String cred_env = "/path/to/svc_account.json";
ServiceAccountCredentials source_credentials = ServiceAccountCredentials
		.fromStream(new FileInputStream(cred_env));

source_credentials = (ServiceAccountCredentials) source_credentials
		.createScoped(Arrays.asList("https://www.googleapis.com/auth/iam"));

ImpersonatedCredentials target_credentials = ImpersonatedCredentials.create(
    source_credentials,
   "[email protected]", 
   null,
   Arrays.asList("https://www.googleapis.com/auth/devstorage.read_only"), 3600);

Storage storage_service = StorageOptions.newBuilder().setProjectId("project-id")
		.setCredentials(target_credentials).build().getService();

for (Bucket b : storage_service.list().iterateAll()) 
  System.out.println(b);