From the JAX-RPC specification: -

"If there is a single unlisted output part that is not a component of an inout parameter, then it is the return type. Otherwise, the return type is void."

A WSDL with the following messages: -

<message name='PhoneBook_lookup'>
<part element='ns1:lookup' name='lookup'/>
</message>
<message name='PhoneBook_lookupResponse'>
<part element='ns1:lookup' name='lookup'/>
</message>

Causes the following method to be generated in the SEI: -

public java.lang.String lookup(javax.xml.rpc.holders.StringHolder lookup) throws java.rmi.RemoteException;

The return type should be void.