👀 Views: 401 💬 Answers: 1 📅 Created: 2025-06-11

java streams collectors exception Java

I'm stuck on something that should probably be simple. I'm sure I'm missing something obvious here, but I'm working with an `IndexOutOfBoundsException` when I attempt to collect elements from a `List` into a custom data structure using Java Streams. I'm using Java 11 and have created a custom collector that aims to aggregate results into a `Map<String, List<String>>`. Here’s the code I'm working with: ```java import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Example { public static void main(String[] args) { List<String> items = List.of("apple", "banana", "apple", "orange", "banana", "kiwi"); Map<String, List<String>> collected = items.stream() .collect(Collectors.toMap( item -> item, item -> List.of(item), // This part seems to be causing the scenario (existing, replacement) -> { existing.addAll(replacement); return existing; } )); } } ``` When I run this code, I receive the following behavior: ``` Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 ``` I suspect the scenario arises from how I'm adding elements to the `existing` list in the merge function of `Collectors.toMap()`. I've also tried initializing a mutable list but keep running into the same `IndexOutOfBoundsException`. Here’s what I tried instead: ```java Map<String, List<String>> collected = items.stream() .collect(Collectors.toMap( item -> item, item -> new ArrayList<>(List.of(item)), (existing, replacement) -> { existing.addAll(replacement); return existing; } )); ``` This still throws an exception. I’m not sure if the question lies in how I’m creating the list or if it’s in how I’m merging the lists. Any insights on how I can resolve this scenario? Is there a better approach? I'm developing on macOS with Java. Any help would be greatly appreciated!