private void checkMutability() {
        if (isImmutable) {
            throw new UnsupportedOperationException(
                    "TransferManager is immutable when created with the builder.");
        }
    }
If the client has been marked as immutable then throw an {@link
UnsupportedOperationException}, otherwise do nothing. Should be called by each mutating
method.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/transfer/TransferManager.java