
    Iterator e = iterator();
    int count = 0;
    if (o==null) {
        while (e.hasNext()) {
        if (e.next()==null) {
            if (count<Integer.MAX_VALUE) {
                count++;
            } else break;
        }
        }
    } else {
        while (e.hasNext()) {
        if ((o.equals(e.next())&&(count<Integer.MAX_VALUE))) {
            if (count<Integer.MAX_VALUE) {
                count++;
            } else break;
        }
        }
    }
    return count;
    }


2) add a method definition to interface java.util.Collection:

    /**
     * Returns the amount of specified element in this collection. If the
     * collection contains more than <tt>Integer.MAX_VALUE</tt> occurrences
of
     * specified element, returns <tt>Integer.MAX_VALUE</tt>.
     *
     * This implementation iterates over the elements in the collection,
     * checking each element in turn for equality with the specified
element.
     *
     * @param o object to be counted for in this collection.
     * @return the amount of specified element in this collection.
     */
    public int elementCount(Object o);


3) The java.util.AbstractSet can be optimized:

    public int elementCount(Object o) {
        if (contains(o)) {
             return 1;
        } else return 0;
    }




any inputs/thoughts?

Thanks


-- 
All of best,
Leszek
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