setSupportActionBar（）总是引发NullPointerException

Question

我试图从ActionBar切换到Toolbars，因为我想应用新的Material设计。

不幸的是，我的方法没有解决，setSupportActionBar()总是引发NullPointerException。

appcompat库包含在库项目中，并且还设置了项目设置中的依赖项。

有什么建议吗？

清单：

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.example.myApp" 
    android:versionCode="1" 
    android:versionName="1.0" > 

    <uses-sdk 
     android:minSdkVersion="15" 
     android:targetSdkVersion="21" /> 

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

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

styles.xml

<resources xmlns:android="http://schemas.android.com/apk/res/android"> 

    <style name="AppTheme.Base" parent="Theme.AppCompat.Light.NoActionBar"> 
     <item name="colorPrimary">@color/ktbl_red</item> 
     <item name="colorPrimaryDark">@color/ktbl_green</item> 
     <item name="colorAccent">@color/ktbl_yellow</item> 

     <item name="android:windowNoTitle">true</item> 
     <item name="windowActionBar">false</item> 
     <item name="windowActionModeOverlay">true</item> 

     <item name="toolbarStyle">@style/Widget.AppCompat.Toolbar</item> 
    </style> 

    <style name="AppTheme" parent="AppTheme.Base"> 
    </style> 

toolbar.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:id="@+id/toolbar" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:minHeight="?attr/actionBarSize" 
    android:background="@color/ktbl_blue" 
    app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar" 
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"> 
</android.support.v7.widget.Toolbar> 

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?> 
<merge> 
    <include layout="@layout/toolbar" /> 
</merge> 

MainActivity

public class MainActivity extends ActionBarActivity{ 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     setContentView(R.layout.activity_main);; 

     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); // error (NPE) 
    } 

    /* ... */ 
} 

NullPointerExecption

02-23 14:21:38.564 3687-3687/com.example.myApp E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myApp/com.example.myApp.view.screens.MainActivity}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
      at android.app.ActivityThread.access$600(ActivityThread.java:123) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
      at android.os.Handler.dispatchMessage(Handler.java:99) 
      at android.os.Looper.loop(Looper.java:137) 
      at android.app.ActivityThread.main(ActivityThread.java:4424) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:95) 
      at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:88) 
      at android.support.v7.internal.app.ToolbarActionBar.<init>(ToolbarActionBar.java:84) 
      at android.support.v7.app.ActionBarActivityDelegateBase.setSupportActionBar(ActionBarActivityDelegateBase.java:175) 
      at android.support.v7.app.ActionBarActivity.setSupportActionBar(ActionBarActivity.java:92) 
      at de.ktbl.com.example.myApp.MainActivity.onCreate(MainActivity.java:33) 
      at android.app.Activity.performCreate(Activity.java:4465) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
            at android.app.ActivityThread.access$600(ActivityThread.java:123) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
            at android.os.Handler.dispatchMessage(Handler.java:99) 
            at android.os.Looper.loop(Looper.java:137) 
            at android.app.ActivityThread.main(ActivityThread.java:4424) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:511) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
            at dalvik.system.NativeStart.main(Native Method) 

Answer 1

尝试使用这样的希望这将消除NPE ...

ActionBar actionBar = getSupportActionBar(); // or getActionBar(); 
    actionBar.setDisplayShowHomeEnabled(true); 
    actionBar.setDisplayUseLogoEnabled(true); 

希望它能帮助.... :)

Answer 2

当对象未知或为null时发生。当您忘记设置对象，源库未知（无法找到）或使用错误的使用类型时，可能会发生这种情况。

你在哪个设备上测试过该应用程序？低于API 15？ 尝试检查库支持

我认为你已经在错误的方式使用了代码。

-1）解决您的清单文件（你忘了活动的结束标志）

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

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

你的Java代码：

public class MainActivity extends ActionBarActivity{ 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); // error (NPE) 

     setContentView(R.layout.activity_main); //set view at end 
    } 

    /* ... */ 
}