Java generics: cannot convert <X> into <? extends X>

debugcn Published at Dev

XDR

Building the following code with Java 10.0.2+13, Gradle 4.9 & Guava 26.0-jre outputs this error:

Error:

argument mismatch; bad return type in method reference

Iterator<Entry<String,ImmutableMap<Path,String>>>

cannot be converted to

Iterator<Entry<? extends String,? extends ImmutableMap<Path,String>>>

Code:

final Map<String, Map<Path, String>> m = new HashMap<>();

// put entries into m

final ImmutableMap<String, ImmutableMap<Path, String>> im =
    ImmutableMap.<String, ImmutableMap<Path, String>>copyOf(
        m.entrySet().stream().map(
            e -> immutableEntry(
                e.getKey(),
                ImmutableMap.copyOf(e.getValue())
            )
        )::iterator
    )
;

Why do String & ImmutableMap<Path,String> not satisfy ? extends String & ? extends ImmutableMap<Path,String>, respectively?

How can I get this to properly compile?

Tomasz Linkowski

Maybe this does not directly answer your question, but is using a Collector not an option?

Like this:

ImmutableMap<String, ImmutableMap<Path, String>> im = m.entrySet().stream()
        .collect(ImmutableMap.toImmutableMap(
                Map.Entry::getKey, e -> ImmutableMap.copyOf(e.getValue())
        ));

EDIT: Just as a curosity, this gets your code to compile (but please use the Collector-version instead, it's much more clear than the code below):

ImmutableMap<String, ImmutableMap<Path, String>> im = ImmutableMap.copyOf(
        m.entrySet().stream()
                .<Map.Entry<? extends String, ? extends ImmutableMap<Path, String>>>map(e -> immutableEntry(e.getKey(), ImmutableMap.copyOf(e.getValue())))
                ::iterator
);

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