There is a warning displayed when the constructor in the following class is uncommented. Because this class is not public this warning should also be displayed when no constructor is present. (The default constructor is given the access modifier of its containing class JLS 2nd edition 8.8.7). class NoConstructor implements Externalizable { //private NoConstructor() {} public void writeExternal(ObjectOutput out) throws IOException { } public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException { } } Also this warning should probably be an inspection (in the "Serialization issues" group) so it can be given a higher severity (or disabled).