getSupportFragmentManager returns null

问题:

I'd like to show Google maps in my android app:

public class DisplayMapActivity extends ActionBarActivity {
    private LbsTracker2 lbsTracker2;
    private GoogleMap googleMap;

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

        try {
            this.initilizeMap();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        initilizeMap();
    }

    private void initilizeMap() {
        if (googleMap == null) {
            MapFragment mapFragment = (MapFragment) getFragmentManager().findFragmentById(R.id.map); // <--- always null! :(
            googleMap = mapFragment.getMap();
        }
    }
    ...
}

But my variable mapFragment is always null inside my method initializeMap() ...

Xml of my activity:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.lbsapp.DisplayMapActivity$PlaceholderFragment" >

    <fragment
            android:id="@+id/map"
            android:name="com.google.android.gms.maps.MapFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="tag_fragment_map" />

</RelativeLayout>

Manifest of my app:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.lbsapp"
    android:versionCode="1"
    android:versionName="1.0" >

    <permission
        android:name="info.androidhive.googlemapsv2.permission.MAPS_RECEIVE"
        android:protectionLevel="signature" />

    <uses-permission android:name="info.androidhive.googlemapsv2.permission.MAPS_RECEIVE" />

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="19" />

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <!-- Required to show current location --> 
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

    <!-- Required OpenGL ES 2.0. for Maps V2 -->
    <uses-feature
        android:glEsVersion="0x00020000"
        android:required="true" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.lbsapp.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="com.example.lbsapp.DisplayLbsActivity"
            android:label="@string/title_activity_display_lbs"
            android:parentActivityName="com.example.lbsapp.MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.lbsapp.MainActivity" />
        </activity>

        <!-- Goolge Maps API Key -->
        <meta-data
            android:name="com.google.android.maps.v2.API_KEY"
            android:value="AIzaSyDnnWGwO8Zfbn5PGKXPF3aspbZTCtJ6j7I" />

        <activity
            android:name="com.example.lbsapp.DisplayMapActivity"
            android:label="@string/title_activity_display_map"
            android:parentActivityName="com.example.lbsapp.MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.lbsapp.MainActivity" />
        </activity>
    </application>    

</manifest>

回答1:

The problem was there:

setContentView(R.layout.activity_main);

Of course it has to be

setContentView(R.layout.activity_display_map);

回答2:

I would recommend using the sopportMapFragment to support older version..

Example:

<fragment
        android:id="@+id/map"
        android:name="com.google.android.gms.maps.SupportMapFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:tag="tag_fragment_map" />

SupportMapFragmentmapFragment = (SupportMapFragment) getFragmentManager().findFragmentById(R.id.map);

回答3:

I think it will work if you use the line you put in comments. Uncomment the MapFragment and comment the one with SupportMapFragment. MapFragment is the modern version for map fragments.

Also, in your layout try class="com.google.android.gms.maps.MapFragment" instead of SupportMapFragment.