This java client is autogenerated by swagger-codegen using this swagger.json file and then, because of errors, using fixed version of it.
Despite official site says updated version of OpenAPI file is invalid, next command successfully generates this client:
java -jar swagger-codegen-cli.jar generate \
-i https://raw.githubusercontent.com/Serg-Maximchuk/heat-client/master/heat-api-swagger.json \
-l java \
--library retrofit2 \
-o .