Mortgage/Loan Overpayment Calculator

A few weeks back I was talking with someone who was looking into over-paying a loan. The premise is that, provided your lender allows overpayments, by over-paying you can save a fortune in interest payments.

The amount you can save isn't all that straight-forward to work out, due to the miracle that is compound interest. I knocked up a quick overpayment calculator using gwt.progressive, which I'm hosting at Amazon S3 with a registered domain name.

Here's one of the classes that's being bound - or "activated" if you like - by gwt.progressive:

public class OverpaymentSummary extends Composite {
    interface MyActivator extends WidgetActivator<OverpaymentSummary>{}
    MyActivator activator = GWT.create(MyActivator.class);

    @RuntimeUiWidget(id="saving") InlineLabel saving;
    @RuntimeUiWidget(id="years") InlineLabel years;
    @RuntimeUiWidget(id="months") InlineLabel months;

    public OverpaymentSummary(Element anElement) {
        initWidget(activator.activate(this, anElement));
    }

    public void setResults(
        Money aStandardTotal, int aStandardMonths, 
        Money anOverpaidTotal, int anOverMonths) {
        saving.setText(aStandardTotal.minus(anOverpaidTotal).toString());

        int _monthsSaved = aStandardMonths - anOverMonths;
        setYears(_monthsSaved/12);
        setMonths(_monthsSaved - ((_monthsSaved/12)*12));

        removeStyleName("hidden");
    }

    private void setYears(int aYears) {
        years.setText(""+aYears);
    }

    private void setMonths(int aMonths) {
        months.setText(""+aMonths);
    }
}

Which is bound onto html that looks like this:

<div class="summary hidden">By overpaying you could save
<span id="saving">???</span>, and reduce your mortgage 
term by <span id="years">?</span> years and 
<span id="months">?</span> months!</div>

Notice the <span> elements with the id's "saving", "years", and "months" - these are bound automatically to the three InlineLabel's annotated with @RuntimeUiWidget.