Android 菜单项上的NullPointerException

我有一个空指针问题。我正在尝试下一步： 当我的应用程序启动时，它会检查用户是否登录，如果是，我会更改我的UIupdateUI（）

因此，我无法更改我的navLogInnavNewAcc和navLogOut

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

private FirebaseAuth mAuth;

MenuItem navLogIn;
MenuItem navNewAcc;
MenuItem navLogOut;
TextView navHeaderName;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.addDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    mAuth = FirebaseAuth.getInstance();

    navLogIn = (MenuItem) findViewById(R.id.nav_log_in);
    navNewAcc = (MenuItem) findViewById(R.id.nav_new_account);
    navLogOut = (MenuItem) findViewById(R.id.nav_log_out);
    navHeaderName = (TextView) findViewById(R.id.nav_header_name);

    // Kreiranje liste
    ListView listView=(ListView)findViewById(R.id.listViewMain);
    CategoryAdapter categoryAdapter = new CategoryAdapter(MainActivity.this);
    listView.setAdapter(categoryAdapter);

}

@Override
public void onStart() {
    super.onStart();
    // Check if user is signed in (non-null) and update UI accordingly.
    FirebaseUser currentUser = mAuth.getCurrentUser();
    updateUI(currentUser);
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_home) {
        Intent intent = new Intent(this,MainActivity.class);
        startActivity(intent);

    } else if (id == R.id.nav_new_ad) {
        Intent intent = new Intent(this,NewAdActivity.class);
        startActivity(intent);

    } else if (id == R.id.nav_log_in) {
        Intent intent = new Intent(this,LogInActivity.class);
        startActivity(intent);

    } else if (id == R.id.nav_search) {

    } else if (id == R.id.nav_new_account) {
        Intent intent = new Intent(this,RegisterActivity.class);
        startActivity(intent);

    } else if (id == R.id.nav_log_out) {
        mAuth.signOut();
        resetUI();
        Toast.makeText(MainActivity.this, "Izlogovani ste", Toast.LENGTH_SHORT).show();

    } else if (id == R.id.nav_about_us) {
        Intent intent = new Intent(this,AboutUsActivity.class);
        startActivity(intent);

    } else if (id == R.id.nav_share) {

    } else if (id == R.id.nav_follow_us) {

    } else if (id == R.id.nav_contact) {
        Intent intent = new Intent(this,ContactActivity.class);
        startActivity(intent);

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

private void resetUI() {

    navLogIn.setVisible(true);
    navNewAcc.setVisible(true);
    navLogOut.setVisible(false);

}

private void updateUI(FirebaseUser currentUser) {
    // TODO

    navLogIn.setVisible(false);
    navNewAcc.setVisible(false);
    navLogOut.setVisible(true);

    Toast.makeText(MainActivity.this, currentUser.getEmail().toString(), Toast.LENGTH_SHORT).show();

}
}

我收到这个错误：

java.lang.NullPointerException:尝试调用接口方法 上的“android.view.MenuItem android.view.MenuItem.setVisible（布尔）”命令 空对象引用

main.xml

    <?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto">

<item
    android:id="@+id/action_settings"
    android:orderInCategory="100"
    android:title="@string/action_settings"
    android:visible="false"
    app:showAsAction="never"></item>
</menu>

活动\u main\u drawer.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:showIn="nav_view">

    <group>
        <item
            android:id="@+id/nav_home"
            android:title="Početna" />
        <item
            android:id="@+id/nav_new_ad"
            android:title="Postavi Oglas" />
        <item
            android:id="@+id/nav_log_in"
            android:title="Prijava" />
        <item
            android:id="@+id/nav_search"
            android:title="Pretraga"
            android:visible="false" />
        <item
            android:id="@+id/nav_new_account"
            android:title="Kreiraj nalog" />
        <item
            android:visible="false"
            android:id="@+id/nav_log_out"
            android:title="Izlogujte se" />
    </group>

    <item android:title="Komunikacija">
        <menu>
            <item
                android:id="@+id/nav_about_us"
                android:title="O Nama" />
            <item
                android:id="@+id/nav_share"
                android:title="Podeli"
                android:visible="false" />
            <item
                android:id="@+id/nav_follow_us"
                android:title="Pratite nas"
                android:visible="false" />
            <item
                android:id="@+id/nav_contact"
                android:title="Kontakt" />
        </menu>
    </item>

</menu>

---

尝试在

oncreateoptions菜单

方法中查找您的

MenuItems

。像这样：

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.activity_main_drawer, menu);
    navLogIn = (MenuItem) menu.findItem(R.id.nav_log_in);
    navNewAcc = (MenuItem) menu.findItem(R.id.nav_new_account);
    navLogOut = (MenuItem) menu.findItem(R.id.nav_log_out);
    return true;
}

你可以用

Menu navMenuLogIn = navigationView.getMenu();
navMenuLogIn.findItem(R.id.nav_log_in).setVisible(true);

确保所有菜单项都与

navigationView.getMenu（）

绑定，我认为这将解决您的问题

在代码中

---

您从oncreate获取菜单项，您需要从OnCreateOptions获取菜单项能否提供更多详细信息？请共享main.xml文件您的菜单在抽屉中，而不是main.xml中，只有在您第一次单击菜单按钮，强制调用OnCreateOptions菜单以填充变量时，这些菜单才会膨胀。您可以在onCreate内部调用InvalidateOptions菜单，因此，在调用OnStart内部的updateUI之前，您将能够获得这些引用，您应该像下面的getMenuInflater（）那样为活动主抽屉充气（R.menu.activity\u main\u drawer，menu）；因为那是你的菜单项所在，我现在很困惑。我应该把代码放在哪里？在onCreate（）中？它解决了我的问题，非常感谢你，伙计：D我正在通过制作一个真正的应用程序来学习android

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

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
    navMenus= navigationView.getMenu();
    ......
    ......

}

private void updateUI(FirebaseUser currentUser) {
    // TODO

    navMenus.findItem(R.id.nav_log_in).setVisible(true);
    navMenus.findItem(R.id.nav_new_account).setVisible(false);
    navMenus.findItem(R.id.nav_log_out).setVisible(true);


    Toast.makeText(MainActivity.this, currentUser.getEmail().toString(), Toast.LENGTH_SHORT).show();

}