JDK-8160406 : Collection.toArray() spec should be explicit about returning precisely an Object[]
  • Type: Bug
  • Component: core-libs
  • Sub-Component: java.util:collections
  • Priority: P4
  • Status: Closed
  • Resolution: Fixed
  • Submitted: 2016-06-27
  • Updated: 2017-12-18
  • Resolved: 2017-11-30
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JDK 10
10 b35Fixed
Related Reports
CSR :  
JDK-8192841 - Collection.toArray() spec should be explicit about returning precisely an Object[]
Relates :  
JDK-6260652 - (coll) Arrays.asList(x).toArray().getClass() should be Object[].class
Relates :  
JDK-8015667 - Stream.toArray(IntFunction) ArrayStoreException should refer to component type of array
Description
Collection.toArray() is specified to return an object of type Object[], but since arrays are covariant, that requirement is met by any array of a subtype of Object, for example, String[].

It's subtle, but it's clearly, if not explicitly, a bug to return an array of a subtype. See JDK-6260652.

Interestingly, the specification for toArray(T[]) says

    Note that toArray(new Object[0]) is identical in function to toArray().

which is a fine statement, but it's in the wrong place. There should be a requirement in the toArray() spec that it returns an instance whose runtime type is precisely Object[].
Comments
Review thread: http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-November/050234.html There is also some imprecision in the toArray() and toArray(T[]) methods about how it describes array types, array component types, and runtime types, that I'll also fix as part of this changeset.
30-11-2017
This has resulted in some confusion on Stack Overflow. See http://stackoverflow.com/questions/38020766/different-behaviour-during-converting-a-list-of-strings-to-string-array
27-06-2016