Sign in existing users

Sign in also looks for two string parameters, which are valid email address and password. However, here, we will make use of another method called signInWithEmailAndPassword. We need to attach addOnCompleteListener and with its callbacks, as shown:

mAuth.signInWithEmailAndPassword(email, password)
        .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    // Successful signin
                    Log.d(TAG, "signInWithEmail:success");
                    FirebaseUser user = mAuth.getCurrentUser();
                    updateUI(user);
                } else {
                    // failed for some reason
                    Log.w(TAG, "signInWithEmail:failure", task.getException());
                    Toast.makeText(MainActivity.this, "Authentication failed.",
                            Toast.LENGTH_SHORT).show();
                    updateUI(null);
                }

                // ...
            }
        });

This code explains that when the task object is successful, we can retrieve the user information through the FirebaseUser object; for instance, consider the following code, which retrieves the information from the authorized user:

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
if (user != null) {
        // These are the information we will be able to retrieve
        // Name, email address, and profile photo Url

        String name = user.getDisplayName();
        String email = user.getEmail();
        Uri photoUrl = user.getPhotoUrl();

        // Check if user's email is verified
        boolean emailVerified = user.isEmailVerified();

        // The user's ID, unique to the Firebase project.
        // UID shall not be used to server purpose
        // FirebaseUser.getToken() instead.
        String uid = user.getUid();
    }