GCE how to add external IP to existing instance at boot - google-compute-engine

Question

I'm using Gcloud-java to manage some VM instances. The code to create a new instance is clear and is the following:
Address externalIp = compute.getAddress(addressId);
InstanceId instanceId = InstanceId.of("us-central1-a", "test-instance");
NetworkId networkId = NetworkId.of("default");
PersistentDiskConfiguration attachConfiguration =
PersistentDiskConfiguration.builder(diskId).boot(true).build();
AttachedDisk attachedDisk = AttachedDisk.of("dev0", attachConfiguration);
NetworkInterface networkInterface = NetworkInterface.builder(networkId)
.accessConfigurations(AccessConfig.of(externalIp.address()))
.build();
MachineTypeId machineTypeId = MachineTypeId.of("us-central1-a", "n1-standard-1");
InstanceInfo instance =
InstanceInfo.of(instanceId, machineTypeId, attachedDisk, networkInterface);
Operation operation = compute.create(instance);
// Wait for operation to complete
operation = operation.waitFor();
if (operation.errors() == null) {
System.out.println("Instance " + instanceId + " was successfully created");
} else {
// inspect operation.errors()
throw new RuntimeException("Instance creation failed");
}
But what should I do if I have en existing instance that I want to start and I want to attach an external IP?
I've tried in this way: first I create a RegionAddressId and get an Address with which I create the networkInterface.
RegionAddressId addressId = RegionAddressId.of("europe-west1", "test-address");
Operation operationAdd = compute.create(AddressInfo.of(addressId));
operationAdd = operationAdd.waitFor();
Address externalIp = compute.getAddress(addressId);
NetworkId networkId = NetworkId.of("default");
NetworkInterface networkInterface = NetworkInterface.builder(networkId)
.accessConfigurations(NetworkInterface.AccessConfig.of(externalIp.address()))
.build();
The I get my instance and add the accessConfig
InstanceId instanceId = InstanceId.of("my-server", "europe-west1-b","my-instance");
Instance instance = compute.getInstance(instanceId);
instance.addAccessConfig("default", NetworkInterface.AccessConfig.of(externalIp.address()));
Operation operation = instance.start();
The result is that my instance is booted with another external IP that I don't know how to obtain.
What is the correct procedure?
Thanks

Answer 1

I've found by myself the solution.
Compute compute = ComputeOptions.defaultInstance().service();
InstanceId instanceId = InstanceId.of("my-server", "europe-west1-b","my-instance");
Operation operation = compute.start(instanceId);
Operation completedOperation = operation.waitFor();
if (completedOperation == null) {
// operation no longer exists
} else if (completedOperation.errors() != null) {
// operation failed, handle error
}
Instance instance = compute.getInstance(instanceId);
String publicIp =
instance.networkInterfaces().get(0).accessConfigurations().get(0).natIp();
I start the instance using the start method of Compute and then (after the operation is completed) I get the instance