public String getContentMd5() {
        return contentMd5;
    }
Returns the Base64-encoded MD5 hash of the object content that was
calculated on the client-side. This method returns null if the MD5
validation is disabled and the caller didn't provide the MD5 hash in the
ObjectMetadata when sending the PutObjectRequest.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/model/PutObjectResult.java