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Keep object order in unmodifiable collection returned by MapObjectMan… #1008

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Keep object order in unmodifiable collection returned by MapObjectMan… #1008

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2 changes: 1 addition & 1 deletion library/src/main/java/com/google/maps/android/collections/MapObjectManager.java
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Original file line number Diff line number Diff line change
Expand Up @@ -122,7 +122,7 @@ public void clear() {
}

protected java.util.Collection<O> getObjects() {
return Collections.unmodifiableCollection(mObjects);
return Collections.unmodifiableSet(mObjects);
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@solcott Thanks for the PR!

Could you explain more why this is needed? Did you still encounter the same issue #772 even after PR #972 was merged?

If so it would be useful to see some code that's generating the error, ideally in the form of unit tests.

My understanding of Collections.unmodifiableX() is that it's just a wrapper around the underlying data structure so it should still respect the ordering enforced by the implementation, in our case the LinkedHashSet.

Collection.iterator and Set.iterator both have effectively the same language in the Javadocs:

There are no guarantees concerning the order in which the elements are returned (unless this collection is an instance of some class that provides a guarantee).

cc @jeffdgr8 as the contributor of PR #972

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@jeffdgr8 jeffdgr8 Oct 26, 2021 •
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Looking at the source for UnmodifiableSet, it subclasses UnmodifiableCollection, additionally overriding equals() and hashCode() to delegate to the wrapped collection.

I would not expect this change to affect the iteration order, which should have been resolved in my PR. But I would expect this change to allow for comparisons between multiple versions of the wrapped collection itself. For example:

...
PolygonManager.Collection polygonCollection = polygonManager.newCollection();
Collection<Polygon> collection1 = polygonCollection.getPolygons();
Collection<Polygon> collection2 = polygonCollection.getPolygons();
Set<PolygonManager.Collection> polygonCollectionSet = new HashSet<>();
polygonCollectionSet.add(collection1);

// currently each of these may be false, but this change should make them true:
collection1.equals(collection2);
polygonCollectionSet.contains(collection2);

@solcott do you have a similar usage that requires comparisons of these MapObject.Collections for equality?

I think this is a good change either way, as it would allow for these kinds of comparisons. Collection itself doesn't stipulate a contract for equals() beyond Object.equals, but Set does, which is the underlying collection we're using here.

If this is the contract we want to support and there are use cases for it, it may make sense to change the signature of getObjects() and related functions that call it to explicitly return a Set rather than the more general Collection type. That way it would enforce this behavior expectation at the API level. This would be a non-breaking safe API change, since Set is a Collection.

}
}
}