chore(samples): update UploadObjectFromMemory to use the most appropriate method (#2495)

Author: BenWhitehead

java-storage/samples/snippets/src/main/java/com/example/storage/object/UploadObjectFromMemory.java

@@ -22,7 +22,6 @@
 import com.google.cloud.storage.BlobInfo;
 import com.google.cloud.storage.Storage;
 import com.google.cloud.storage.StorageOptions;
-import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 
@@ -45,7 +44,25 @@ public static void uploadObjectFromMemory(
     BlobId blobId = BlobId.of(bucketName, objectName);
     BlobInfo blobInfo = BlobInfo.newBuilder(blobId).build();
     byte[] content = contents.getBytes(StandardCharsets.UTF_8);
-    storage.createFrom(blobInfo, new ByteArrayInputStream(content));
+
+    // Optional: set a generation-match precondition to enable automatic retries, avoid potential
+    // race
+    // conditions and data corruptions. The request returns a 412 error if the
+    // preconditions are not met.
+    Storage.BlobTargetOption precondition;
+    if (storage.get(bucketName, objectName) == null) {
+      // For a target object that does not yet exist, set the DoesNotExist precondition.
+      // This will cause the request to fail if the object is created before the request runs.
+      precondition = Storage.BlobTargetOption.doesNotExist();
+    } else {
+      // If the destination already exists in your bucket, instead set a generation-match
+      // precondition. This will cause the request to fail if the existing object's generation
+      // changes before the request runs.
+      precondition =
+          Storage.BlobTargetOption.generationMatch(
+              storage.get(bucketName, objectName).getGeneration());
+    }
+    storage.create(blobInfo, content, precondition);
 
     System.out.println(
         "Object "