Control flow analysis fails within a stream collector

[wbadam]

I think this may be the same as #332 which was closed recently.

NullAway is unable to infer nullity on a dereference made inside a stream collector.

In the example below, streams1() passes when the filter is added, and fails otherwise. In streams2(), there is a dereference warning for foo.bar even though the filter makes sure the value is non-null.

public class Streams {
    record Foo(@Nullable String bar, String baz) {}

    // NullAway check passes ✅ 
    List<Integer> streams1() {
        List<Foo> foos = new ArrayList<>();
        return foos.stream()
                .filter(foo -> foo.bar != null)
                .map(foo -> foo.bar.length())
                .toList();
    }

    // NullAway check fails on dereferencing `foo.bar` within the collector ❌ 
    Map<Integer, String> streams2() {
        List<Foo> foos = new ArrayList<>();
        return foos.stream()
                .filter(foo -> foo.bar != null)
                .collect(Collectors.toMap(foo -> foo.bar.length(), foo -> foo.baz));
    }
}

NullAway version: 0.10.24

[msridhar]

Hi @wbadam I was able to reproduce this. Support for this kind of reasoning on streams is limited to particular constructs where we were confident the handling was correct. I think we could probably extend this to handle Collectors.toMap, but it doesn't quite fit the patterns we were matching before, so it may take a bit of work. @lazaroclapp do you have any thoughts here?

[msridhar]

@wbadam I have an initial PR up for this case in #938. Can you think of any related stream scenarios involving filters where NullAway is reporting false positives for you all?

[wbadam]

@msridhar After a quick search I've found a couple of similar cases:

• .collect(Collectors.groupingBy(foo -> foo.bar.length()));
• .collect(ImmutableMap.toImmutableMap(foo -> foo.bar.length(), foo -> foo.baz)); (less important I guess as it could be replaced by native api)

There may be more that I couldn't immediately find, but these I reckon would cover most of our cases.

[msridhar]

@wbadam I've added support for those cases to #938.

[msridhar]

FYI @wbadam this fix was part of the 0.10.25 release from today