Android；BottomNavigationView = null

Question

我不知道为什么BottomNavigationView为空。我使用Firebase，并添加了一些代码将数据添加到JSON文件中。在我这么做之后，我的一项活动就停止了。甚至不是我所从事的活动。我的导航等于null，我检查了XML文件。名称是相同的，调试也没有帮助，因为它向我展示了设置BottomNavigationView并给我一个NullPointer的一行代码。

这是正在创建此问题的导航：

private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener

    = new BottomNavigationView.OnNavigationItemSelectedListener() 

  {

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId()) {
            case R.id.navigation_to_dos:
                return true;
            case R.id.navigation_categories:
                intent = new Intent(ToDos.this, Categories.class);
                intent.setFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
                startActivity(intent);
                return true;
            case R.id.navigation_account:
                intent = new Intent(ToDos.this, Account.class);
                intent.setFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
                startActivity(intent);
                return true;
        }
        return false;
    }
};

这是我在onCreate()函数中初始化与Navigaion有关的所有内容的部分：

BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
        navigation.setSelectedItemId(R.id.navigation_to_dos);

我知道这个错误：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: lb335.bbbaden.ch.pendenzenverwaltung, PID: 25288
              java.lang.RuntimeException: Unable to start activity ComponentInfo{lb335.bbbaden.ch.pendenzenverwaltung/lb335.bbbaden.ch.pendenzenverwaltung.ToDos}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.BottomNavigationView.setOnNavigationItemSelectedListener(android.support.design.widget.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3184)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3279)
                  at android.app.ActivityThread.-wrap12(Unknown Source:0)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1881)
                  at android.os.Handler.dispatchMessage(Handler.java:108)
                  at android.os.Looper.loop(Looper.java:166)
                  at android.app.ActivityThread.main(ActivityThread.java:7406)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:926)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.BottomNavigationView.setOnNavigationItemSelectedListener(android.support.design.widget.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
                  at lb335.bbbaden.ch.pendenzenverwaltung.ToDos.onCreate(ToDos.java:76)
                  at android.app.Activity.performCreate(Activity.java:7365)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3137)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3279) 
                  at android.app.ActivityThread.-wrap12(Unknown Source:0) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1881) 
                  at android.os.Handler.dispatchMessage(Handler.java:108) 
                  at android.os.Looper.loop(Looper.java:166) 
                  at android.app.ActivityThread.main(ActivityThread.java:7406) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:926) 

我的活动XML：

<android.support.design.widget.BottomNavigationView
    android:id="@+id/navigation"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:background="?android:attr/windowBackground"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:menu="@menu/navigation" />

我在这个问题上迷失了方向，希望能得到更有经验的程序员的帮助。

编辑：

多亏了snachmsm，我才发现，我的函数顺序完全搞砸了。在实际设置内容之前，我执行了findViewById()。我不得不把它换掉，现在起作用了。

Answer 1

由: java.lang.NullPointerException:试图在空对象引用上调用虚拟方法'void 引起的

你的BottomNavigationView是null，这是事实。这一行不是获取您的View

BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);

您是否认为您的BottomNavigationView属于充气布局？R.id.navigation是否正确地设置为BottomNavigationView？显示来自onCreate方法的一些XML和更多代码。也许是不合适的秩序？(例如，findViewById在setContentView之前调用)

编辑:好的，id被正确地设置为XML内部的View，但是您只发布了其中的一小部分，还展示了如何在Activity中膨胀这个XML。

Answer 2

您的导航返回false，因为您将其设置为false

请检查导航的末尾，并将返回类型从return false;更改为return true;

private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener

= new BottomNavigationView.OnNavigationItemSelectedListener() 

 {

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    switch (item.getItemId()) {
        case R.id.navigation_to_dos:
            return true;
        case R.id.navigation_categories:
            intent = new Intent(ToDos.this, Categories.class);
            intent.setFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
            startActivity(intent);
            return true;
        case R.id.navigation_account:
            intent = new Intent(ToDos.this, Account.class);
            intent.setFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
            startActivity(intent);
            return true;
    }
    return false; // Here is the problem. Change it from false to true
}};

还有，BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);

Answer 3

在我的案例中，我也面临着这个问题，我通过替换正确的家庭活动来解决这个问题

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_seller_home);//here was problem,i was  inflating wrong home activity