从活动启动 Fragment 活动会导致 NullPointerException

Question

我正在尝试将片段添加到我的项目应用程序的仪表板。但是，我得到了 nullpointerexception 在bottomMenu函数的.setOnItemSelectedListener中，代码如下：

仪表板.java

public class Dashboard extends AppCompatActivity {

ChipNavigationBar chipNavigationBar;

@Override
protected void onCreate(Bundle savedInstanceState) {
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    super.onCreate(savedInstanceState);

    chipNavigationBar = findViewById(R.id.bottom_nav_menu);

    getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new DashboardFragment()).commit();
    
    bottomMenu();

}

private void bottomMenu() {

    chipNavigationBar.setOnItemSelectedListener(new ChipNavigationBar.OnItemSelectedListener() {
        @Override
        public void onItemSelected(int i) {
                Fragment fragment = null;
            switch (i){
                case R.id.bottom_nav_dashboard:
                    fragment = new DashboardFragment();
                    break;
                case R.id.bottom_nav_explore:
                    fragment = new ExploreFragment();
                    break;
                case R.id.bottom_nav_profile:
                    fragment = new ProfileFragment();
                    break;
            }
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, fragment).commit();
        }
    });

}

}

LogCat：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.desikhao, PID: 11751
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.desikhao/com.example.desikhao.LocationOwner.Dashboard}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.ismaeldivita.chipnavigation.ChipNavigationBar.setOnItemSelectedListener(com.ismaeldivita.chipnavigation.ChipNavigationBar$OnItemSelectedListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3308)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3457)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2044)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:224)
    at android.app.ActivityThread.main(ActivityThread.java:7562)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.ismaeldivita.chipnavigation.ChipNavigationBar.setOnItemSelectedListener(com.ismaeldivita.chipnavigation.ChipNavigationBar$OnItemSelectedListener)' on a null object reference
    at com.example.desikhao.LocationOwner.Dashboard.bottomMenu(Dashboard.java:39)
    at com.example.desikhao.LocationOwner.Dashboard.onCreate(Dashboard.java:33)
    at android.app.Activity.performCreate(Activity.java:7893)
    at android.app.Activity.performCreate(Activity.java:7880)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1307)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3283)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3457) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2044) 
    at android.os.Handler.dispatchMessage(Handler.java:107) 
    at android.os.Looper.loop(Looper.java:224) 
    at android.app.ActivityThread.main(ActivityThread.java:7562) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950) 

起初，我认为问题是因为我在片段中存储了一个空值，如果是这种情况，那么我该如何解决它，我是新手，所以请让我知道我在这里做错了什么.

Answer 1

您的 chipNavigationBar 变量为 null，因此 OnItemSelectedListener 被实例化为 null，这是不可能的。

你试图在Dashboard中找到你的chipNavigationBar，但你甚至没有设置setContentView(R.layout.yourLayout);，这意味着无法找到ViewById。

你必须在你的片段中实例化变量，在那里你有一个有效的视图。

chipNavigationBar = findViewById(R.id.bottom_nav_menu);