public class MainActivity extends Activity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mainactivity);
    }
}

 <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity 
            android:name="lianxi.MainActivity">  //包名+类名
  
          <intent-filter
            >
           <action android:name="android.intent.action.MAIN"/>
　　　　　　 <category android:name="android.intent.category.LAUNCHER" /> //是程序能显示在手机程序列表中
            </intent-filter>
        </activity>
    </application>