code: public Policy listBucketIamMembers(String bucketName) {
      // [START view_bucket_iam_members]
      // Initialize a Cloud Storage client
      Storage storage = StorageOptions.getDefaultInstance().getService();
  
      // Get IAM Policy for a bucket
      Policy policy = storage.getIamPolicy(bucketName);
  
      // Print Roles and its identities
      Map<Role, Set<Identity>> policyBindings = policy.getBindings();
      for (Map.Entry<Role, Set<Identity>> entry : policyBindings.entrySet()) {
        System.out.printf("Role: %s Identities: %s\n", entry.getKey(), entry.getValue());
      }
      // [END view_bucket_iam_members]
      return policy;
    }
docs: Example of listing the Bucket-Level IAM Roles and Members