如何正确设置另一个活动的布局可见性？

Question

I have a LinearLayout in my BottomNavgigation's MainActivity, and I'm trying to hide the layout from one of the fragments, but I keep getting a nullpointer exception, and i'm quite certain everything is set right

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:orientation="vertical"
        android:background="@color/colorGrey"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        app:defaultNavHost="true"
        app:navGraph="@navigation/mobile_navigation" />

    <LinearLayout
        android:id="@+id/create_transaction_layout"  \\view I'm trying to set visibility on
        android:layout_width="match_parent"
        android:layout_height="@dimen/margin_padding_size_60"
        android:layout_gravity="bottom"
        android:layout_marginBottom="@dimen/margin_padding_size_24"
        android:gravity="center"
        android:visibility="visible">

        <LinearLayout
            android:id="@+id/send_receive_layout"
            android:layout_width="wrap_content"
            android:layout_height="@dimen/margin_padding_size_40"
            android:background="@drawable/card_bg"
            android:elevation="4dp"
            android:gravity="center_vertical"
            android:orientation="horizontal">

            <LinearLayout
                android:id="@+id/fab_send"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:background="@drawable/card_bg"
                android:clickable="true"
                android:focusable="true"
                android:gravity="center"
                android:paddingStart="@dimen/margin_padding_size_16"
                android:paddingEnd="@dimen/margin_padding_size_16">

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:drawableStart="@drawable/ic_communication_call_made"
                    android:drawablePadding="8dp"
                    android:gravity="center_vertical"
                    android:text="Send"
                    android:textColor="@color/colorPrimaryDark"
                    android:textSize="18sp"
                    app:fontFamily="@font/source_sans_pro" />

            </LinearLayout>

            <View
                android:layout_width="1dp"
                android:layout_height="24dp"
                android:background="@color/colorPrimary" />

            <LinearLayout
                android:id="@+id/fab_receive"
                android:layout_width="match_parent"
                android:layout_height="@dimen/margin_padding_size_40"
                android:background="@drawable/card_bg"
                android:clickable="true"
                android:focusable="true"
                android:gravity="center"
                android:paddingStart="@dimen/margin_padding_size_16"
                android:paddingEnd="@dimen/margin_padding_size_16">

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:drawableStart="@drawable/ic_communication_call_received"
                    android:drawablePadding="8dp"
                    android:gravity="center_vertical"
                    android:text="Receive"
                    android:textColor="@color/colorPrimaryDark"
                    android:textSize="18sp"
                    app:fontFamily="@font/source_sans_pro" />

            </LinearLayout>

        </LinearLayout>

    </LinearLayout>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:labelVisibilityMode="labeled"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_nav_menu" />

</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static BottomNavigationView navView;
    private LinearLayout createTransaction;

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

        createTransaction = findViewById(R.id.create_transaction_layout);

        navView = findViewById(R.id.nav_view);
        addBadgeView();
        AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
                R.id.navigation_home, R.id.navigation_transactions, R.id.navigation_wallets, R.id.navigation_account)
                .build();
        NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
//        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
        NavigationUI.setupWithNavController(navView, navController);
    }

    private void addBadgeView() {
        navView = findViewById(R.id.nav_view);
        navView.getOrCreateBadge(R.id.navigation_transactions); // Show badge
    }

    public void hideFAB() {
        createTransaction.setVisibility(View.GONE);
    }

}

and in my Fragment I try setting visibility like this

WalletFragment.java

private MainActivity mainActivity;

public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    walletsViewModel =
            ViewModelProviders.of(this).get(WalletsViewModel.class);
    View root = inflater.inflate(R.layout.fragment_wallets, container, false);

    mainActivity.hideFAB();

    return root;
}

please what could I have done wrong? as everything seems fine to me

Answer 1

Inside your fragment you can call

((MainActivity) requiredActivity()).hideFAB()

Answer 2

Use getActivity to get the instance of hosting activity, cast it to its class and then call the public methods.

( (MainActivity) getActivity() ).hideFAB()