How to group objects from a list which can belong to two or more groups?

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I have a list of Items where each Item can belong to one or more category. For a limited set of categories(string) I want to create a map with category as key and list of Items as value.

Assume my Item class is defined as shown below:

public static class Item{
    long id;
    List<String> belongsToCategories;

    public List<String> getBelongsToCategories() {
        return belongsToCategories;
    }

    public void setBelongsToCategories(List<String> belongsToCategories) {
        this.belongsToCategories = belongsToCategories;
    }

    public Item(long id,List<String> belongsToCategories) {
        this.id = id;
        this.belongsToCategories = belongsToCategories;
    } 

    @Override
    public String toString() {
        return "Item{" + "id=" + id + '}';
    }        
}

and a list of Items:

public static void main(String[] args) {
    List<Item> myItemList   = new ArrayList<>();

    myItemList.add(new Item(1,Arrays.asList("A","B")));
    myItemList.add(new Item(2,Arrays.asList("A","C")));
    myItemList.add(new Item(3,Arrays.asList("B","C")));
    myItemList.add(new Item(4,Arrays.asList("D")));
    myItemList.add(new Item(5,Arrays.asList("D","E")));
    myItemList.add(new Item(6,Arrays.asList("A","F")));

    Map<String,List<Item>> myMap= new HashMap<>();

How can i fill myMap from myList?

I thought Stream API could help, but I don't know which classifier to put in the groupingBy method when an Item can belong to one or more categories

 myItemList.stream().collect(Collectors.groupingBy(classifier));

This

myItemList.stream().collect(Collectors.groupingBy(Item::getBelongsToCategories));

produces

[D, E]=[Item{id=5}]
[B, C]=[Item{id=3}]
[A, B]=[Item{id=1}]
[D]=[Item{id=4}]
[A, C]=[Item{id=2}]
[A, F]=[Item{id=6}]

expected is tough something like:

A=[Item{id=1}, Item{id=2}, Item{id=6}]
B=[Item{id=1}, Item{id=3}]
C=[Item{id=2}, Item{id=3}]
D=[Item{id=4}, Item{id=5}]
E=[Item{id=5}]
F=[Item{id=6}]

You can use flatMap to map to SimpleEntry and then groupingBy as :

return items.stream()
        .flatMap(p -> p.getBelongsToCategories()
                .stream()
                .map(l -> new AbstractMap.SimpleEntry<>(l, p)))
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.mapping(Map.Entry::getValue,
                        Collectors.toList())));

Blender 2.8: How to Group Objects – Simply Explained, In Blender, a large scene with many objects can make it easy to lose track of things. Organizational Needs; Grouping: Demystified; Locating Groups; Moving Objects into a Collection Note that it's also possible to place objects in more than one group, if need be. Select “Move to Collection” from the list. At the bottom of the worksheet, name the category that these items belong in. Sorting Objects into Groups - Sort the pictures into three groups and write each objects name under the appropriate category in the table. You Be the Referee - Categorize the items by placing an A for animal, F for food, S for sports, or C for camping in the empty box.

You can use groupByEach from Eclipse Collections.

Multimap<String, Item> itemsByCategory =
        ListIterate.groupByEach(myItemList, Item::getBelongsToCategories);

System.out.println(itemsByCategory);

Output:

{D=[Item{id=4}, Item{id=5}], 
E=[Item{id=5}], 
F=[Item{id=6}], 
A=[Item{id=1}, Item{id=2}, Item{id=6}], 
B=[Item{id=1}, Item{id=3}], 
C=[Item{id=2}, Item{id=3}]}

You can also use the Collectors2 utility class and groupByEach Collector with a Java Stream.

Multimap<String, Item> itemsByCategory = myItemList.stream().collect(
        Collectors2.groupByEach(
                Item::getBelongsToCategories,
                Multimaps.mutable.list::empty));

Note: I am a committer for Eclipse Collections.

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Here is the solution by ListMultimap, either from Google Guava or my library Abacus-Util

ListMultimap<String, Item> result = Multimaps.newListMultimap(new HashMap<>(), () -> new ArrayList<>()); // by Google Guava.
// Or result = N.newListMultimap(); // By Abacus-Util
items.forEach(item -> item.getBelongsToCategories().forEach(c -> result.put(c, item)));

I think it will be a little more efficient and concise without creating the unnecessary temporary Entry Objects by flatMap.

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Group By: split-apply-combine — pandas 0.25.0.dev0+752 , By “group by” we are referring to a process involving one or more of the return a sensibly combined result if it doesn't fit into either of the above two categories. On a DataFrame, we obtain a GroupBy object by calling groupby() . unique groups and corresponding values being the axis labels belonging to each group. Microsoft 365 Groups is the cross-application membership service in Office 365. At the basic level, a Microsoft 365 Group is an object in Azure Active Directory with a list of members and a loose coupling to related workloads including a SharePoint team site, Yammer Group, shared Exchange mailbox resources, Planner, Power BI and OneNote.

Grouping-class function, We call grouping an arbitrary mapping from a collection of NO objects to a Objects mapped to a given group are said to belong to, or to be assigned to, or to be on the mapping e.g. any object can be mapped to 0, 1, or more groups, and can and two additional slots to re-order and re-list the object to a related mapping. To group two or more objects together in a worksheet, follow these steps: Select two or more objects. Hold down the Shift key or the Ctrl key to select multiple objects. On the Drawing Tools Format tab, choose Group→Group in the Arrange group.