code: public Bucket setRetentionPolicy(String bucketName, Long retentionPeriod)
        throws StorageException {
      // [START storage_set_retention_policy]
      // Instantiate a Google Cloud Storage client
      Storage storage = StorageOptions.getDefaultInstance().getService();
  
      // The name of a bucket, e.g. "my-bucket"
      // String bucketName = "my-bucket";
  
      // The retention period for objects in bucket
      // Long retentionPeriod = 3600L; // 1 hour in seconds
  
      Bucket bucketWithRetentionPolicy =
          storage.update(
              BucketInfo.newBuilder(bucketName).setRetentionPeriod(retentionPeriod).build());
  
      System.out.println(
          "Retention period for "
              + bucketName
              + " is now "
              + bucketWithRetentionPolicy.getRetentionPeriod());
      // [END storage_set_retention_policy]
      return bucketWithRetentionPolicy;
    }
docs: Example of setting a retention policy on a bucket