There are two DatagramSocket ctors accepting port number.
One of them even has "The local port must be between 0 and 65535 inclusive" assertion.

It turns out that "A port number of zero will let the system pick up an ephemeral port in a bind operation" - assertion from the InetSocketAddress constructor used by the implementation.

Would be good for DatagramSocket ctors to specify that zero port number tells the system to pick any appropriate port number.