Re: Determine index from array reference?

From:

Lew <lew@nowhere.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 24 Jan 2007 22:32:50 -0500

Message-ID:

<ocudnRga9tx-uSXYnZ2dnUVZ_sCinZ2d@comcast.com>

Knute Johnson wrote:

Is there a way to determine the index of an array element given a
reference?

Patricia Shanahan wrote:

This type of question always seems a little backwards to me. I tend to
think the other way round. Not "How do I do this with an array?" but "I
need to do these accesses. What data structure should I use?".

Why an array? What other operations are being done on it?

Knute Johnson wrote:

I asked about an array because that's what I have now. An array of
JTextFields that have ActionListeners attached. In the AL I need to
update another array. So what I had done in the past was to extend
JTextField and add an int variable to hold an index value for the
JTextField. I added a getIndex() method and in my ActionListener I use
that method to acquire the index to modify my other array.

So I could have checked the JTextField reference against all the others
in the array and gotten an index that way but that didn't sound a whole
lot better than the way I was getting it now.

That is exactly what your posted code does.

The little program below appears to work with a Vector. What do you
think of that approach?

Why ignore the advice to use ArrayList?

public class test1 {
    public static void main(String[] args) {
        Vector<JTextField> v = new Vector<JTextField>();
        JTextField test = null;

        for (int i=0; i<10; i++) {
            JTextField tf = new JTextField(" ",10);
            v.add(tf);
            if (i == 3)
                test = tf;
        }

        System.out.println(v.indexOf(test));
        System.out.println(v.indexOf(null));
    }
}

Bear in mind that these are first occurrences of these values in the List.

If you plan to use the result of your "indexOf()" to locate another object,
rather than just println() it, you might consider using a
Map <JTextField, OtherType>. That would have the benefit of constant time
lookup (if you use HashMap) instead of O(n). You also avoid bugs caused by
"parallel" Lists going non-Euclidean.

- Lew