### 17.4 P3 — 죽은 매개변수 하나가 유일한 비기본값에서 NPE 를 낳는다

```java
private static BrokerCredentialProfile credential(
    String broker, String role, BrokerSecuritySettings.Credential credential, Supplier<Boolean> required) {
  if (credential == null && Boolean.TRUE.equals(required.get())) {
    throw configurationError(key("security", broker, role), "a configured broker needs a %s credential; …");
  }
  String type = credential.type() == null ? "" : credential.type().toUpperCase(Locale.ROOT);
```

호출처가 셋이고 전부 `() -> true` 다.

```java
credential(name, "producer", security.producer(), () -> true),