Ensure that getAll() always returns a mutable list

[aahlenst]

If a box is empty, getAll() returns Collections.emptyList(). This empty list is immutable which can only be noticed at runtime (UnsupportedOperationException is thrown). This is surprising. This pull request changes getAll() in such a way that a mutable list is returned in every case.

[rocboronat]

Let me downvote this PR. Having immutable objects leads to easier to understand code.

[aahlenst]

@rocboronat That‘s mostly true but that‘s not what this change is about. The current API makes no promises regarding immutability. It promises a java.util.List, which is a mutable container. And in most of the cases, you actually get a mutable list. But if the box is empty, you suddenly get an immutable list. This is bonkers because it turns a compile-time error into a runtime problem.

In my opinion, It‘d be perfectly fine if ObjectBox consistently returned immutable collections. But this would require a change to the public API and the introduction of a proper immutable list type without any mutators. Otherwise the compiler cannot help.

[greenrobot-team]

Thanks for reporting. Starting with 2.4.0-RC Box.getAll() always returns a mutable list.