### Question:
HelloMessageImpl implements HelloMessage { @Override public Set<String> getChannelIds() { return Collections.unmodifiableSet(channelIds); } HelloMessageImpl();  HelloMessageImpl(final String uaid);  HelloMessageImpl(final String uaid, final Set<String> channelIds); @Override String getUAID(); @Override Set<String> getChannelIds(); @Override Type getMessageType(); @Override String toString();  }### Answer:
@Test(expected = UnsupportedOperationException.class) public void channelIdsUnmodifiable() { final HelloMessageImpl handshake = new HelloMessageImpl(newUAID(), channelIds("123abc", "efg456")); handshake.getChannelIds().remove("123abc"); }