How to wait for an asynchronous method

Question

I need to return the value uId. I am getting the correct value in the first log statement inside the onResponse() function. But when it comes to the return statement it returns null.

I think that onResponse() is running on another thread. If so, how can I make the getNumber() function wait till the onResponse() function finishes execution.(Like thread.join())

Or is there any other solution?

code :

String uId;
public String getNumber() {

    ApiInterface apiInterface = ApiClient.getClient().create(ApiInterface.class);
    Call<TopLead> call = apiInterface.getTopLead();
    call.enqueue(new Callback<TopLead>() {
        @Override
        public void onResponse(Call<TopLead> call, Response<TopLead> response) {
            String phoneNumber;
            TopLead topLead = response.body();
            if (topLead != null) {
                phoneNumber = topLead.getPhoneNumber().toString();
                uId = topLead.getUId().toString();
                //dispaly the correct value of uId
                Log.i("PHONE NUMBER, UID", phoneNumber +", " + uId);
                onCallCallback.showToast("Calling " + phoneNumber);
            } else {
                onCallCallback.showToast("Could not load phone number");
            }
        }

        @Override
        public void onFailure(Call<TopLead> call, Throwable t) {
            t.printStackTrace();
        }
    });
    //output: Return uid null
    Log.i("Return"," uid" + uId);
    return uId; 

Answer 1

Your method performs an async request. So the action "return uId; " doesn't wait until your request finishes because they are on different threads.

There are several solutions I can suggest

1. Use an interface callback

    public void getNumber(MyCallback callback) {
      ...
       phoneNumber = topLead.getPhoneNumber().toString();
       callback.onDataGot(phoneNumber);
    }
   

Your callback interface

     public interface MyCallback {

        void onDataGot(String number);
     }

And finally, calling the method

getNumber(new MyCallback() {
    @Override
    public void onDataGot(String number) {
    // response
    }
});

2. When using Kotlin (I think it's time for you to use Kotlin instead of Java :))

   fun getNumber(onSuccess: (phone: String) -> Unit) {
     phoneNumber = topLead.getPhoneNumber().toString()
     onSuccess(phoneNumber)
   }
   

Calling the method

    getNumber {
      println("telephone $it")
    }