On Wed, 25 Nov 2020 14:07:07 GMT, Peter Levart <plev...@openjdk.org> wrote:
>> This is the default implementation in the Stream interface, which is >> overridden by an implementation in the ReferencePipeline class, so it will >> rarely be used in normal operation. The ReferencePipeline version (part of >> this changeset) is based on toArray() and avoids any copying. I'm thus not >> inclined to add new interfaces in order to support this default >> implementation. >> >> As written it's true that the default implementation does perform apparently >> redundant copies, but we can't be assured that toArray() actually returns a >> freshly created array. Thus, we wrap it using Arrays.asList and then copy it >> using the ArrayList constructor. This is unfortunate but necessary to avoid >> situations where someone could hold a reference to the internal array of a >> List, allowing modification of a List that's supposed to be unmodifiable. > > An alternative with similar performance would be to do a Stream.toArray() and > then copy that array into new Object[] and then wrap that copy with > listFromTrustedArrayNullsAllowed(). The difference would be in the > serialization format of the resulting List and maybe also in the access > performance of resulting List (no indirection via the unmodifiableList > wrapper and different type for JIT to speculate about). So if we want the > resulting List to behave exactly the same in both implementations of > toList(), then this alternative might be preferable. WDYT? Such alternative might also be faster using intrisified array copying method (here measuring just copying overhead): Benchmark (len) Mode Cnt Score Error Units ToListBench.toList1 10 avgt 10 14.213 ± 0.061 ns/op ToListBench.toList1 1000 avgt 10 541.883 ± 3.845 ns/op ToListBench.toList1 1000000 avgt 10 753223.523 ± 4656.664 ns/op ToListBench.toList2 10 avgt 10 8.810 ± 0.052 ns/op ToListBench.toList2 1000 avgt 10 264.748 ± 0.807 ns/op ToListBench.toList2 1000000 avgt 10 349518.502 ± 3242.061 ns/op https://gist.github.com/plevart/974b67b65210f8dd122773f481c0a603 ------------- PR: https://git.openjdk.java.net/jdk/pull/1026
