Grouping and sorting nested collections in Groovy

Question

I have the following POGO:

class Widget {
    String name
    Integer order
    // lots of other fields

    Widget(Integer order) {
        super()
        this.order = order
    }
}

I have a List<Widget> and am trying to put them into a List<Set<Widget>> where they are grouped by the order field. So if I have 3 widgets with the following respective orders: { 3, 1, 2 } then I would have an outer List of size 3 and each element in that list would be a Set of size 1. I could manually produce this in code via:

Widget w1 = new Widget(3)
Widget w2 = new Widget(1)
Widget w3 = new Widget(2)

// Now sort them by order manually:
Set<Widget> firstOrderWidgets = []
firstOrderWidgets << w2 // order = 1

Set<Widget> secondOrderWidgets = []
secondOrderWidgets << w3 // order = 2

Set<Widget> thirdOrderWidgets = []
thirdOrderWidgets << w1 // order = 3

List<Set<Widget>> sortedCorrectly = []
sortedCorrectly << firstOrderWidgets    // All widgets w/ order = 1
sortedCorrectly << secondOrderWidgets   // All widgets w/ order = 2
sortedCorrectly << thirdOrderWidgets    // All widgets w/ order = 3

So the idea here is that the "unsorted/ungrouped" List<Widget> could be quite large, and many widgets could contain the same order. We want to group all widgets with the same order (order = 1, order = 2, etc.) into the same inner Set and then add those sets to the outer List in ascending order. So if we were to add a 4th widget to the example above:

Widget w4 = new Widget(2)

This widget belongs with the other 2nd order widget:

Set<Widget> secondOrderWidgets = []
secondOrderWidgets << w3 // order = 2
secondOrderWidgets << w4 // order = 2

So I'm trying to write a method that take List<Widget> as input, groups them by order and then sorts those groups/sets in ascending order. The order field is guaranteed to be non-null, but could be any valid positive (1+) integer. My best attempt is causing all sorts of runtime/dynamic exceptions:

List<Set<Widget>> sortWidgets(List<Widget> toSort) {
  def groupedByOrder = toSort.groupBy({ widget -> widget.order })

  groupedByOrder = groupedByOrder.sort()

  List<Set<Widget>> sortedList = []
  groupedByOrder.each { order, widgets ->
      sortedList << new HashSet(widgets)
  }

    sortedList
}

Can anyone spot where I'm going awry?

Answer 1

I couldn't manage to reproduce your error. It can happen due to how equals and hashCode are implemented in your Widget class. I managed to get a working example using @Canonical :

@groovy.transform.Canonical
class Widget {
    int order
    String toString() { "Widget(order=$order, ${hashCode()})" }
}

widget = { new Widget(order: it) }

w1 = widget(3)
w2 = widget(1)
w3 = widget(2)
w4 = widget(2)
w5 = widget(2)

allWidgets = [w4, w1, w2, w5, w3]

def sortWidgets(widgets) {
    widgets.sort(false) { it.order }.groupBy { it.order }.values() as List
}

assert sortWidgets(allWidgets) == [
    [w2],
    [w3, w4, w5],
    [w1]
]