谷歌地图polyline项目显示java.lang.RuntimeException

Question

我正在一个简单的Android项目上使用一组硬编码的经度和纬度值显示一条折线。

MainActivity.java如图所示： package com.flip.GoogleMaps;

import android.app.Activity; 
import android.app.FragmentManager; 
import android.os.Bundle; 
import com.google.android.gms.maps.GoogleMap; 
import com.google.android.gms.maps.MapFragment; 
import com.google.android.gms.maps.model.LatLng; 
import com.google.android.gms.maps.model.Polyline; 
import com.google.android.gms.maps.model.PolylineOptions; 

public class MainActivity extends Activity { 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    GoogleMap myMap = null; 
    FragmentManager myFragmentManager = getFragmentManager(); 
    MapFragment myMapFragment = MapFragment)myFragmentManager.findFragmentById(R.id.map); 
    myMap = myMapFragment.getMap(); 
    myMap.setMyLocationEnabled(true); 
    myMap.setMapType(GoogleMap.MAP_TYPE_HYBRID); 
    super.onCreate(savedInstanceState); 


    PolylineOptions rectOptions = new PolylineOptions() 
    .add(new LatLng(37.35, -122.0)) 
    .add(new LatLng(37.45, -122.0)) 
    .add(new LatLng(37.45, -122.2)) 
    .add(new LatLng(37.35, -122.2)) 
    .add(new LatLng(37.35, -122.0)); 

    @SuppressWarnings("unused") 
    Polyline polyline = myMap.addPolyline(rectOptions);   
    } 
} 

的activity_main.xml是folllows：

<LinearLayout 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:orientation="vertical" 
tools:context=".MainActivity" > 

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

</LinearLayout> 

而且AndroidManifest.xml如下：

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
package="com.flip.GoogleMaps" 
android:versionCode="1" 
android:versionName="1.0" > 
<uses-feature 
    android:glEsVersion="0x00020000" 
    android:required="true"/> 
<uses-sdk 
    android:minSdkVersion="8" 
    android:targetSdkVersion="17" /> 
<uses-permission android:name="android.permission.INTERNET"/> 
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/> 
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/> 
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/> 
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> 
<permission  android:name="com.flip.GoogleMaps.permission.MAPS_RECEIVE" 
    android:protectionLevel="signature"></permission> 
<uses-permission android:name="com.flip.GoogleMaps.permission.MAPS_RECEIVE"/> 
<uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES"/> 


<application 
    android:allowBackup="true" 
    android:icon="@drawable/ic_launcher" 
    android:label="@string/app_name" 
    android:theme="@style/AppTheme" > 
    <meta-data 
     android:name="com.google.android.maps.v2.API_KEY" 
     android:value="AIzaSyBD9fsiMd_G9Pzeq2Eqas2FwJjzMGbSOnA"/> 
    <uses-library android:name="com.google.android.maps" /> 
    <activity 
     android:name="com.flip.GoogleMaps.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> 

</application> 
</manifest> 

以下错误显示在logcat的时候启动失败：

03-14 12:22:13.573: E/AndroidRuntime(1120): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.flip.GoogleMaps/com.flip.GoogleMaps.MainActivity}: java.lang.NullPointerException 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.ActivityThread.access$600(ActivityThread.java:130) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.os.Looper.loop(Looper.java:137) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at  android.app.ActivityThread.main(ActivityThread.java:4745) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at java.lang.reflect.Method.invoke(Method.java:511) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at dalvik.system.NativeStart.main(Native Method) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  Caused by: java.lang.NullPointerException 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at com.flip.GoogleMaps.MainActivity.onCreate(MainActivity.java:20) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.Activity.performCreate(Activity.java:5008) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023) 
03-14 12:22:13.573: E/AndroidRuntime(1120):  ... 11 more 

我现在无可救药。请帮忙！

Answer 1

您的代码中存在拼写错误。更改

MapFragment myMapFragment = MapFragment)myFragmentManager.findFragmentById(R.id.map); 

到

MapFragment myMapFragment = (MapFragment)myFragmentManager.findFragmentById(R.id.map);