/** Return {@link io.grpc.Channel} which is used by Cloud Pub/Sub gRPC API's. */
public static Channel getChannel(CredentialsProvider credentialsProvider) throws IOException {
  ManagedChannel channelImpl =
      NettyChannelBuilder.forAddress(ENDPOINT, 443)
          .negotiationType(NegotiationType.TLS)
          // Maximum Pub/Sub message size is 10MB.
          .maxInboundMessageSize(10 * 1024 * 1024)
          .build();
  final ClientAuthInterceptor interceptor =
      new ClientAuthInterceptor(
          credentialsProvider.getCredentials(),
          Executors.newCachedThreadPool());
  return ClientInterceptors.intercept(channelImpl, interceptor);
}
 

/** Return channel with interceptor for returning credentials. */
private Channel newChannel() throws IOException {
  checkState(publisherChannel != null, "PubsubGrpcClient has been closed");
  ClientAuthInterceptor interceptor =
      new ClientAuthInterceptor(credentials, Executors.newSingleThreadExecutor());
  return ClientInterceptors.intercept(publisherChannel, interceptor);
}
 

public static void main(final String[] args) throws Exception {

        if (args.length == 0) {
            System.err.println("Please specify your project name.");
            System.exit(1);
        }
        final String project = args[0];
        ManagedChannelImpl channelImpl = NettyChannelBuilder
            .forAddress("pubsub.googleapis.com", 443)
            .negotiationType(NegotiationType.TLS)
            .build();
        GoogleCredentials creds = GoogleCredentials.getApplicationDefault();
        // Down-scope the credential to just the scopes required by the service
        creds = creds.createScoped(Arrays.asList("https://www.googleapis.com/auth/pubsub"));
        // Intercept the channel to bind the credential
        ExecutorService executor = Executors.newSingleThreadExecutor();
        ClientAuthInterceptor interceptor = new ClientAuthInterceptor(creds, executor);
        Channel channel = ClientInterceptors.intercept(channelImpl, interceptor);
        // Create a stub using the channel that has the bound credential
        PublisherGrpc.PublisherBlockingStub publisherStub = PublisherGrpc.newBlockingStub(channel);
        ListTopicsRequest request = ListTopicsRequest.newBuilder()
                .setPageSize(10)
                .setProject("projects/" + project)
                .build();
        ListTopicsResponse resp = publisherStub.listTopics(request);
        System.out.println("Found " + resp.getTopicsCount() + " topics.");
        for (Topic topic : resp.getTopicsList()) {
            System.out.println(topic.getName());
        }
    }