The sublist that stopped working
A hard Java puzzle in the modifying while iterating family. One line below contains a real bug. Try to spot it first, then read why it fails and how to fix it.
Highlighted: line 7. Play this one as a puzzle without the answer.
What is wrong
subList returns a live view of the original list, not a copy. Structurally modifying the parent afterwards invalidates the view, and reading it throws ConcurrentModificationException.
The fix
How to avoid it next time
Copy a sublist if it must outlive changes to the source. Treat views as temporary.
Want the background? Read Collections: List, Set, Map in the Java concepts library.
About modifying while iterating
Collections detect structural changes made during iteration and throw ConcurrentModificationException, but only some of the time, and not at all for certain patterns. Removing in a loop is a classic way to skip elements or crash.
In code review: Any call to add or remove on the collection being looped over deserves suspicion. Use Iterator.remove, removeIf, or build a new collection.
More puzzles in this family
Frequently asked questions
What is the bug in "The sublist that stopped working"?
subList returns a live view of the original list, not a copy. Structurally modifying the parent afterwards invalidates the view, and reading it throws ConcurrentModificationException. The faulty line is line 7: all.add(6);
How do I fix it?
Replace it with: List
How do I catch modifying while iterating in code review?
Any call to add or remove on the collection being looped over deserves suspicion. Use Iterator.remove, removeIf, or build a new collection.
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