It's not really about the implementation of Java (might be bad, I don't know). It is the specification.

- People talked about null being an issues and that is a big one.

- The entire idea of OOP extremism Java implemented was a mistake - though just a consequence of the time it was born in. Much has been written about this topic by many people.

- Lacking facilities and really design for generic programming (also related to the OOP extremism and null issue

So much more more you can find out with Google or any LLM