我是
android新手,我正在关注Material Design Toolbar的以下教程：
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：
04-01 19:16:10.214 2246-2253/com.example.bhaskar.ddit_results E/art﹕ Failed sending reply to debugger: broken pipe
04-01 19:16:11.985 2246-2246/com.example.bhaskar.ddit_results E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.bhaskar.ddit_results,PID: 2246
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
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()' on a null object reference
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 com.example.bhaskar.ddit_results.MainActivity.onCreate(MainActivity.java:41)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
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：
<resources>
<!-- Base application theme. -->
<style name="MyTheme" parent="MyTheme.Base">
</style>
<style name="MyTheme.Base" parent="Theme.AppCompat.Light.NoActionBar">
<item name="colorPrimary">#ff32cd32</item>
<item name="colorPrimaryDark">#ff04cd2c</item>
<item name="colorAccent">#ff4081</item>
</style>
V21 / styles.xml：
<resources>
<style name="MyTheme" parent="MyTheme.Base">
<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>
在toolbar.xml中我有：
<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:background="?attr/colorPrimary" app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar" app:popupTheme="@style/ThemeOverlay.AppCompat.Light" android:elevation="4dp"/>
我还在activity_main.xml文件中包含了工具栏布局.
在MainActivity.java中：
public class MainActivity extends ActionBaractivity {
....
private Toolbar toolbar;
public static final int RECORD_ENTRY_REQUEST_CODE=3;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Set our view from the "main" layout resource
toolbar = (Toolbar)findViewById(R.id.toolbar);
//Toolbar will Now take on default actionbar characteristics
setContentView(R.layout.activity_main);
setSupportActionBar(toolbar);
setTitle("DDIT_Results");
....
}
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.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); // <-- call this first
toolbar = (Toolbar) findViewById(R.id.toolbar); // <-- setup views after
setSupportActionBar(toolbar);
setTitle("DDIT_Results");
}
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:
<android.support.v7.widget.Toolbar
android:id=”@+id/my_awesome_toolbar”
android:layout_height=”wrap_content”
android:layout_width=”match_parent”
android:minHeight=”?attr/actionBarSize”
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:
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.blah);
final Toolbar toolbar = (Toolbar) findViewById(R.id.my_awesome_toolbar);
setSupportActionBar(toolbar);
}
From this point on,all menu items are displayed in your Toolbar,populated via the standard options menu callbacks.