code: public Blob composeBlobs(
        String bucketName, String blobName, String sourceBlob1, String sourceBlob2) {
      // [START composeBlobs]
      BlobId blobId = BlobId.of(bucketName, blobName);
      BlobInfo blobInfo = BlobInfo.newBuilder(blobId).setContentType("text/plain").build();
      ComposeRequest request =
          ComposeRequest.newBuilder()
              .setTarget(blobInfo)
              .addSource(sourceBlob1)
              .addSource(sourceBlob2)
              .build();
      Blob blob = storage.compose(request);
      // [END composeBlobs]
      return blob;
    }
docs: [VARIABLE "source_blob_2"]