Make google-cloud-java forward compatible with Guava 23+

[garrettjonesgoogle]

I haven't personally tried this yet, but I have evidence that google-cloud-java is not currently forward compatible with Guava 23+. We need to make sure all code can run against both Guava 20 and Guava 23 (basically only depend on classes/methods that exist in the intersection of those versions).

[broady]

/cc @jadekler

[marshallpierce]

It does appear to be incompatible. I'm using Guava 26 and seeing:

ava.lang.NoSuchMethodError: com.google.common.util.concurrent.Futures.transform(Lcom/google/common/util/concurrent/ListenableFuture;Lcom/google/common/base/Function;)Lcom/google/common/util/concurrent/ListenableFuture;
	at com.google.api.core.ApiFutures.transform(ApiFutures.java:99)
	at com.google.cloud.firestore.UpdateBuilder.commit(UpdateBuilder.java:618)
	at com.google.cloud.firestore.WriteBatch.commit(WriteBatch.java:41)
	at com.google.cloud.firestore.DocumentReference.set(DocumentReference.java:171)

[jeanbza]

Yikes. I think this only gets solved when we go to Java 8 (it seems guava 21.0 is last supported Java 7 version?).

[marshallpierce]

ApiFutures is in https://github.com/googleapis/api-common-java, which has already released a version (1.7.0) that doesn't use the deprecated Guava features. That is, in turn, depended on by com.google.api:gax-grpc, which has also released an updated version (1.30.0) that depends on api-common-java 1.7.0. So, using api-common 1.7.0 (possibly via gax-grpc 1.30.0) should do the trick.

[chingor13]

Guava upgraded to 26.0-android in https://github.com/googleapis/google-cloud-java/releases/tag/v0.71.0