Filter.addGreaterThan(prop, number) throws exception

Filter filter = hstQuery.createFilter();
filter.addGreaterThan("myhippoproject:availability", 0); // in order to query only available products
hstQuery.setFilter(filter);
HstQueryResult result = hstQuery.execut();

    public String getStringValue(Object value) throws FilterException{
        if(value instanceof String || value instanceof Boolean) {
            return "'" + value.toString() + "'";
        } else if(value instanceof Long || value instanceof Double) {
            return value.toString();
        } else if(value instanceof Calendar){
            return DateTools.createXPathConstraint(session, (Calendar)value);
        } else if(value instanceof Date){
            Calendar cal = new GregorianCalendar();
            cal.setTime((Date)value);
            return DateTools.createXPathConstraint(session, cal);
        }
        throw new FilterException("Unsupported Object type '"+value.getClass().getName()+"' to query on.");
    }