空对象引用上的navigationView setText致命异常

Question

好的，所以我一直在寻找解决此问题的方法，而我发现的所有解决方案都无法正常工作。 我正在尝试将我的navigationView中的“登录”按钮的文本更改为“注销/登录”，具体取决于它们是否已登录，然后在按下按钮时将其发送以登录或注销。 出于某种原因，我的应用程序在空对象引用（以下某些日志）上发生致命异常而崩溃。 我究竟做错了什么？

我创建了“ signInOutText（）;” 所以我可以尝试将其放置在不同的位置，它目前在onResume中，但我也尝试过在onCreate中，最初将其全部放在onResume中。 希望我只是忽略一些简单的事情。 我从下面的MainActivity.java中裁剪了一些代码，虽然时间不长，但是让我知道您是否需要查看全部内容。

MainActivity.java

public class MainActivity extends AppCompatActivity {

//Defining Variables
private Toolbar toolbar;
private NavigationView navigationView;
private DrawerLayout drawerLayout;
String category;
MenuItem mPreviousMenuItem;

private ProgressDialog mProgressDialog;

String providerId,uid,name,email;
Uri photoUrl;

private static final String TAG = "MainActivity";

// set name of your preferences list
private static String MY_PREFERENCES = "Settings";
String userNameLoad, emailLoad, profileLoad;
TextView usernameTextView, emailTextView, signInTextView;

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

    // Initializing Toolbar and setting it as the actionbar
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    //Initializing NavigationView
    navigationView = (NavigationView) findViewById(R.id.navigation_view);


    //Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {

        // This method will trigger on item Click of navigation menu
        @Override
        public boolean onNavigationItemSelected(MenuItem menuItem) {

            menuItem.setCheckable(true);
            menuItem.setChecked(true);
            if (mPreviousMenuItem != null) {
                mPreviousMenuItem.setChecked(false);
            }
            mPreviousMenuItem = menuItem;

            //Closing drawer on item click
            drawerLayout.closeDrawers();


            //Check to see which item was being clicked and perform appropriate action
            switch (menuItem.getItemId()){

                //Replacing the main content with fragments                    
                case R.id.home:
                    //code for button
                    break;
                case R.id.cat1:
                    //code for button
                    break;
                case R.id.signIn:                        
                    signInOut();
                    break;
                default:                        
                    break;
            }               

            return true;
        }
    });

    // Initializing Drawer Layout and ActionBarToggle
    drawerLayout = (DrawerLayout) findViewById(R.id.drawer);
    ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this,drawerLayout,toolbar,R.string.openDrawer, R.string.closeDrawer){

        @Override
        public void onDrawerClosed(View drawerView) {
            // Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
            super.onDrawerClosed(drawerView);
        }

        @Override
        public void onDrawerOpened(View drawerView) {
            // Code here will be triggered once the drawer open as we don't want anything to happen so we leave this blank

            super.onDrawerOpened(drawerView);
        }
    };

    //Setting the actionbarToggle to drawer layout
    drawerLayout.setDrawerListener(actionBarDrawerToggle);

    //calling sync state is necessary or else your hamburger icon wont show up
    actionBarDrawerToggle.syncState();
}

private void signInOutText() {

    //Initializing NavigationView
    navigationView = (NavigationView) findViewById(R.id.navigation_view);
    //Get header xml from navigationView
    View header = navigationView.getHeaderView(0);

    //Check if user logged in, change sign in sign out button to correct text
    FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
    if (user != null) {
        signInTextView = (TextView) header.findViewById(R.id.signIn);
        signInTextView.setText(R.string.sign_out);
    } else {
        signInTextView = (TextView) header.findViewById(R.id.signIn);
        signInTextView.setText(R.string.sign_in);
    }
}

private void signInOut(){ 
    //TODO add Code .... if logged in logout
    Intent intent = new Intent(MainActivity.this, ChooserActivity.class);
    startActivity(intent);
    finish();

}

public void setActionBarTitle(String title) {
    if(getSupportActionBar() != null){
        getSupportActionBar().setTitle(title);
    }
}

@Override
public void onResume() {
    super.onResume();
    // Set title
    setActionBarTitle(getString(R.string.app_name));
    signInOutText();
}

}

日志记录

Unable to resume activity {com.test.app/com.test.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void
android.widget.TextView.setText(int)' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4156)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4250)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3361)
at android.app.ActivityThread.access$1100(ActivityThread.java:222)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7229)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(int)' on a null object reference
at com.test.app.MainActivity.signInOutText(MainActivity.java:203)
at com.test.app.MainActivity.onResume(MainActivity.java:307)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1286)
at android.app.Activity.performResume(Activity.java:6987)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4145)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4250) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3361) 
at android.app.ActivityThread.access$1100(ActivityThread.java:222) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1795) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:158) 
at android.app.ActivityThread.main(ActivityThread.java:7229) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

activity_main.xml

<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawer"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context=".MainActivity">

<LinearLayout
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:orientation="vertical">
    <include
        layout="@layout/toolbar"
        />
    <FrameLayout
        android:id="@+id/frame"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </FrameLayout>

</LinearLayout>

<android.support.design.widget.NavigationView
    android:id="@+id/navigation_view"
    android:layout_height="match_parent"
    android:layout_width="wrap_content"
    android:layout_gravity="start"
    app:headerLayout="@layout/header"
    app:menu="@menu/drawer"
    />

menu / drawer.xml

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

    <group android:id="@+id/drawer_group"
        android:checkable="true">


        <item
            android:id="@+id/home"
            android:checked="false"
            android:icon="@drawable/ic_home_black_24dp"
            android:title="@string/home_string" />
    </group>

    <group android:id="@+id/group_cat"
        android:checkable="true">

        <item
            android:id="@+id/menuCategories"
            android:title="@string/categories_string"
            android:layout_gravity="bottom">

            <menu>

                <item
                    android:id="@+id/cat1"
                    android:checked="false"
                    android:icon="@drawable/ic_inbox_black"
                    android:title="@string/cat1_string" />

            </menu>

        </item>

    </group>


    <group android:id="@+id/group_signInOut"
        android:checkable="true">

        <item
            android:id="@+id/signIn"
            android:checked="false"
            android:icon="@drawable/ic_power_settings_new_black_24dp"
            android:title="@string/sign_in" />

    </group>

</menu>

谢谢您的帮助！

Answer 1

好的，所以我意识到我正在尝试更新菜单“项目”而不是textView，因此起了很大的作用。 这是我在signInOutText（）中使用的代码，以使其正常工作：

private void signInOutText() {

        // get menu from navigationView
        Menu menu = navigationView.getMenu();

        // find MenuItem you want to change
        MenuItem nav_signIn = menu.findItem(R.id.signIn);

        //Check if user logged in, change sign in/out button to correct text
        FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
        if (user != null) {
            nav_signIn.setTitle(R.string.sign_out);
        } else {
            nav_signIn.setTitle(R.string.sign_in);
        }
    }

希望对您有所帮助！ 谢谢您的帮助！