private com.google.protobuf.SingleFieldBuilderV3<
              com.google.storagetransfer.v1.proto.TransferTypes.GcsData,
              com.google.storagetransfer.v1.proto.TransferTypes.GcsData.Builder,
              com.google.storagetransfer.v1.proto.TransferTypes.GcsDataOrBuilder>
          getGcsDataSourceFieldBuilder() {
        if (gcsDataSourceBuilder_ == null) {
          if (!(dataSourceCase_ == 1)) {
            dataSource_ =
                com.google.storagetransfer.v1.proto.TransferTypes.GcsData.getDefaultInstance();
          }
          gcsDataSourceBuilder_ =
              new com.google.protobuf.SingleFieldBuilderV3<
                  com.google.storagetransfer.v1.proto.TransferTypes.GcsData,
                  com.google.storagetransfer.v1.proto.TransferTypes.GcsData.Builder,
                  com.google.storagetransfer.v1.proto.TransferTypes.GcsDataOrBuilder>(
                  (com.google.storagetransfer.v1.proto.TransferTypes.GcsData) dataSource_,
                  getParentForChildren(),
                  isClean());
          dataSource_ = null;
        }
        dataSourceCase_ = 1;
        onChanged();
        return gcsDataSourceBuilder_;
      }
<pre>
The Cloud Storage bucket from which to replicate objects.
</pre>

<code>.google.storagetransfer.v1.GcsData gcs_data_source = 1;</code>
java-storage-transfer/proto-google-cloud-storage-transfer-v1/src/main/java/com/google/storagetransfer/v1/proto/TransferTypes.java