public interface Hello<T> {

	   @POST
	   @Path("/hello")
	   @Produces("text/plain")
	   @Consumes("text/plain")
	   String sayHi(T in);
}

@Path(value = "/say")
public interface HelloString extends Hello<String> {

}

ResteasyClient client = new ResteasyClientBuilder().build();
		   ResteasyWebTarget target = client.target("http://localhost:9095");
		   HelloString proxy = target.proxy(HelloString.class);
		   String hello = proxy.sayHi("hello");

java.lang.IllegalArgumentException: The type is incompatible with the class of the entity.
	at javax.ws.rs.core.GenericEntity.checkTypeCompatibility(GenericEntity.java:173)
	at javax.ws.rs.core.GenericEntity.<init>(GenericEntity.java:152)
	at org.jboss.resteasy.client.jaxrs.internal.proxy.processors.invocation.MessageBodyParameterProcessor.process(MessageBodyParameterProcessor.java:35)
	at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientInvoker.createRequest(ClientInvoker.java:135)
	at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientInvoker.invoke(ClientInvoker.java:101)
	at org.jboss.resteasy.client.jaxrs.internal.proxy.ClientProxy.invoke(ClientProxy.java:62)