private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;

private String email;
private String password;


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

    final TextView emailEditText = (EditText) findViewById(R.id.emailField);
    final TextView passwordEditText = (EditText) findViewById(R.id.passwordField);
    Button loginButton = (Button) findViewById(R.id.loginButton);
    Button registerButton = (Button) findViewById(R.id.registerButton);

loginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            // Reterives user inputs
            email = emailEditText.getText().toString();
            password = passwordEditText.getText().toString();

            // trims the input
            email = email.trim();
            password = password.trim();
            // When a user signs in to your app, pass the user's email address and password to signInWithEmailAndPassword
            mAuth.signInWithEmailAndPassword(email, password)
                    .addOnCompleteListener(LoginActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            Log.d("TAG", "signInWithEmail:onComplete:" + task.isSuccessful());

                            // If sign in fails, display a message to the user. If sign in succeeds
                            // the auth state listener will be notified and logic to handle the
                            // signed in user can be handled in the listener.
                            if (!task.isSuccessful()) {
                                Log.w("TAG", "signInWithEmail", task.getException());
                                Toast.makeText(LoginActivity.this, "Authentication failed.",
                                        Toast.LENGTH_SHORT).show();
                            }
                        }
                    });

        }
    });
    // responds to changes in the user's sign-in state
    mAuthListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            FirebaseUser user = firebaseAuth.getCurrentUser();
            if (user != null) {
                // User is signed in
                Log.d("TAG", "onAuthStateChanged:signed_in:" + user.getUid());

                // Authenticated successfully with authData
                Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                startActivity(intent);

            } else {
                // User is signed out
                Log.d("TAG", "onAuthStateChanged:signed_out");
            }
        }
    };
    registerButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(LoginActivity.this, RegisterActivity.class));
        }
    });
}
@Override
public void onStart() {
    super.onStart();
    mAuth.addAuthStateListener(mAuthListener);
}

@Override
public void onStop() {
    super.onStop();
    if (mAuthListener != null) {
        mAuth.removeAuthStateListener(mAuthListener);
    }
}

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="false"
    android:theme="@style/AppTheme">
    <activity android:name=".LoginActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".MainActivity"></activity>
    <activity android:name=".RegisterActivity"></activity>
</application>