java - 无法创建底部导航 View - 尝试膨胀时变空

Question

我正在尝试使底部导航 View 与 Activity 而不是 fragment 一起使用。

我试图在我的 Activity 中调用它，如下所示:

...
    private TextView mTextMessage;
    BottomNavigationView navigation;

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

        navigation = findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

        // Ensure correct menu item is selected (where the magic happens)
        Menu menu = navigation.getMenu();
        MenuItem menuItem = menu.getItem(0);
        menuItem.setChecked(true);

        searchData = new ArrayList<>();
        searchAdapter = new SearchResultsAdapter(this, searchData);
        ListView listView = findViewById(R.id.list);
        listView.setAdapter(searchAdapter);
    }
    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.navigation_home:
                    mTextMessage.setText(R.string.title_home);
                    startActivity(new Intent(SearchDownload.this, PlayerActivity.class));
                    return true;
                case R.id.navigation_dashboard:
                    mTextMessage.setText(R.string.title_dashboard);
                    return true;
                case R.id.navigation_notifications:
                    mTextMessage.setText(R.string.title_notifications);
                    return true;
            }
            return false;
        }
    };
...

我收到此错误:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
        at com.example.liudas.bakalaurinis40.SearchDownload.onCreate(SearchDownload.java:77)

我的导航 Activity 中确实有正确的标签。

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".NavigationActivity">

<TextView
    android:id="@+id/message"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="@dimen/activity_horizontal_margin"
    android:layout_marginLeft="@dimen/activity_horizontal_margin"
    android:layout_marginTop="@dimen/activity_vertical_margin"
    android:text="@string/title_home"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintTop_toTopOf="parent" />
<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/navigation"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginStart="0dp"
    android:layout_marginEnd="0dp"
    android:background="?android:attr/windowBackground"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:menu="@menu/navigation" />

看来我正在使用正确的库: 导入 com.google.android.material.bottomnavigation.BottomNavigationView;

不确定还需要检查什么。

Answer 1

与:

setContentView(R.layout.activity_main);

您膨胀了activity_main.xml的布局。
包含 BottomNavigationView 的 xml 文件不是 activity_main.xml，对吗？
所以这个:

navigation = findViewById(R.id.navigation);

返回null，因为无法找到id为navigation的小部件。