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Using `stream::iterator` to create an `Iterable` results in an `Iterable` which
can only be iterated once, and will throw an `IllegalArgumentException` on
subsequent attempts.
The contract of `Iterable` is poorly defined, but many APIs will assume that
`Iterable`s allow multiple iteration.
To give a concrete example using protocol buffers,
```java
MyProto construct(Stream<Integer> ints) {
return MyProto.newBuilder()
.addAllSubMessage(
ints.map(i -> SubMessage.newBuilder().setVal(i).build())::iterator)
.build();
}
```
With the current implementation of the protocol buffer generated code, this will
work, but the following minor change will lead to re-iteration and failure,
```java
MyProto construct(Stream<Integer> ints) {
MyProto.Builder builder = MyProto.newBuilder();
builder.addSubMessageBuilder().setVal(0);
return builder
.addAllSubMessage(
ints.map(i -> SubMessage.newBuilder().setVal(i).build())::iterator)
.build(); // build iterates twice, and throws.
}
```
To avoid such pitfalls, the `Stream` can either be terminated with
`forEachOrdered`
```java
MyProto construct(Stream<Integer> ints) {
MyProto.Builder builder = MyProto.newBuilder();
ints.map(i -> SubMessage.newBuilder().setVal(i).build())
.forEachOrdered(builder::addSubMessage);
return builder.build();
}
```
or collected (with the caveat that the contents of the `Stream` will now be
materialized into memory at once),
```java
MyProto construct(Stream<Integer> ints) {
return MyProto.newBuilder()
.addAllSubMessage(
ints.map(i -> SubMessage.newBuilder().setVal(i).build())
.collect(toImmutableList()))
.build();
}
```
or suppressed using `@SuppressWarnings("StreamToIterable")`. Only suppress if
you're sure the API you're using will only iterate the resulting one-shot
`Iterable` once, and can't accept a `Stream` or an `Iterator`.
|
