java.lang.IndexOutOfBoundsExce...

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indexes

Hi folks, hoping someone can help.

I have an arraylist of Integers. When there is a button click the data in the text field gets converted and added to the array list as an int. You can see, once there has been ten submissions (counter takes care of that) it calls the function train();. Here is my code for that below

public void onClick(View v) {
            // TODO Auto-generated method stub

            String rate = happy.getText().toString();
            int rating = Integer.valueOf(rate);
            counter += 1;

            if (counter == 10) {
                train();
            }

            if (rating > 0 && rating < 11) {
                happyRating.add(rating);

                test = happyRating.get(0);
                Log.d("Value: ", Integer.toString(test));

            } else {

                InputAlertHandler.post(new Runnable() {
                    public void run() {
                        int duration = Toast.LENGTH_LONG;
                        CharSequence text = "Please enter Happiness Indicator between 1 and 10";
                        Context context = getApplicationContext();
                        inputAlert = Toast
                                .makeText(context, text, duration);
                        inputAlert.show();
                    }
                });
            }
        }

In the function train(); I am getting data from this array list and using it in calculations.

public void train() {

    for (int i = 2; i < 12; i++) {

        if (happyRating.get(i - 1) < happyRating.size()){

            int x, x1, x2, y, y1, y2;
            double learningRate = -0.00002;

            x1 = happyRating.get(i - 1);
            x2 = happyRating.get(i - 2);
            y1 = iteration[i - 1];
            y2 = iteration[i - 2];

            x = x2 - x1;
            y = y2 - y1;

            if (x == 0) {
                slope = 0;
            } else {
                slope = (y2 - y1) / (x2 - x1);
            }
            j++;

            Log.d("J:", + Integer.toString(j));

            double weightAdj = happyRating.get(j) * slope * learningRate;

            weighting = (weighting + weightAdj);

            Log.d("WEIGHTING:", + Double.toString(weighting));

        }
        else{
            break;
        }
    }

When the train() gets called I get the error. Now I know this looks strange and you'll ask why set i=2 and i<12, the reason for this is I need to calculate the slope of the two previous points, and use [i-2] and [i-1] to get those points, if I had i = 0 then had [i-1] there would be an error. I know why I'm getting the index error because when i hits 11 and i-1 = 10 and there's no index ten in the array list. However I thought that by including my check

if (happyRating.get(i - 1) < happyRating.size())

That this would handle that error. I was obviously wrong.
Could anyone suggest an alternative to my method?

Regards,
Gary