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There must be a good reason, or some history behind this design decision. Or perhaps I misunderstand the use case.

Background: Google Guava has a Multimap interface. The get method has signature: Collection<V> get(@Nullable K key). I expected: Collection<V> get(@Nullable Object key)

Normally generic map-like interfaces accept Object to get methods as it helps with wildcards. See Java's Map interface (and What are the reasons why Map.get(Object key) is not (fully) generic).

I have a method that accepts a Multimap with wildcards, such as: void doWork(Multimap<? extends MyKeyType, ? extends MyValueType>). However, even with a MyKeyType reference, I cannot call (effectively) Multimap.get(? extends MyKeyType). (The code will not compile.)

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kevinarpe
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I guess, just because this eliminates a lot of dumb errors when you supply something wrong to get() method and just have null returned without any compilation error or runtime exception. Such problems in the code can only be discovered using static analysis tools like FindBugs (see GC_UNRELATED_TYPES pattern), but even FindBugs cannot detect every possible such bug.

As for your case: probably you have to introduce the generic parameter instead using

void <T extends MyKeyType> doWork(Multimap<T, ? extends MyValueType> mmap) {}

And every instance of your key reference should actually have the T type. If it's impossible for you, then you may use ugly unchecked cast like this:

((Multimap<MyKeyType, ? extends MyValueType>)mmap).get(key)

Probably it's a good idea to assign casted multimap to the intermediate variable.

Tagir Valeev
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    As an alternative, you can use `asMap` to get a `Map` view of the `Multimap`. – Sotirios Delimanolis Dec 18 '15 at 05:51
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    Ironically, Kevin Bourrillion (Guava co-lead) wrote about this matter here: http://smallwig.blogspot.com/2007/12/why-does-setcontains-take-object-not-e.html, and said: _So what to do about this vexing source of bugs, as illustrated at top? Well, when I typed that code into IntelliJ, it flagged a warning for me right away. This let me know to either fix the problem or add an annotation/comment to suppress it. Problem solved._ – kevinarpe Dec 18 '15 at 06:08
  • @kevinarpe, Link you have posted is little distorted. Here is the updated link: http://smallwig.blogspot.in/2007/12/why-does-setcontains-take-object-not-e.html – milanchandna Jun 01 '16 at 09:39
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    BTW he is not the author of MultiMap interface. @Tagir, As kevinarpe rightly mentioned that dumb bugs can be caught using good Editors but logically API has to be correct. – milanchandna Jun 01 '16 at 09:46