How at a time one row can expand in <p:dataTable>?

As of 2015, and this question is first in Google search results, I want to add that for PrimeFaces 5.1, there is dataTable attribute rowExpandMode, when set to single - allows only one row to be shown. Example:

<p:dataTable var="line" value="#{bean.lines}" rowExpandMode="single">

It's not exactly what was asked, but I hope that it'll help to others.

You can use (I have tested it in mojarra 2.1.20 and Primefaces 3.5 and it works fine) the following solution which calls a JavaScript function when the row is expanded. When clicking on a second row, and there is another expanded row, it will trigger a click event, which will in turn collapse the previously opened row.

xhtml:

<p:ajax event="rowToggle" onstart="test();"/>  

Javascript:

<script type="text/javascript">
    function test(){
        var i = $('.ui-row-toggler.ui-icon-circle-triangle-s').length;
        if(i == 1){return;}
            $('.ui-row-toggler.ui-icon-circle-triangle-s').trigger('click');
    }
</script>