從ActionBar調用新活動

嘗試從操作欄調用新活動（設置）時，出現錯誤java.lang.NullPointerException。我不太確定自己出錯的地方，下面是我正在使用的代碼。一切都很好，當我測試應用程序並按下「設置」按鈕時，應用程序崩潰。從ActionBar調用新活動

MainActivity.java

package liam.lbee.com.test1; 

import android.content.Intent; 
import android.support.v7.app.ActionBarActivity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.Toast; 



public class MainActivity extends ActionBarActivity implements View.OnClickListener { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
    } 


    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      startActivity(new Intent(getApplicationContext(),SettingsActivity.class)); 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

}

Menu_Main.xml

<menu xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    xmlns:tools="http://schemas.android.com/tools" tools:context=".MainActivity"> 



    <item android:id="@+id/action_settings" 
     android:title="@string/action_settings" 
     app:showAsAction="never" /> 


</menu>

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="liam.lbee.com.test1" > 

    <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> 
     <activity 
      android:name=".SettingsActivity" 
      android:label="@string/title_activity_settings" 
      android:parentActivityName=".MainActivity" > 
      <meta-data 
       android:name="android.support.PARENT_ACTIVITY" 
       android:value="liam.lbee.com.test1.MainActivity" /> 
     </activity> 
    </application> 

</manifest>

logcat的文件

java.lang.RuntimeException: Unable to start activity ComponentInfo{liam.lbee.com.test1/liam.lbee.com.test1.SettingsActivity}: java.lang.NullPointerException 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2292) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2350) 
      at android.app.ActivityThread.access$800(ActivityThread.java:163) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1257) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:157) 
      at android.app.ActivityThread.main(ActivityThread.java:5335) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:515) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081) 
      at dalvik.system.NativeStart.main(Native Method) 
    Caused by: java.lang.NullPointerException 
      at liam.lbee.com.test1.SettingsActivity.setupActionBar(SettingsActivity.java:58) 
      at liam.lbee.com.test1.SettingsActivity.onCreate(SettingsActivity.java:48) 
      at android.app.Activity.performCreate(Activity.java:5389) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2256) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2350) 
            at android.app.ActivityThread.access$800(ActivityThread.java:163) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1257) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:157) 
            at android.app.ActivityThread.main(ActivityThread.java:5335) 
            at java.lang.reflect.Method.invokeNative(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:515)

setupActionBar（如需要）

private void setupActionBar() { 
     if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) { 
      // Show the Up button in the action bar. 
      getActionBar().setDisplayHomeAsUpEnabled(true); 
     } 
    }

來源

2015-02-09 SilverShotBee

哪一行給你NullPointerException？ – MFazio23 2015-02-09 20:59:19

@ MFazio23 - ** 8585-8585/liam.lbee.com.test1 E/ViewRootImpl：sendUserActionEvent（）mView == null **是測試時日誌文件中的第一個錯誤 – SilverShotBee 2015-02-09 21:03:19

在您的logcat中，應該有一個堆棧顯示NullPointerException和其發生的類+方法的跟蹤：例如： * java.lang.NullPointerException at [Class Name]。[Method Name]（[File name] .java：74）* – MFazio23 2015-02-09 21:07:09

最後的總結：這個問題是與SettingsActivity不使用從ActionBarActivity的getSupportActionBar（）方法。一旦這個類被更新爲從ActionBarActivity擴展並且被調用的方法被改變了，這就起作用了。

這樣的另一個例子：getActionBar() returns Null (AppCompat-v7 21)

來源

2015-02-09 21:46:12 MFazio23

從ActionBar調用新活動

回答

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