当我尝试初始化GoogleMap对象时，我得到一个NullPointerException

我正在尝试在我的一个片段类中显示一个地图。 我在地图的XML类中创建了一个片段，并使用它如下：

@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。 问题是我不断得到一个NullPointerException，我初始化mapFragment，我不知道为什么。 有谁知道为什么会这样？

非常感谢任何帮助。

logcat的：

 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) 

Answer 1

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

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

但是，虽然这样可行，但最好让Fragment扩展SupportMapFragment，这样你就不需要嵌套片段了：

 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的完全工作的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：

结果：

编辑2：

以下是使用ViewPager和三个选项卡的完整类代码，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); } } } 

layout xml：

结果，我可以成功切换到所有选项卡，并且地图显示没有错误：