public class MainActivity extends AppCompatActivity {

    private FirebaseAuth mAuth;

    EditText email, pass;
    private String TAG = MainActivity.class.getName();
    AuthCredential emailAuthProvider;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAuth = FirebaseAuth.getInstance();

        email = findViewById(R.id.editText);
        pass = findViewById(R.id.editText2);
    }

    public void sign(View view){
        String email2 = email.getText().toString();
        String passs = pass.getText().toString();


        mAuth.signInWithEmailAndPassword(email2, passs)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            // Sign in success, update UI with the signed-in user's information
                            Log.d(TAG, "signInWithEmail:success");
                            FirebaseUser user = mAuth.getCurrentUser();



                        } else {
                            // If sign in fails, display a message to the user.
                            Log.w(TAG, "signInWithEmail:failure", task.getException());
                            Toast.makeText(MainActivity.this, "Authentication failed.",
                                    Toast.LENGTH_SHORT).show();

                        }

                        // ...
                    }
                });

        linkUserAuth(email2, passs);
    }



    private void linkUserAuth(String email, String pass){
        AuthCredential credential = EmailAuthProvider.getCredential(email, pass);
        mAuth.getCurrentUser().linkWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener<AuthResult>(){
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if(task.isSuccessful()){
                    Log.d(TAG,"linkWithCredential:success");
                    FirebaseUser mergeAuthUser=task.getResult().getUser();

                }else{
                    Log.w(TAG,"linkWithCredential:failure",task.getException());
                }
            }
        });
    }
}