Why this code compiles with jdk8u45 and above but not with jdk8u25?

问题

Please, could someone help me to figure out why the following code compiles with jdk8u45 and above but fails with jdk8u25? I looked through the JDK release notes but didn't find anything related to the issue or maybe missed it.

The code

public class Main {

    static class Param {
        final int id;

        Param(int id) {
            this.id = id;
        }
    }

    static class Subtask {
        final Param param;

        Subtask(Param param) {
            this.param = param;
        }
    }

    public static void main(String[] args) {
        List<? extends Param> params = IntStream.range(1, 100).mapToObj(Param::new).collect(Collectors.toList());
        NavigableMap<String, Subtask> map = params.stream()
                .collect(Collectors.toMap(p -> UUID.randomUUID().toString(), Subtask::new, (a, b) -> a, TreeMap::new));
    }
}

jdk8u25 exception:

Error:(33, 17) java: no suitable method found for collect(java.util.stream.Collector<org.ka.Main.Param,capture#1 of ?,java.util.TreeMap<java.lang.String,org.ka.Main.Subtask>>)
    method java.util.stream.Stream.<R>collect(java.util.function.Supplier<R>,java.util.function.BiConsumer<R,? super capture#2 of ? extends org.ka.Main.Param>,java.util.function.BiConsumer<R,R>) is not applicable
      (cannot infer type-variable(s) R
        (actual and formal argument lists differ in length))
    method java.util.stream.Stream.<R,A>collect(java.util.stream.Collector<? super capture#2 of ? extends org.ka.Main.Param,A,R>) is not applicable
      (cannot infer type-variable(s) R,A,capture#3 of ?,T,K,U,M,K,V
        (argument mismatch; java.util.stream.Collector<capture#2 of ? extends org.ka.Main.Param,capture#4 of ?,java.util.TreeMap<java.lang.Object,org.ka.Main.Subtask>> cannot be converted to java.util.stream.Collector<? super capture#2 of ? extends org.ka.Main.Param,capture#4 of ?,java.util.TreeMap<java.lang.Object,org.ka.Main.Subtask>>))

回答1:

I had similar problems with type inferencing which broke somewhere between 8u5 and 8u25 and was fixed in 8u40. The bug fix list in 8u40 has some javac fixes having to do with nested lambda bodies incorrectly ruling out some methods in overload resolution which is what I think your problem is.

Here is the list of all bug fixes in 8u40

来源：https://stackoverflow.com/questions/37368060/why-this-code-compiles-with-jdk8u45-and-above-but-not-with-jdk8u25