<!--透明activity的style-->
  <style name="transparent_activity">
    <item name="android:windowBackground">@color/transparent_background</item>
    <item name="android:windowNoTitle">true</item>
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowAnimationStyle">@android:style/Animation.Translucent</item>
  </style>

<color name="transparent_background">#50000000</color>

 <activity
      android:name=".TransparentActivity"
      android:theme="@style/transparent_activity" />

android:theme="@android:style/Theme.Translucent"

//改成activity继承自Activity：
public class MainActivity extends Activity