setSupportActionBar() 总是引发 NullPointerException

Question

我尝试从 ActionBar 切换到 Toolbars，因为我想应用新的 Material 设计。

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

appcompat 库作为库项目包含在内，项目设置中的依赖项也已设置。

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>

工具栏.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

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

主活动

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

尝试像这样使用，希望它会删除 NPE ...

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

希望对您有所帮助.... :)

Answer 2

当对象未知或为空时发生。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。

您在哪台设备上测试了该应用？低于 API 15？ 尝试检查库支持

我认为您以错误的方式使用了代码。

-1) 修复你的清单文件（你忘记了活动的结束标记）

<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
    }

    /* ... */
}