why Google map view not appear in linear layout? it appear in linear layout pick preview layout from "fragment layout" context menu

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Mobile

Android

activity_main

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android">
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:id="@+id/mainView">

<fragment>
android:id="@+id/mvMain"
android:name="com.google.android.gms.maps.MapFragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:clickable="true"
android:layout_alignParentTop="true" />


_______________________________________________________
manifest
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<manifest xmlns:android="http://schemas.android.com/apk/res/android">
package="com.example.ppll"
android:versionCode="1"
android:versionName="1.0" >
<uses-permission android:name="android.permission.INTERNET">

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE">
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE">
<!-- The following two permissions are not required to use
Google Maps Android API v2, but are recommended. -->
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION">
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION">

<uses-feature>
android:glEsVersion="0x00020000"
android:required="true"/>
<uses-sdk>
android:minSdkVersion="8"
android:targetSdkVersion="21" />


<application>
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme">
<uses-library android:name="com.google.android.maps">

<meta-data
android:name="com.google.android.maps.v2.API_KEY"
android:value="AIzaSyC8pJ9YFkGrcCF1Z37BBP2v92q8SwYBaH8"/>

<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">





_____________________________________________________________________
main.java
_____________________________________________________________________
package com.example.ppll;



import com.google.android.maps.MapActivity;


import com.google.android.maps.MapView;

import android.os.Bundle;

public class MainActivity extends MapActivity {

MapView map;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
map=(MapView)findViewById(R.id.mvMain);
}

@Override
protected boolean isRouteDisplayed() {
// TODO Auto-generated method stub
return false;
}
}