如何应对实现Comparable的类 <?> 不可比 <E> ？

Question

I have created an Iterator wrapper which returns elements until a certain threshold is reached as following:

public class MyIterators {
    public static <E extends Comparable<E>> Iterator<E> threshold(final Iterator<? extends E> iterator, final E threshold) {
        ...
    }
}

I want to use this for an Iterator<ChronoZonedDateTime> in a utility function as following:

public static Iterator<ZonedDateTime> oneYear(final Iterator<ZonedDateTime> iterator) {
    return MyIterators.threshold(iterator, ZonedDateTime.now().plusYears(1));
}

I am getting:

method threshold in class MyIterators cannot be applied to given types;
[ERROR]   required: java.util.Iterator<? extends E>,E
[ERROR]   found: java.util.Iterator<java.time.chrono.ChronoZonedDateTime>,java.time.ZonedDateTime
[ERROR]   reason: cannot infer type-variable(s) E
[ERROR]     (argument mismatch; java.util.Iterator<java.time.chrono.ChronoZonedDateTime> cannot be converted to java.util.Iterator<? extends java.time.chrono.ChronoZonedDateTime<?>>)

The compiler is inferring ChronoZonedDateTime instead of ZonedDateTime , while I am not declaring/using that class at all.

How do I cope with this? Do I introduce a manual cast, or is there still a way to do all this in a type-safe manner?

I am working with JDK 8.x.

Answer 1

I believe the issue is caused by the raw type, ChronoZonedDateTime . If you specify a wildcard for the generic type, ChronoZonedDateTime<?> , then your code compiles and runs:

final Iterator<ChronoZonedDateTime<?>> iterator = null;
MyIterators.threshold(iterator, ZonedDateTime.now().plusYears(1));

See: What is a raw type and why shouldn't we use it?

---

Editing this answer as OP removed some information from the question.

Regarding your second problem, you should change <E extends Comparable<E>> to <E extends Comparable<? super E>> <E extends Comparable<? super E>> in the signature of threshold , which will allow you to pass in Iterator objects whose generic type is one that compares against its parent type.

The new method signature should look like the following:

public static <E extends Comparable<? super E>> Iterator<E> threshold(final Iterator<? extends E> iterator, final E threshold) {