Asserts a method returns expected value

assert_isgrouped.dart

void main() {
  BvItem test1Item = BvItem(netSize: 1);
  BvItem test2Item = BvItem(netSize: 6);
  BvItem test3Item = BvItem(netSize: 7);
  BvItem test4Item = BvItem(netSize: 8);
  BvItem test5Item = BvItem(netSize: 14);
  BvItem test6Item = BvItem(netSize: 15);
  BvItem test7Item = BvItem(); // yes, null.
  
  num? netSizeA = isGrouped(test1Item) ?? (test1Item.netSize ?? -1);
  num? netSizeB = isGrouped(test2Item) ?? (test2Item.netSize ?? -1);
  num? netSizeC = isGrouped(test3Item) ?? (test3Item.netSize ?? -1);
  num? netSizeD = isGrouped(test4Item) ?? (test4Item.netSize ?? -1);
  num? netSizeE = isGrouped(test5Item) ?? (test5Item.netSize ?? -1);
  num? netSizeF = isGrouped(test6Item) ?? (test6Item.netSize ?? -1);
  num? netSizeG = isGrouped(test7Item) ?? (test7Item.netSize ?? -1);
  
  assert(netSizeA == 1); print('${test1Item.netSize} works');
  assert(netSizeB == 6); print('${test2Item.netSize} works');
  assert(netSizeC == 7); print('${test3Item.netSize} works');
  assert(netSizeD == 7); print('${test4Item.netSize} works');
  assert(netSizeE == 15); print('${test5Item.netSize} works');
  assert(netSizeF == 15); print('${test6Item.netSize} works');
  assert(netSizeG == -1); print('${test7Item.netSize} works');
}

class BvItem {
  final int? netSize;
  
  BvItem({this.netSize});
}

int? isGrouped(BvItem item) {
  if (item.netSize == null) return null;
  
  // the ! symbol here says, "I firmly believe this value will not be null. Trust me."
  // If it is null, we will crash.
  if (item.netSize! >= 7 && item.netSize! < 14) return 7;
  if (item.netSize == 14) return 15;
  
  return null;
}