Summary
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`java.nio.channels.DatagramChannel::socket` returns a `java.net.DatagramSocket` that allows code using the `DatagramSocket` API to access the channel's socket using the legacy API. This CSR proposes to remove a statement from the `socket` method's javadoc to allow for changes to the implementation, in particular to allow the implementation to return a `java.net.MulticastSocket` (a sub-class of `DatagramSocket`).


Problem
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`DatagramChannel::socket` specifies that the returned object (a non-exposed subclass of `DatagramSocket`) does not declare any public methods beyond those specified by `DatagramSocket`. This statement is testable and prevents implementations from implementing additional interfaces beyond those implemented by `DatagramSocket`, or extending it in other ways that define additional public methods.


Solution
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Remove the statement from the javadoc that the returned object doesn't declare any public methods beyond those declared by DatagramSocket.

Change the implementation to return a `java.net.MulticastSocket` (a sub-class of `DatagramSocket`). We do not propose to document (in an `@implNote` or elsewhere) that the returned object is an instance of `MulticastSocket`. Users of the API may of course discover this and cast to `java.net.MulticastSocket` of course but it seems unlikely given that `MulticastSocket` is legacy with many deprecated methods.


Specification
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Remove the following statement from the DatagramChannel:socket specification:

"The returned object will not declare any public methods that are not declared in 
the java.net.DatagramSocket class."