What is the alternative to the deprecated 'GoogleCredential'?

I had to solve similar task when using Google Admin SDK API and
com.google.api.services.admin.directory.Directory was needed.

    final ServiceAccountCredentials serviceAccountCredentials = ServiceAccountCredentials.fromPkcs8(
            clientId,
            clientEmail,
            serviceAccountPkcs8Key,
            serviceAccountPkcs8Id,
            Arrays.asList(DirectoryScopes.ADMIN_DIRECTORY_USER_READONLY));
    final GoogleCredentials delegatedCredentials = serviceAccountCredentials.createDelegated(delegatedUserEmail);
    HttpRequestInitializer requestInitializer = new HttpCredentialsAdapter(delegatedCredentials);

    Directory directory = new Directory.Builder(httpTransport, JSON_FACTORY, requestInitializer)
            .setApplicationName(applicationName)
            .build();

After a while of looking around, I managed to fix it, using GoogleCredentials and HttpRequestInitializer. The code changes are as follows.

final GoogleCredential googleCredential = GoogleCredential
  .fromStream(Objects.requireNonNull(classloader.getResourceAsStream("Key.json")))
  .createScoped(Collections.singletonList(StorageScopes.DEVSTORAGE_FULL_CONTROL));

final com.google.api.services.storage.Storage myStorage = new com.google.api.services.storage.Storage.Builder(
      new NetHttpTransport(), new JacksonFactory(), googleCredential).build();

becomes

final GoogleCredentials googleCredentials = serviceAccountCredentials
                    .createScoped(Collections.singletonList(StorageScopes.DEVSTORAGE_FULL_CONTROL));
            HttpRequestInitializer requestInitializer = new HttpCredentialsAdapter(googleCredentials);        

final com.google.api.services.storage.Storage myStorage = new com.google.api.services.storage.Storage.Builder(
                new NetHttpTransport(), new JacksonFactory(), requestInitializer).build();

You can find an alternative solution as being posted on the Google APIs Github repository commits.

Please use Google Auth Library for Java for handling Application Default Credentials and other non-OAuth2 based authentication.

Explicit Credential Loading sample code:

To get Credentials from a Service Account JSON key use GoogleCredentials.fromStream(InputStream) or GoogleCredentials.fromStream(InputStream, HttpTransportFactory). Note that the credentials must be refreshed before the access token is available.

GoogleCredentials credentials = GoogleCredentials.fromStream(new FileInputStream("/path/to/credentials.json"));
credentials.refreshIfExpired();
AccessToken token = credentials.getAccessToken();
// OR
AccessToken token = credentials.refreshAccessToken();