Android Question Strange problem with a Map

LucaMs · Dec 3, 2019

Test project attached (its design also is strange, but it is similar to my real project).

Error occurred on line: 37 (Main)
Line 37 is an empty line.

java.util.NoSuchElementException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:759)

Erel · Dec 3, 2019

B4X:

For Each Index As Int In mapX.Keys
   AddOrRemove(Index, False)
Next

You are modifying the map inside the For Each loop so the iterator tries to access an invalid item.

You can call mapX.Clear instead of this code.

LucaMs · Dec 3, 2019

Erel said:
You are modifying the map inside the For Each loop so the iterator tries to access an invalid item.

Why? I'm using the map key. I could understand if I was using an array and a "for next" loop.

Erel said:
You can call mapX.Clear instead of this code.

No, because I have to "handle" (GUI) every single removal.

Erel · Dec 3, 2019

LucaMs said:
Why? I'm using the map key. I could understand if I was using an array and a "for next" loop.

The iterator doesn't copy the list of keys so when you change the underlying data you are making it invalid.

You need to make a copy yourself:

B4X:

Dim KeysToRemove As List
KeysToRemove.Initialize
For Each Index As Int In mapX.Keys
   KeysToRemove.Add(Index)
Next
For Each k As Int In KeysToRemove
   mapX.Remove(k)
Next

LucaMs · Dec 3, 2019

Erel said:
The iterator doesn't copy the list of keys so when you change the underlying data you are making it invalid.

Is it a java hashmap "behaviour" (I should say "bug")?

Erel · Dec 3, 2019

It is not a bug. It is a designed limitation in favor of performance.

It is not directly related to Java though HashMap native iterator will also fail if you modify the collection like this.

LucaMs · Dec 3, 2019

It sounds very strange to me. I would like to try with vb.net... if it didn't take about 8 hours just to start VS

Thanks, Erel

Erel · Dec 3, 2019

BTW, if you switch to B4XOrderedMap from B4XCollections then you will be able to simplify it a bit:

B4X:

Dim KeysToRemove As List
KeysToRemove.Initialize
KeysToRemove.AddAll(mapX.Keys)
For Each Key As Int In KeysToRemove
   mapX.Remove(Key)
Next

It will also allow you to access elements based on an index, without using the keys for that:

B4X:

mapX.Get(mapX.Keys.Get(3)) 'get the 4th element value

Android Question Strange problem with a Map

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