java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ActionBar.setNavigationMode(int)”

Posted 2023-02-19

【中文标题】java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“ActionBar.setNavigationMode(int)”

【英文标题】：java.lang.NullPointerException: Attempt to invoke virtual method 'ActionBar.setNavigationMode(int)' on a null object reference

【发布时间】：2015-08-17 23:02:41

【问题描述】：

我在运行时收到此错误。

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setNavigationMode(int)' on a null object reference

主Activity.java

    public class MainActivity <T extends Fragment> extends ActionBarActivity implements ListView.OnItemClickListener
      
    @Override
    protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ActionBar actionBar = getSupportActionBar();

    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    actionBar.setDisplayShowTitleEnabled(false);

    ActionBar.Tab tab;
    tab = actionBar.newTab()
            .setText("Categories")
            .setTabListener(new TabListener<CategoriesFragment>(
                    this, "Categories", CategoriesFragment.class));
    actionBar.addTab(tab);

    tab = actionBar.newTab()
            .setText("Acceuil")
            .setTabListener(new TabListener<AcceuilFragment>(
                    this, "Acceuil", AcceuilFragment.class));
    actionBar.addTab(tab);
    tab = actionBar.newTab()
            .setText("Topics")
            .setTabListener(new TabListener<TopicsFragment>(
                    this, "Topics", TopicsFragment.class));
    actionBar.addTab(tab);

    tab = actionBar.newTab()
            .setText("Tendances")
            .setTabListener(new TabListener<TendancesFragment>(
                    this, "Tendances", TendancesFragment.class));
    actionBar.addTab(tab);

    setRef();

    //Set the custom toolbar
    if (toolbar != null)
    
        toolbar.setTitle(R.string.app_name);
        setSupportActionBar(toolbar);
    

    drawerToggle = new ActionBarDrawerToggle(
            this,
            drawerLayout,
            toolbar,
            R.string.open,
            R.string.close
    )

    
        public void onDrawerClosed(View view)
        
            Log.d("MainActivity", "OnDrawerClosed");
            super.onDrawerClosed(view);
            invalidateOptionsMenu();
        

        public void onDrawerOpened(View drawerView)
        
            Log.d("MainActivity", "OnDrawerOpened");
            super.onDrawerOpened(drawerView);
            invalidateOptionsMenu();
        
    ;

    drawerLayout.setDrawerListener(drawerToggle);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    // use this setting to improve performance if you know that changes
    // in content do not change the layout size of the RecyclerView
    mRecyclerView.setHasFixedSize(true);

    // use a linear layout manager
    mLayoutManager = new LinearLayoutManager(this);
    mRecyclerView.setLayoutManager(mLayoutManager);
    mRecyclerView.setItemAnimator(new DefaultItemAnimator());

    // specify an adapter (see also next example)
    mAdapter = new MyAdapter(myDataset);
    mRecyclerView.setAdapter(mAdapter);




@Override
protected void onPostCreate(Bundle savedInstanceState) 
    super.onPostCreate(savedInstanceState);
    drawerToggle.syncState();


private void setRef()

    // create reference to the xml views
    mRecyclerView = (android.support.v7.widget.RecyclerView) findViewById(R.id.my_recycler_view);
    fab = (Button) findViewById(R.id.addButton);
    drawerLayout = (android.support.v4.widget.DrawerLayout) findViewById(R.id.drawer_layout);
    drawerLayout.setStatusBarBackground(R.color.primarydark);

    toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.toolbar);
    leftDrawerList = (ListView) findViewById(R.id.list_drawer);

    View list_header = getLayoutInflater().inflate(R.layout.drawerlist_header, null);
    leftDrawerList.addHeaderView(list_header);
    navigationDrawerAdapter = new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_expandable_list_item_1, leftSliderData);
    leftDrawerList.setAdapter(navigationDrawerAdapter);
    leftDrawerList.setOnItemClickListener(this);


@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id)

    Toast.makeText(getApplicationContext(), "Clicked on " + leftSliderData[position - 1], Toast.LENGTH_LONG).show();


public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) 
    // Check if the fragment is already initialized
    if (mFragment == null) 
        // If not, instantiate and add it to the activity
        mFragment = Fragment.instantiate(mActivity, mClass.getName());
        ft.add(android.R.id.content, mFragment, mTag);
     else 
        // If it exists, simply attach it in order to show it
        ft.attach(mFragment);
    


public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) 
    if (mFragment != null) 
        // Detach the fragment, because another one is being attached
        ft.detach(mFragment);
    


public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) 
    // User selected the already selected tab. Usually do nothing.
    

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<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_layout"
android:layout_
android:layout_
android:fitsSystemWindows="true"
tools:context=".MainActivity">

<!-- Setting this one to true makes the status bar
    to have a non-transparent shitty background -->

<!--
     As the main content view, the view below consumes the entire
     space available using match_parent in both dimensions.
-->

<FrameLayout
    android:layout_
    android:layout_
    android:fitsSystemWindows="false">

    <include layout="@layout/actionbarlayout"></include>

    <include layout="@layout/content_view"></include>

    <Button
        android:id="@+id/addButton"
        android:text="+"
        android:textSize="25sp"
        android:textColor="@android:color/white"
        android:textAlignment="center"
        android:layout_marginRight="15dp"
        android:layout_marginBottom="15dp"
        android:background="@drawable/circlebtn"
        android:layout_
        android:layout_
        android:stateListAnimator="@animator/anim"
        android:elevation="4dp"
        style="?android:buttonStyleSmall"
        android:layout_gravity="right|bottom" >
    </Button>

</FrameLayout>

<com.androprogrammer.test.materialapp1.ScrimInsetsFrameLayout
    android:id="@+id/container"
    android:layout_
    android:layout_
    android:fitsSystemWindows="true"
    android:elevation="8dp"
    android:layout_gravity="start"
    app:insetForeground="#4000">

    <ListView
        android:id="@+id/list_drawer"
        android:layout_
        android:layout_
        android:choiceMode="singleChoice"
        android:background="@android:color/white"
        android:fitsSystemWindows="true">
    </ListView>

</com.androprogrammer.test.materialapp1.ScrimInsetsFrameLayout>
</android.support.v4.widget.DrawerLayout>

values/styles.xml

<resources>

<!-- Base application theme. -->
<style name="AppTheme" parent="AppTheme.Base"/>

<style name="AppTheme.Base" parent="Theme.AppCompat.Light">
    <item name="colorPrimary">@color/primary</item>
    <item name="colorPrimaryDark">@color/primarydark</item>
    <item name="colorAccent">@color/primary</item>
    <item name="colorControlHighlight">@color/primary</item>
    <item name="android:windowNoTitle">true</item>
    <item name="windowActionBar">false</item>
    <item name="drawerArrowStyle">@style/DrawerArrowStyle</item>
    <item name="windowActionModeOverlay">true</item>

</style>

<style name="DrawerArrowStyle" parent="Widget.AppCompat.DrawerArrowToggle">
    <item name="spinBars">true</item>
    <item name="color">@android:color/white</item>
</style>
</resources>

v21/styles.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="AppTheme" parent="AppTheme.Base">
    <item name="android:windowDrawsSystemBarBackgrounds">true</item>
    <item name="android:statusBarColor">@android:color/transparent</item>
    <item name="android:windowContentTransitions">true</item>
    <item name="android:windowAllowEnterTransitionOverlap">true</item>
    <item name="android:windowAllowReturnTransitionOverlap">true</item>
    <item name="android:windowSharedElementEnterTransition">@android:transition/move</item>
    <item name="android:windowSharedElementExitTransition">@android:transition/move</item>

</style>
</resources>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.androprogrammer.test.materialapp1" >

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

请帮忙:)

【问题讨论】：

你确定你不需要做任何其他事情来让它工作吗？ ActionBar actionBar = getSupportActionBar();该方法似乎没有返回您期望的对象。

我知道 actionbar 返回 Null 我认为错误来自 Styles.xml 但我无法更正

你确定这个<T extends Fragment>？

是的，我很确定

您在哪个版本的 android 中看到此错误？ 3.*?

【参考方案1】：

尝试改变父样式

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

而ActionBarActivity 已弃用，您应该使用AppCompatActivity

【参考方案2】：

在您的 style.xml 中，您必须启用操作栏。如果 actionbar 被禁用（设置为 false），那么它不会在运行时添加到活动中（即getActionBar() 返回null）。

【讨论】：

当我将它设置为 true 运行时显示另一个错误 此 Activity 已经有一个由窗口装饰提供的操作栏。不要在主题中请求 Window.FEATURE_ACTION_BAR 并将 windowActionBar 设置为 false 以使用工具栏。