Android 从片段隐藏/显示导航元素

我想在用户未登录时从应用程序导航栏中隐藏“Profile”菜单项。当用户最终登录时，我想做两件事：

隐藏“登录”按钮

显示“配置文件”按钮

这应该是直截了当的，尽管我不确定如何访问这些我希望关闭的菜单项，而且当我尝试关闭时，会不断收到NullPointerException

我不能像我尝试过的那样，仅按id选择元素

MenuItem profileBtn=view.findViewById（R.id.nav_profile）；
profileBtn.setVisible（true）当它抛出一个空点异常时：

java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.MenuItem.setVisible(boolean)' on a null object reference

下面是loginScreen.java片段，我从中查看如何在底部的“onClick”方法中显示profile按钮和隐藏login按钮


public class loginScreen extends Fragment implements View.OnClickListener{

    private loginScreenText loginScreenText;
    EditText email;
    EditText password;
    NavigationView navigationView;
    Button button;
    SimpleAccountManager accountManager;
    Context mContext;

    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        setHasOptionsMenu(true);

        mContext = getActivity().getApplicationContext();
        accountManager = new SimpleAccountManager(mContext);
        button = (Button) inflater.inflate(R.layout.login_page, container, false).findViewById(R.id.loginBtn);

        loginScreenText =
        ViewModelProviders.of(this).get(loginScreenText.class);
        View root = inflater.inflate(R.layout.login_page, container, false);
        final TextView textView = root.findViewById(R.id.text_share);
        button = root.findViewById(R.id.loginBtn);
        email = root.findViewById(R.id.email);
        navigationView = root.findViewById(R.id.nav_view);
        password = root.findViewById(R.id.password);
        button.setOnClickListener(this);

        return root;
    }

    public void onClick(View view){

        if(view == button){
            User user;
            if(accountManager.getUserByEmail(email.getText().toString()) != null){
                user = accountManager.getUserByEmail(email.getText().toString());


/*              Attempt to reference the profile button which results in NullPointerException as shown 
                above*/

                MenuItem profileBtn= view.findViewById(R.id.nav_profile);
                profileBtn.setVisible(true);

                Fragment profile = new profileScreen(mContext);
                FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
                FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
                fragmentTransaction.replace(((ViewGroup)(getView().getParent())).getId(), profile);
                fragmentTransaction.commit();


                //kill current fragment
                getFragmentManager().beginTransaction()
                        .remove(loginScreen.this).commit();


                accountManager.login(user, password.getText().toString());
            }   else{
                loginScreenText.setmText("No user found with email: " + email.getText().toString());
            }

        }
    }
}


以下是activity\u main\u drawer.xml中的菜单项



提前感谢任何对如何解决此问题提出建议的人
 您需要先获取导航视图，然后获取其菜单，然后从中获取菜单项，而不是直接使用findViewById获取菜单项

换言之：

NavigationView navigationView = findViewById(R.id.navigation_view); //use the proper id
Menu menu = navigationView.getMenu();
MenuItem menuItem = menu.findItem(R.id.nav_profile);

我在一个空对象引用上运行了虚拟方法“android.view.Menu com.google.android.material.navigation.NavigationView.getMenu（）”后得到了这个结果？
NavigationView navigationView = findViewById(R.id.navigation_view); //use the proper id
Menu menu = navigationView.getMenu();
MenuItem menuItem = menu.findItem(R.id.nav_profile);