引起：java.lang.NullPointerException：尝试调用虚拟方法'void android.support.v7.app.ActionBar.setTitle（java.lang.CharSequence）'[重复]

Question

谁能帮我解决这个错误

这是 MainActivity.java 文件：

public class MainActivity extends AppCompatActivity {
    private FirebaseAuth mAuth;
    private Toolbar mToolbar;
    private ViewPager myViewPager;
    private TabLayout myTabLayout;
    private TabsPagerAdapter mTabsPagerAdapter;

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

        myViewPager = (ViewPager)findViewById(R.id.main_tabs_pager);
        mTabsPagerAdapter = new TabsPagerAdapter(getSupportFragmentManager());
        myViewPager.setAdapter(mTabsPagerAdapter);
        myTabLayout = (TabLayout)findViewById(R.id.main_tabs);
        myTabLayout.setupWithViewPager(myViewPager);


        mToolbar = (Toolbar)findViewById(R.id.main_app_bar);
        setSupportActionBar(mToolbar);

        getSupportActionBar().setTitle("myChat");
    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        if (currentUser == null){
            LogOutUser();
        }
    }

    private void LogOutUser() {
        Intent startPageIntent = new Intent(MainActivity.this, StartPageActivity.class);
        startPageIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(startPageIntent);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.menu,menu);
        return true;
    }

    public boolean onOptionsItemSelected(MenuItem item){
        super.onOptionsItemSelected(item);
        if(item.getItemId()== R.id.mainlogout_button){
            mAuth.signOut();
            LogOutUser();

        }
        return true;


    }
}

这是错误：

原因：java.lang.NullPointerException：尝试调用虚拟 方法'无效 android.support.v7.app.ActionBar.setTitle(java.lang.CharSequence)' 开启 空对象引用 yassine.com.mychat.MainActivity.onCreate(MainActivity.java:40)

Answer 1

这里是 MainActivity.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="yassine.com.mychat.MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/appBarLayout">
        <include
            layout="@layout/app_bar_layout"
            android:id ="@+id/main_page_toolbar">
        </include>

        <android.support.design.widget.TabLayout
            android:id="@+id/main_tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"></android.support.design.widget.TabLayout>


    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/main_tabs_pager"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_below="@+id/appBarLayout">

    </android.support.v4.view.ViewPager>


</RelativeLayout>