片段内的谷歌地图 V2 的空指针异常

Question

我最近开始用 android 开发一些东西。 我正在尝试使用抽屉导航和谷歌地图创建一个活动。

我已经阅读了如何做，我搜索了一个解决方案但未能解决问题。

这是主要活动。和'也对错误负责：

package it.kytech.linkess;


import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.support.v4.widget.DrawerLayout;

import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;

public class MainActivity extends ActionBarActivity
        implements NavigationDrawerFragment.NavigationDrawerCallbacks {

    /**
     * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
     */
    private NavigationDrawerFragment mNavigationDrawerFragment;

    /**
     * Used to store the last screen title. For use in {@link #restoreActionBar()}.
     */
    private CharSequence mTitle;

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

        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
        mTitle = getTitle();

        // Set up the drawer.
        mNavigationDrawerFragment.setUp(
                R.id.navigation_drawer,
                (DrawerLayout) findViewById(R.id.drawer_layout));

        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    @Override
    public void onNavigationDrawerItemSelected(int position) {
    }

    public void restoreActionBar() {
        ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
        actionBar.setDisplayShowTitleEnabled(true);
        actionBar.setTitle(mTitle);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        if (!mNavigationDrawerFragment.isDrawerOpen()) {
            // Only show items in the action bar relevant to this screen
            // if the drawer is not showing. Otherwise, let the drawer
            // decide what to show in the action bar.
            getMenuInflater().inflate(R.menu.main, menu);
            restoreActionBar();
            return true;
        }
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        static final LatLng HAMBURG = new LatLng(53.558, 9.927);
        static final LatLng KIEL = new LatLng(53.551, 9.993);
        private GoogleMap map;

        public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {

            View v = inflater.inflate(R.layout.fragment_main, null, false);

            FragmentTransaction ft = getFragmentManager().beginTransaction();


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

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

            if (fmap == null) {
                fmap = SupportMapFragment.newInstance();
                ft.add(R.id.map, fmap);
            }

            ft.commit();

            Marker hamburg = map.addMarker(new MarkerOptions().position(HAMBURG)
                    .title("Hamburg"));
            Marker kiel = map.addMarker(new MarkerOptions()
                    .position(KIEL)
                    .title("Kiel")
                    .snippet("Kiel is cool"));

            // Move the camera instantly to hamburg with a zoom of 15.
            map.moveCamera(CameraUpdateFactory.newLatLngZoom(HAMBURG, 15));

            // Zoom in, animating the camera.
            map.animateCamera(CameraUpdateFactory.zoomTo(10), 2000, null);


            return v;
        }
    }

}

这是主要的 xml。

<!-- A DrawerLayout is intended to be used as the top-level content view using match_parent for both width and height to consume the full space available. -->
<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="it.kytech.linkess.MainActivity">

    <!-- As the main content view, the view below consumes the entire
         space available using match_parent in both dimensions. -->
    <FrameLayout
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <!-- android:layout_gravity="start" tells DrawerLayout to treat
         this as a sliding drawer on the left side for left-to-right
         languages and on the right side for right-to-left languages.
         If you're not building against API 17 or higher, use
         android:layout_gravity="left" instead. -->
    <!-- The drawer is given a fixed width in dp and extends the full height of
         the container. -->
    <fragment android:id="@+id/navigation_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:name="it.kytech.linkess.NavigationDrawerFragment" />

</android.support.v4.widget.DrawerLayout>

这是地图的xml：

<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"
    tools:context="it.kytech.linkess.MainActivity$PlaceholderFragment">

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

</RelativeLayout>

这是抽屉的xml：

<ListView 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:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp"
    android:background="#cccc"
    tools:context="it.kytech.myapplication2.app.NavigationDrawerFragment" />

我还附上了我的清单：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="it.kytech.linkess" >

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

    <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="my hidden key"/>

        <meta-data android:name="com.google.android.gms.version"
            android:value="@integer/google_play_services_version" />

        <activity
            android:name="it.kytech.linkess.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>

错误发生在这一行：

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

完整日志：

04-15 22:21:08.325  30251-30251/it.kytech.linkess E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: it.kytech.linkess, PID: 30251
    java.lang.RuntimeException: Unable to start activity ComponentInfo{it.kytech.linkess/it.kytech.linkess.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2215)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
            at android.app.ActivityThread.access$800(ActivityThread.java:145)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1206)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5149)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at it.kytech.linkess.MainActivity$PlaceholderFragment.onCreateView(MainActivity.java:110)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:938)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1115)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1478)
            at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:570)
            at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
            at android.app.Activity.performStart(Activity.java:5241)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2178)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2265)
            at android.app.ActivityThread.access$800(ActivityThread.java:145)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1206)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5149)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
            at dalvik.system.NativeStart.main(Native Method)

显然生成代码的关键已被隐藏。 我希望你能帮助我。

Answer 1

问题是您在初始化您的 sopportmapfragment 之前获取地图，这就是您的地图为空的原因。尝试切换它首先初始化地图片段并获取地图。

Answer 2

你的空指针很可能是因为在当前布局（activity_main.xml）中找不到R.id.map。

查看您的代码后，我看不到您在哪里扩充了地图 xml 文件并将其添加到您的布局中。

也许您打算添加一个包含语句 <include layout="@layout/map"/> 而不是框架布局，以便实际实例化？

要么这样，要么扩充 map.xml 文件，将其附加到您的布局中，然后调用 findViewById。