What exactly does the onComplete handler return after the operation is completed?

The onComplete handler in Android, for example in the context of Task from Firebase or other asynchronous operations, is called after the operation completes and returns a Task<T> object. This object contains information about the result of the operation: whether it was successful, if an error occurred, and if successful — the result itself.

Example of usage:

someTask.addOnCompleteListener(task -> {
    if (task.isSuccessful()) {
        // Get the result
        T result = task.getResult();
    } else {
        // Handle error
        Exception e = task.getException();
    }
});

Thus, onComplete does not directly return the result but provides a Task object from which the status and data of the operation can be obtained.