At first I got this working as documented in the Jersey User guide
Authenticator.setDefault (authinstance);
However I did not like this as it relied on setting a global authenticator. After some research I discovered that Jersey has a HTTPBasicAuthFilter
which is even easier to use.
Client c = Client.create();
c.addFilter(new HTTPBasicAuthFilter(user, password));
See:
https://jersey.github.io/nonav/apidocs/1.10/jersey/com/sun/jersey/api/client/filter/HTTPBasicAuthFilter.html
https://jersey.github.io/nonav/apidocs/1.10/jersey/com/sun/jersey/api/client/filter/Filterable.html#addFilter(com.sun.jersey.api.client.filter.ClientFilter)