private com.google.protobuf.SingleFieldBuilderV3<
            com.google.cloud.tpu.v2alpha1.CustomerEncryptionKey,
            com.google.cloud.tpu.v2alpha1.CustomerEncryptionKey.Builder,
            com.google.cloud.tpu.v2alpha1.CustomerEncryptionKeyOrBuilder>
        getCustomerEncryptionKeyFieldBuilder() {
      if (customerEncryptionKeyBuilder_ == null) {
        customerEncryptionKeyBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.cloud.tpu.v2alpha1.CustomerEncryptionKey,
                com.google.cloud.tpu.v2alpha1.CustomerEncryptionKey.Builder,
                com.google.cloud.tpu.v2alpha1.CustomerEncryptionKeyOrBuilder>(
                getCustomerEncryptionKey(), getParentForChildren(), isClean());
        customerEncryptionKey_ = null;
      }
      return customerEncryptionKeyBuilder_;
    }
<pre>
Optional. Customer encryption key for boot disk.
</pre>

<code>
.google.cloud.tpu.v2alpha1.CustomerEncryptionKey customer_encryption_key = 1 [(.google.api.field_behavior) = OPTIONAL];
</code>
java-tpu/proto-google-cloud-tpu-v2alpha1/src/main/java/com/google/cloud/tpu/v2alpha1/BootDiskConfig.java