How to save objects in list without unique ID (belongsTo, hasMany)

Question

When I have collection, where each object is unique but they belong to some parentId, how should I store it?

My Idea is

• ArrayList <MyType> objects_list; // to store those objects
• ArrayList <int[]> parents_list // to store parent_id vs int[] object_list.id's

So connection would be

• object_list.item belongsTo parents_list.item
• parents_list.item hasMany object_list.item

Isn't there some more efficient, more Java, solution?

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Little more explain:

I have collection of object, where every object has parent_id in some variable within.
I need to store those objects, so that I could easily select all objects by their parent_id

And I cannot use simple one ArrayList with parent_id as key, because key has to be unique.

So how to store them, to fetch all objects by their parent_id like Collection.getByParentId(parent_id) ?

Answer 1

Like Dave said before, store the parent ID in MyType.

// all MyType objects
List<MyType> objects;

// This way you could track the relations
// (you would have to update this on change)
Map<Integer, List<MyType>> relations;