Java only promises to protect integrity of JVM itself, not your data. If you accidentally write to a non-concurrent collection from multiple threads, it's not a program-corrupting Undefined Behavior like in C/C++/golang, but it can still misbehave by corrupting the collection itself, causing it to mangle or lose data, or in the best case throw an exception. Either way you don't get the intended program behavior, and you may be unaware of using a wrong collection type until you create an invalid access at compile time.

This is different from Rust, where thread-safety is part of the type system, so multi-threaded code that could write to a shared HashMap instead of a ConcurrentHashMap just won't compile with a type error.