Java ConcurrentModificationException: Causes and Fixes
ConcurrentModificationException usually has nothing to do with threads. Here is exactly what modCount does, why it fires, and the four fixes that actually work.
java-collections
3 PostsComparison Method Violates Its General Contract in Java
IllegalArgumentException: Comparison method violates its general contract! comes from TimSort, not your code. Here is why it fires, and how to fix the comparator.
Java IllegalStateException: Duplicate Key in Collectors.toMap
Collectors.toMap throws IllegalStateException: Duplicate key when two elements map to the same key. Exact causes, JDK 8-25 message differences, and the fixes.