I can't compile a Guava's Cache's builder because of Java generics

generics, guava, java

Solution

A solution with some ignored warning :

  static LoadingCache<Class<? extends Resource<?>>, ResourceType<? extends Resource<?>>> cache = CacheBuilder.newBuilder().build(
      new CacheLoader<Class<? extends Resource<?>>, ResourceType<? extends Resource<?>>>() {
        @SuppressWarnings({ "rawtypes", "unchecked" })
        @Override public ResourceType<? extends Resource<?>> load(Class<? extends Resource<?>> key) throws Exception {
          return new ResourceType(key);
        }
    });

As suggested by @John B in a comment. This code is not different at run-time than the code in the question due to type-erasure.

Problem

I have the following short, self-contained code that shows an error at compile time. I've desperately tried to have it compiling. I usually don't have trouble with generics anymore, but well I give up on this one and I ask the help of the team. ``` import com.google.common.cache.CacheBuilder; import com.google.common.cache.CacheLoader; import com.google.common.cache.LoadingCache; public class CacheWithGenericsDoesNotCompile { // Class definition can't be modified static class Resource<T extends Resource<T>> {} // Class definition can't be modified static class ResourceType<T extends Resource<T>> { public ResourceType(Class<T> type) {} } // Variable definition may be modified static LoadingCache<Class<? extends Resource<?>>, ResourceType<? extends Resource<?>>> cache = CacheBuilder.newBuilder().build( new CacheLoader<Class<? extends Resource<?>>, ResourceType<? extends Resource<?>>>() { @Override public ResourceType<? extends Resource<?>> load(Class<? extends Resource<?>> key) throws Exception { return new ResourceType<? extends Resource<?>>(key); } }); // Method definition can't be modified, method content may. @SuppressWarnings("unchecked") static <T extends Resource<T>> ResourceType<T> getResourceType(Class<T> type) { return (ResourceType<T>)cache.getUnchecked(type); } } ``` The line that fails to compile is: ``` return new ResourceType<? extends Resource<?>>(key); ``` I know why it fails: I may not write `new Xxxx<...>` with a question mark (`?`). I just can't write this line differently to have the other lines compiling. I have a fallback solutions in the case where `Resource` has no generics, but in the limit of the possible I'd like to keep `Resource` with generics. I have no restriction about the generics of the `LoadingCache` except that I need it to be called as in `getResourceType(Class)`. So... how can I fix this code?

Original source