Write an OpenAPI spec with Java code (instead of YAML or JSON directly)

example.java

//usr/bin/env jbang "$0" "$@" ; exit $?
//DEPS io.smallrye:smallrye-open-api-core:2.0.3

import static org.eclipse.microprofile.openapi.OASFactory.*;

import org.eclipse.microprofile.openapi.models.OpenAPI;

import io.smallrye.openapi.runtime.io.Format;
import io.smallrye.openapi.runtime.io.OpenApiSerializer;

public class example {

    public static void main(String... args) throws Exception {
        OpenAPI spec = createSpec();
        String content = OpenApiSerializer.serialize(spec, Format.YAML);
        System.out.println(content);
    }

    public static OpenAPI createSpec() {
        return createOpenAPI()
            .openapi("3.0.1")
            .info(
                createInfo()
                    .title("Ping Specification")
                    .version("1.0")
            )
            .addServer(
                createServer()
                    .url("http://localhost:8080/")
            )
            .paths(
                createPaths()
                    .addPathItem(
                        "/ping", createPathItem()
                            .GET(
                                createOperation()
                                    .operationId("pingGet")
                                    .responses(
                                        createAPIResponses()
                                            .addAPIResponse(
                                                "200", createAPIResponse()
                                                    .description("OK")
                                            )
                                    )
                            )
                    )
            );
    }
}

See the corresponding article:
Write your OpenAPI with Java instead of YAML