code: public Bucket setDefaultKmsKey(String bucketName, String kmsKeyName) throws StorageException {
      // [START storage_set_bucket_default_kms_key]
      // Instantiate a Google Cloud Storage client
      Storage storage = StorageOptions.getDefaultInstance().getService();
  
      // The name of the existing bucket to set a default KMS key for, e.g. "my-bucket"
      // String bucketName = "my-bucket"
  
      // The name of the KMS-key to use as a default
      // Key names are provided in the following format:
      // 'projects/<PROJECT>/locations/<LOCATION>/keyRings/<RING_NAME>/cryptoKeys/<KEY_NAME>'
      // String kmsKeyName = ""
  
      BucketInfo bucketInfo =
          BucketInfo.newBuilder(bucketName).setDefaultKmsKeyName(kmsKeyName).build();
  
      Bucket bucket = storage.update(bucketInfo);
  
      System.out.println("Default KMS Key Name: " + bucket.getDefaultKmsKeyName());
      // [END storage_set_bucket_default_kms_key]
      return bucket;
    }
docs: Example of setting a default KMS key on a bucket.