driver-legacy/src/main/com/mongodb/MongoClient.java
@Nullable
    public MongoCredential getCredential() {
        if (getCredentialsList().size() > 1) {
            throw new IllegalStateException("Instance constructed with more than one MongoCredential");
        } else if (getCredentialsList().isEmpty()) {
            return null;
        } else {
            return getCredentialsList().get(0);
        }
    }
@Nullable
    public MongoCredential getCredential() {
        if (getCredentialsList().size() > 1) {
            throw new IllegalStateException("Instance constructed with more than one MongoCredential");
        } else if (getCredentialsList().isEmpty()) {
            return null;
        } else {
            return getCredentialsList().get(0);
        }
    }
Gets the credential that this client authenticates all connections with

@return the credential, which may be null in unsecured deployments
@since 3.9