Sorting and group two nested lists in Java based on a field in the inner list

Question

Java 8 here. I have the following classes:

@Getter
@Setter
public class InventoryItem {

  // For all instances inside the same manufacturer, this value will be the same
  private String manufacturer;
  private String model;
  private BigDecimal price;

}

public class Inventory extends ArrayList<InventoryItem> {
}

public class InventoryProcessor {

  public void processInventoriesForAllManufacturers() {

    // Essentially get a List<List<InventoryItem>>, where in the
    // inner list is a list of all InventoryItems for a particular
    // manufacturer, and the outer list is a list of all manufacturer's
    // inventories
    List<Inventory> inventoriesForAllManufacturers =
      inventoryService.getAllInventories();

    // Now I need to sort this list based on manufacturer name, but how???

  }

}

So the problem here is, that inventoryService.getAllInventories(), might return the equivalent of:

public List<Inventory> getAllInventories() {

  List<Inventory> allInventories = new ArrayList<>();

  List<InventoryItem> manufacturer1Inv = new ArrayList<InventoryItem>();
  InventoryItem item1 = new InventoryItem(
    "Lorem Ipsum Inc", "FK10", BigDecimal.valueOf(1.99));
  InventoryItem item2 = new InventoryItem(
    "Lorem Ipsum Inc", "FK11", BigDecimal.valueOf(9.99));
  manufacturer1Inv.add(item1);
  manufacturer1Inv.add(item2);

  List<InventoryItem> manufacturer2Inv = new ArrayList<InventoryItem>();
  InventoryItem item3 = new InventoryItem(
    "Ice Station Zebra Associates", "00djd94", BigDecimal.valueOf(12.99));
  InventoryItem item4 = new InventoryItem(
    "Ice Station Zebra Associates", "00djd95", BigDecimal.valueOf(22.99));
  List<InventoryItem> manufacturer2Inv = new ArrayList<InventoryItem>();
  manufacturer2Inv.add(item3);
  manufacturer2Inv.add(item4);

  List<InventoryItem> manufacturer3Inv = new ArrayList<InventoryItem>();
  InventoryItem item5 = new InventoryItem(
    "ACME Corp", "1234A", BigDecimal.valueOf(2.99));
  InventoryItem item6 = new InventoryItem(
    "ACME Corp", "1234B", BigDecimal.valueOf(6.99));
  List<InventoryItem> manufacturer3Inv = new ArrayList<InventoryItem>();
  manufacturer3Inv.add(item5);
  manufacturer3Inv.add(item6);

  Inventory m1Inv = new Inventory(manufacturer1Inv);
  Inventory m2Inv = new Inventory(manufacturer2Inv);
  Inventory m3Inv = new Inventory(manufacturer3Inv);

  allInventories.add(m1Inv);
  allInventories.add(m2Inv);
  allInventories.add(m3Inv);

  return allInventories;

}

All InventoryItems belonging to the same manufacturer will have the same exact manufacturer value.

So, given this List<Inventory>, how could I sort it based on the unique manufacturer values of the inner InventoryItems?

Meaning, a resulting list that would contain elements in the following order (given the example above):

"ACME Corp", "1234A", BigDecimal.valueOf(2.99));
"ACME Corp", "1234B", BigDecimal.valueOf(6.99));
"Ice Station Zebra Associates", "00djd94", BigDecimal.valueOf(12.99));
"Ice Station Zebra Associates", "00djd95", BigDecimal.valueOf(22.99));
"Lorem Ipsum Inc", "FK10", BigDecimal.valueOf(1.99));
"Lorem Ipsum Inc", "FK11", BigDecimal.valueOf(9.99));

My best attempt thus far:

  public void processInventoriesForAllManufacturers() {

    // Essentially get a List<List<InventoryItem>>, where in the
    // inner list is a list of all InventoryItems for a particular
    // manufacturer, and the outer list is a list of all manufacturer's
    // inventories
    List<Inventory> inventoriesForAllManufacturers =
      inventoryService.getAllInventories();

    // Now I need to sort this list based on manufacturer name, but how???
    List<Inventory> sorted = inventoriesForAllManufacturers.stream().map(inv ->
        Collections::sort(in)).collect(Collectors.toList());

  }

Produces the compilter error "Object is not a functional interface." Any ideas where I'm going awry?

Again, they only need to be grouped + sorted alphabetically by the manufacturer name; within each manufacturer's inventory, they should appear in order of insertion onto the list.

Ousmane D. · Accepted Answer · 2019-11-22 20:35:24Z

2

First, Inventory shouldn't extend ArrayList. It should have a List<InventoryItem>.

Sorting by manufacturer should be as easy as

list.sort(Comparator.comparing(Inventory::getManufacturer));

All you need for that to work is to have a getManufacturer() method in Inventory. Since all the items of an Inventory have the same manufacturer, all you need to return from this method is the manufacturer of the first item.

You should of course decide if it's legal for an Inventory to have no item at all, and decide how to place such inventories in your ordeing.

edited Nov 22, 2019 at 20:35

Ousmane D.

56.6k8 gold badges101 silver badges134 bronze badges

answered Nov 22, 2019 at 20:32

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Sorting and group two nested lists in Java based on a field in the inner list

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related