java - 当我尝试初始化 GoogleMap 对象时，出现 NullPointerException

标签 java android google-maps google-api fragment

我正在尝试在我的 fragment 类之一中显示 map 。我在 XML 类中为 map 创建了一个 fragment ，并按如下方式使用它:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_profile, container, false);
    GoogleMap mapFragment = ((SupportMapFragment) getFragmentManager()
            .findFragmentById(R.id.map)).getMap();

    Marker hamburg = mapFragment.addMarker(new MarkerOptions().position(HAMBURG)
            .title("Hamburg"));
    Marker kiel = mapFragment.addMarker(new MarkerOptions()
            .position(KIEL)
            .title("Kiel")
            .snippet("Kiel is cool")
            .icon(BitmapDescriptorFactory
                    .fromResource(R.drawable.icon)));

这是基于我在网上找到的教程。问题是我在初始化 mapFragment 时不断收到 NullPointerException，但我不确定为什么。有谁知道为什么会发生这种情况？

非常感谢您的帮助。

日志猫:

java.lang.NullPointerException
        at com.example.sv_laptop03.snapchain.Profile.onCreateView(Profile.java:65)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
        at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:490)
        at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1105)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:951)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1473)
        at android.view.View.measure(View.java:16831)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:16831)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1410)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
        at android.view.View.measure(View.java:16831)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5245)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2586)
        at android.view.View.measure(View.java:16831)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2189)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1352)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1535)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1249)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6364)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)
        at android.view.Choreographer.doCallbacks(Choreographer.java:591)
        at android.view.Choreographer.doFrame(Choreographer.java:561)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)
        at android.os.Handler.handleCallback(Handler.java:730)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:176)
        at android.app.ActivityThread.main(ActivityThread.java:5419)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:525)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
        at dalvik.system.NativeStart.main(Native Method)

最佳答案

由于您在 Fragment 内使用嵌套的 SupportMapFragment，因此需要使用 getChildFragmentManager(): 

GoogleMap mapFragment = ((SupportMapFragment) getChildFragmentManager()
            .findFragmentById(R.id.map)).getMap();

但是，虽然这可行，但您最好让 Fragment 扩展 SupportMapFragment，这样您就不需要嵌套 Fragment:

public class MyMapFragment extends SupportMapFragment 
                                    implements OnMapReadyCallback {

    private GoogleMap mMap;
    private Marker marker;


    public MyMapFragment () {
    }

    @Override
    public void onResume() {
        super.onResume();

        setUpMapIfNeeded();
    }

    private void setUpMapIfNeeded() {

        if (mMap == null) {

            getMapAsync(this);
        }
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {

        mMap = googleMap;
        setUpMap();
    }

    private void setUpMap() {

        mMap.setMyLocationEnabled(true);
        mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
        mMap.getUiSettings().setMapToolbarEnabled(false);


        mMap.setOnMapClickListener(new GoogleMap.OnMapClickListener() {

            @Override
            public void onMapClick(LatLng point) {

                //remove previously placed Marker
                if (marker != null) {
                    marker.remove();
                }

                //place marker where user just clicked
                marker = mMap.addMarker(new MarkerOptions().position(point).title("Marker")
                        .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_MAGENTA)));

            }
        });

    }

}

编辑:

这是一个带有嵌套 SupportMapFragment 的完整工作 fragment :

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.BitmapDescriptorFactory;
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 MainActivityFragment extends Fragment {

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

    public MainActivityFragment() {
    }

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

        View rootView = inflater.inflate(R.layout.fragment_main, container, false);

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

        map.setMyLocationEnabled(true);
        map.setMapType(GoogleMap.MAP_TYPE_HYBRID);
        map.getUiSettings().setMapToolbarEnabled(false);

        Marker hamburg = map.addMarker(new MarkerOptions().position(HAMBURG)
                .title("Hamburg"));
        Marker kiel = map.addMarker(new MarkerOptions()
                .position(KIEL)
                .title("Kiel")
                .snippet("Kiel is cool")
                .icon(BitmapDescriptorFactory
                        .fromResource(R.mipmap.ic_launcher)));

        map.moveCamera(CameraUpdateFactory.newLatLngZoom(HAMBURG, 15)); 

        map.animateCamera(CameraUpdateFactory.zoomTo(10), 2000, null);

        return rootView;
    }
}

fragment_main.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" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivityFragment">

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


</RelativeLayout>

结果:

enter image description here

编辑2:

这里是使用 ViewPager 和三个选项卡对我有用的完整类代码，基于此答案中的第一个解决方案，它不使用嵌套的 SupportMapFragment，而是具有 Fragment 范围 SupportMapFragment:

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

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

import java.util.Locale;


public class MainActivity extends AppCompatActivity implements ActionBar.TabListener{


    SectionsPagerAdapter mSectionsPagerAdapter;

    ViewPager mViewPager;

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

        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        final ActionBar actionBar = getSupportActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);
            }
        });

        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            actionBar.addTab(actionBar.newTab().setText(mSectionsPagerAdapter.getPageTitle(i)).setTabListener(this));
        }

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {

    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {

    }


    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {

            switch (position) {
                case 0:
                    return PlaceholderFragment.newInstance(position + 1);
                case 1:
                    return PlaceholderFragment.newInstance(position + 1);
                case 2:
                    return MapTabFragment.newInstance(position + 1);
            }

            return null;
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            Locale l = Locale.getDefault();

            switch (position) {
                case 0:
                    return getString(R.string.title_section1).toUpperCase(l);
                case 1:
                    return getString(R.string.title_section2).toUpperCase(l);
                case 2:
                    return getString(R.string.title_section3).toUpperCase(l);
            }
            return null;
        }
    }


    public static class PlaceholderFragment extends Fragment {

        private static final String ARG_SECTION_NUMBER = "section_number";

        TextView text;

        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);

            text = (TextView) rootView.findViewById(R.id.section_label);
            text.setText("placeholder");

            return rootView;
        }
    }

    public static class MapTabFragment extends SupportMapFragment implements
            OnMapReadyCallback {

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

        private static final String ARG_SECTION_NUMBER = "section_number";

        private GoogleMap mMap;
        private Marker marker;


        public static MapTabFragment newInstance(int sectionNumber) {
            MapTabFragment fragment = new MapTabFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        public MapTabFragment() {
        }

        @Override
        public void onResume() {
            super.onResume();

            Log.d("MyMap", "onResume");
            setUpMapIfNeeded();
        }

        private void setUpMapIfNeeded() {

            if (mMap == null) {

                Log.d("MyMap", "setUpMapIfNeeded");

                getMapAsync(this);
            }
        }

        @Override
        public void onMapReady(GoogleMap googleMap) {
            Log.d("MyMap", "onMapReady");
            mMap = googleMap;
            setUpMap();
        }

        private void setUpMap() {

            mMap.setMyLocationEnabled(true);
            mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
            mMap.getUiSettings().setMapToolbarEnabled(false);


            mMap.setMyLocationEnabled(true);
            mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
            mMap.getUiSettings().setMapToolbarEnabled(false);

            Marker hamburg = mMap.addMarker(new MarkerOptions().position(HAMBURG)
                    .title("Hamburg"));
            Marker kiel = mMap.addMarker(new MarkerOptions()
                    .position(KIEL)
                    .title("Kiel")
                    .snippet("Kiel is cool")
                    .icon(BitmapDescriptorFactory
                            .fromResource(R.mipmap.ic_launcher)));

            mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(HAMBURG, 15));

            mMap.animateCamera(CameraUpdateFactory.zoomTo(10), 2000, null);


        }
    }
}

布局 XML:

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:id="@+id/pager"
    android:layout_width="match_parent" android:layout_height="match_parent"
    tools:context=".MainActivity" />

结果，我能够成功切换到所有选项卡，并且 map 显示没有错误:

enter image description here

关于java - 当我尝试初始化 GoogleMap 对象时，出现 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31461056/

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