Replace List<@Nullable Object> with List<?>

[vlsi]

Overview

The latter is easier to read, and it forbids list.add(...) at the compile-time.

See https://jspecify.dev/docs/user-guide/#wildcard-bounds

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[vlsi]

It looks like AssertJ can't assert on values like List<?> and List<? extends Object>:

List<?> values = List.of("1", 2)

assertThat(values).containsExactly("1", 2); // this does not compile
Assertions.<Object>assertThat(values).containsExactly("1", 2); // this compiles

Currently, the build fails because some of the JUnit tests attempt things like the following:

var values = readFieldValues(fields, new ClassWithFields());
assertThat(values).containsExactly("enigma", 3.14, "text", 2.5, null, 42, "constant", 99);

I'm not sure if AssertJ's assertThat is the reason to refrain from List<?>.

[vlsi]

A funny case is that an illegal .set was there in the test code:

// Modify local copy:
assertThrows(UnsupportedOperationException.class, () -> copy.set(0, "Boom!"));

The code fails to compile with List<?>, so it is a good case: it moves a runtime failure to a compile-time failure.

[marcphilipp]

Since this is a breaking change, I'm not convinced we should do it for public APIs. Could you please revert all changes except those in ReflectionUtils and its test class?

[vlsi]

I'm not fond of this cast, however, it seems to be the least evil as the added suppression holds for a single statement only rather than a method.

[marcphilipp]

Thanks, @vlsi! 👍