A batch of minor incompatible style changes coming

[Eugene Kirpichov <ki...@google.com>]

Hello,

Recently we introduced a style guide for developing PTransforms
https://beam.apache.org/contribute/ptransform-style-guide/ and a natural
consequence of that was making Beam itself comply with its own API best
practices.

That work is tracked in JIRA https://issues.apache.org/jira/browse/BEAM-1353
and many of the changes are (trivially) backward-incompatible; we're trying
to finish this work sooner, before declaring Beam stable API.

The changes so far are:
- TextIO.Read now always returns a PCollection<String> and does *not* take
.withCoder() to parse the strings. Instead, parse the strings by applying a
ParDo or MapElements to the collection.
- Likewise, TextIO.Write now always takes a PCollection<String>, and to
write something else to TextIO, convert it to String using a ParDo or
MapElements.
- Class Write.Bound is now simply Write. This matters only if you were
extracting applications of Write.to(Sink) into a variable - its type used
to be Write.Bound<...>, now it'll be Write<...>.
- Likewise, classes Flatten.FlattenIterables and
Flatten.FlattenPCollectionList are renamed respectively to
Flatten.Iterables and Flatten.PCollections.
- GroupByKey.create(boolean fewKeys) is now simply GroupByKey.create() and
GroupByKey.createWithFewKeys().
- Classes Count.PerElement, PerKey, Globally are now private so you have to
use the factory functions such as Count.perElement() (whereas previously
you could use "new Count.PerElement()"). Additionally, if you want to e.g.
use .withHotKeyFanout() or whatnot, then you can't do that directly on a
result of .apply(Count.perElement() etc) anymore - instead Count exposes
its combine function as Count.combineFn() and you should apply
Combine.globally(Count.combineFn()) yourself.
- Same as for Count, applies to Latest and Sample
- ToString.of() renamed to ToString.elements(), kv to kvs, iterable to
iterables.
- BufferedExternalSorter.Options setter methods are renamed from setBlah to
withBlah.
- In KafkaIO, you now have to specify type parameters explicitly: e.g.
KafkaIO.<Foo, Bar>read().

More changes like this will follow. Please follow the JIRA item for updates.