android – setSupportActionBar()抛出Nullpointer异常

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我是 android新手,我正在关注Material Design Toolbar的以下教程： 
  1. http://www.android4devs.com/2014/12/how-to-make-material-design-app.html

但是在执行代码之后. logcat中显示以下错误：

  1. 04-01 19:16:10.214    2246-2253/com.example.bhaskar.ddit_results E/art﹕ Failed sending reply to debugger: Broken pipe
  2. 04-01 19:16:11.985    2246-2246/com.example.bhaskar.ddit_results E/AndroidRuntime﹕ FATAL EXCEPTION: main
  3. Process: com.example.bhaskar.ddit_results,PID: 2246
  4. java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bhaskar.ddit_results/com.example.bhaskar.ddit_results.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()' on a null object reference
  5.         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
  6.         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
  7.         at android.app.ActivityThread.access$800(ActivityThread.java:144)
  8.         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
  9.         at android.os.Handler.dispatchMessage(Handler.java:102)
  10.         at android.os.Looper.loop(Looper.java:135)
  11.         at android.app.ActivityThread.main(ActivityThread.java:5221)
  12.         at java.lang.reflect.Method.invoke(Native Method)
  13.         at java.lang.reflect.Method.invoke(Method.java:372)
  14.         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
  15.         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
  16.  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()' on a null object reference
  17.         at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:95)
  18.         at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:88)
  19.         at android.support.v7.internal.app.ToolbarActionBar.<init>(ToolbarActionBar.java:84)
  20.         at android.support.v7.app.ActionBarActivityDelegateBase.setSupportActionBar(ActionBarActivityDelegateBase.java:175)
  21.         at android.support.v7.app.ActionBarActivity.setSupportActionBar(ActionBarActivity.java:92)
  22.         at com.example.bhaskar.ddit_results.MainActivity.onCreate(MainActivity.java:41)
  23.         at android.app.Activity.performCreate(Activity.java:5933)
  24.             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
  25.         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)   
  26.         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)    
  27.         at android.app.ActivityThread.access$800(ActivityThread.java:144)
  28.         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
  29.         at android.os.Handler.dispatchMessage(Handler.java:102)
  30.         at android.os.Looper.loop(Looper.java:135)
  31.         at android.app.ActivityThread.main(ActivityThread.java:5221)
  32.         at java.lang.reflect.Method.invoke(Native Method)
  33.         at java.lang.reflect.Method.invoke(Method.java:372)
  34.         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
  35.         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

这是由NullPointerException引起的.我正在使用Android Studio 1.0.1.

styles.xml：

  1. <resources>
  2.  
  3. <!-- Base application theme. -->
  4. <style name="MyTheme" parent="MyTheme.Base">
  5.  
  6. </style>
  7.  
  8. <style name="MyTheme.Base" parent="Theme.AppCompat.Light.NoActionBar">
  9.  
  10.     <item name="colorPrimary">#ff32cd32</item>
  11.     <item name="colorPrimaryDark">#ff04cd2c</item>
  12.     <item name="colorAccent">#ff4081</item>
  13. </style>

V21 / styles.xml：

  1. <resources>
  2. <style name="MyTheme" parent="MyTheme.Base">
  3.     <item name="android:windowContentTransitions">true</item>
  4.     <item name="android:windowAllowEnterTransitionOverlap">true</item>
  5.     <item name="android:windowAllowReturnTransitionOverlap">true</item>
  6.     <item name="android:windowSharedElementEnterTransition">@android:transition/move</item>
  7.     <item name="android:windowSharedElementExitTransition">@android:transition/move</item>

在toolbar.xml中我有：

  1. <android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
  2. xmlns:app="http://schemas.android.com/apk/res-auto"
  3. android:id="@+id/toolbar"
  4. android:layout_width="match_parent"
  5. android:layout_height="wrap_content"
  6. android:background="?attr/colorPrimary"
  7. app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
  8. app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
  9. android:elevation="4dp"/>

我还在activity_main.xml文件中包含了工具栏布局.
在MainActivity.java中：

  1. public class MainActivity extends ActionBarActivity {
  2.  
  3. ....
  4.  
  5.  
  6.  
  7. private Toolbar toolbar;
  8. public static final int RECORD_ENTRY_REQUEST_CODE=3;
  9.  
  10. @Override
  11. protected void onCreate(Bundle savedInstanceState) {
  12.     super.onCreate(savedInstanceState);
  13.     // Set our view from the "main" layout resource
  14.  
  15.      toolbar = (Toolbar)findViewById(R.id.toolbar);
  16.  
  17.     //Toolbar will now take on default actionbar characteristics
  18.     setContentView(R.layout.activity_main);
  19.     setSupportActionBar(toolbar);
  20.     setTitle("DDIT_Results");
  21.     ....
  22.  
  23. }

我搜索了这个问题并且没有提出任何解决方案.希望有人认识到这个错误并且有一个快速的答案 – 否则我可以提供更多的信息,如果要求.任何帮助将是appriciated.

解决方法

您需要在添加任何视图(如工具栏)之前调用setContentView. 
  1. protected void onCreate(Bundle savedInstanceState) {
  2.     super.onCreate(savedInstanceState);
  3.     setContentView(R.layout.activity_main);         // <-- call this first
  4.  
  5.     toolbar = (Toolbar) findViewById(R.id.toolbar); // <-- setup views after
  6.  
  7.     setSupportActionBar(toolbar);
  8.     setTitle("DDIT_Results");
  9. }

请参阅官方Android开发者博客：http://android-developers.blogspot.com/2014/10/appcompat-v21-material-design-for-pre.html

行动吧

To use Toolbar as an Action Bar,first disable the decor-provided
Action Bar. The easiest way is to have your theme extend from
Theme.AppCompat.NoActionBar (or its light variant).

Second,create a Toolbar instance,usually via your layout XML:

  1. <android.support.v7.widget.Toolbar
  2.     android:id=”@+id/my_awesome_toolbar”
  3.     android:layout_height=”wrap_content”
  4.     android:layout_width=”match_parent”
  5.     android:minHeight=”?attr/actionBarSize”
  6.     android:background=”?attr/colorPrimary” />

The height,width,background,and so on are totally up to you; these
are just good examples. As Toolbar is just a ViewGroup,you can style
and position it however you want.

Then in your Activity or Fragment,set the Toolbar to act as your
Action Bar:

  1. @Override
  2. public void onCreate(Bundle savedInstanceState) {
  3.     super.onCreate(savedInstanceState);
  4.     setContentView(R.layout.blah);
  5.  
  6.     final Toolbar toolbar = (Toolbar) findViewById(R.id.my_awesome_toolbar);
  7.     setSupportActionBar(toolbar);
  8. }

From this point on,all menu items are displayed in your Toolbar,populated via the standard options menu callbacks.

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