From their GitHub documentation "OpenAPI Generator allows generation of API client libraries (SDK generation), server stubs, documentation and configuration automatically given an OpenAPI Spec".
Below is an example of using the openapi-generator-cli to generate a java client from the octopus-energy-api.yaml file:
cd openapi
docker run \
--rm \
-v "${PWD}:/files" \
openapitools/openapi-generator-cli \
generate \
-i /files/spec.yaml \
-g java \
-o /files/client/
See https://github.com/OpenAPITools/openapi-generator/ for full documentation on the openapi-generator.