Add nullability annotations for ConcurrentHashMap.

This is an unusual case. Normally, even if a Map does not accept null
keys, calling containsKey(null) is legitimate, and returns false, so
Map.containsKey() takes @Nullable Object. On a CHM, calling
containsKey(null) throws NPE. (This behaviour is allowed by the
contract of Map, which says "Attempting to query the presence of an
ineligible key or value may throw an exception, or it may simply
return false; some implementations will exhibit the former behavior
and some will exhibit the latter.") This change therefore declares the
argument to CHM.containsKey as @NonNull Object. This relationship
breaks the normal 'rules' that an implementation can't narrow the type
of an argument to an interface method it's implementing, but it is the
best representation of reality. The practical consequence is that a
warning or error will be generated on chm.containsKey(null) if chm is
statically typed as CHM, but not if it is a CHM instance that is
statically types as Map. This gap in null-safety checking is an
inevitable consequence of the optionality in the contract of Map.

There are some rather non-obvious nullability contracts on the
functional programming methods on CHM. For example, consider the
forEach overload which takes a BiFunction<? super K, ? super V, ?
extends U> and a Consumer<? super U>: the BiFunction is allowed to
return null, so its third parameter is @Nullable, but the Consumer is
not applied in that case, so its parameter is @NonNull. See the
class-level and method-level javadoc.

Test: make core-current-stubs-nullability-validation-check-nullability-warnings
Bug: 64930165
Change-Id: Ia36ef9f841df1dae4ce06b24acd34290d25e415b

0 files changed, 0 insertions, 0 deletions