public class MainActivity extends AppCompatActivity {
static ArrayList<String>places;
static ArrayList<String>lat;
static ArrayList<String>lng;
static ArrayAdapter arrayAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ListView listView = (ListView)findViewById(R.id.listView);
lat = new ArrayList<String>();
lng = new ArrayList<String>();
SharedPreferences sharedPreference = this.getSharedPreferences("com.example.inbal.memorableplaces", Context.MODE_PRIVATE);
try {
places =(ArrayList<String>) ObjectSerializer.deserialize(sharedPreference.getString("Places",ObjectSerializer.serialize(new ArrayList<String>())));
lat =(ArrayList<String>) ObjectSerializer.deserialize(sharedPreference.getString("Lat",ObjectSerializer.serialize(new ArrayList<String>())));
lng =(ArrayList<String>) ObjectSerializer.deserialize(sharedPreference.getString("Lng",ObjectSerializer.serialize(new ArrayList<String>())));
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
if (lat.size()==0)
lat.add("");
if (lng.size()==0)
lng.add("");
if (places.size()==0)
places.add("Add a new place");
arrayAdapter= new ArrayAdapter(this,android.R.layout.simple_list_item_1,places);
listView.setAdapter(arrayAdapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
Intent intent = new Intent(getApplicationContext(), MapsActivity.class);
intent.putExtra("placeName", i);
startActivity(intent);
}
});
}
}
public class MapsActivity extends FragmentActivity implements
OnMapReadyCallback,GoogleMap.OnMapLongClickListener{
private GoogleMap mMap;
LocationManager locationManager;
LocationListener locationListener;
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
super.onRequestPermissionsResult(requestCode, permissions, grantResults);
if (grantResults.length>0&&grantResults[0]==PackageManager.PERMISSION_GRANTED){
if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)== PackageManager.PERMISSION_GRANTED)
locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER,1,10,locationListener);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_maps);
// Obtain the SupportMapFragment and get notified when the map is ready to be used.
SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
.findFragmentById(R.id.map);
mapFragment.getMapAsync(this);
}
/**
* Manipulates the map once available.
* This callback is triggered when the map is ready Gto be used.
* This is where we can add markers or lines, add listeners or move the camera. In this case,
* we just add a marker near Sydney, Australia.
* If Google Play services is not installed on the device, the user will be prompted to install
* it inside the SupportMapFragment. This method will only be triggered once the user has
* installed Google Play services and returned to the app.
*/
@Override
public void onMapReady(GoogleMap googleMap) {
Intent intent = getIntent();
int num = intent.getIntExtra("placeName", 0);
if (num == 0) {
mMap = googleMap;
mMap.setOnMapLongClickListener(this);
locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
locationListener = new LocationListener() {
@Override
public void onLocationChanged(Location location) {
LatLng userLocation = new LatLng(location.getLatitude(), location.getLongitude());
mMap.clear();
mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(userLocation, 15));
}
@Override
public void onStatusChanged(String s, int i, Bundle bundle) {
}
@Override
public void onProviderEnabled(String s) {
}
@Override
public void onProviderDisabled(String s) {
}
};
if (Build.VERSION.SDK_INT < 23)
locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1, 10, locationListener);
else {
if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED)
locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 20, 10, locationListener);
else
ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);
}
}
else {
String lat = MainActivity.lat.get(num);
String lng = MainActivity.lng.get(num);
LatLng location= new LatLng(Double.parseDouble(lat),Double.parseDouble(lng));
mMap.moveCamera(CameraUpdateFactory.newLatLng(location));
}
}
@Override
public void onMapLongClick(LatLng latLng)
{
Geocoder geocoder = new Geocoder(getApplicationContext(), Locale.getDefault());
try {
List<Address> listAddress = geocoder.getFromLocation(latLng.latitude,latLng.longitude,1);
String address = "";
if (listAddress!=null && listAddress.size()>0) {
if (listAddress.get(0).getLocality() != null) {
address+=listAddress.get(0).getLocality();
if (listAddress.get(0).getThoroughfare() != null)
address=address+", "+listAddress.get(0).getThoroughfare();
if (listAddress.get(0).getSubThoroughfare() != null)
address=address +" "+listAddress.get(0).getSubThoroughfare();
}
mMap.clear();
mMap.addMarker(new MarkerOptions().position(latLng).title(address));
}
MainActivity.places.add(address);
MainActivity.lat.add(Double.toString(latLng.latitude));
MainActivity.lng.add(Double.toString(latLng.longitude));
SharedPreferences sharedPreferences = this.getSharedPreferences("com.example.inbal.memorableplaces",Context.MODE_PRIVATE);
sharedPreferences.edit().putString("Places",ObjectSerializer.serialize(MainActivity.places)).apply();
sharedPreferences.edit().putString("Lat",ObjectSerializer.serialize(MainActivity.lat)).apply();
sharedPreferences.edit().putString("Lng",ObjectSerializer.serialize(MainActivity.lng)).apply();
MainActivity.arrayAdapter.notifyDataSetChanged();
} catch (IOException e) {
e.printStackTrace();
}
}
}
嘿，我建立了一个应该保存位置的项目。唯一的问题是，在我按下位置并保存后，我再也无法显示位置了。
行mMap.moveCamera(CameraUpdateFactory.newLatLng(location));会使应用崩溃，因为它说
java.lang.NullPointerException：尝试调用虚方法＆＃39; void com.google.android.gms.maps.GoogleMap.moveCamera（com.google.android.gms.maps.CameraUpdate）＆＃39; 在空对象引用上
答案 0 :(得分：1)
这是因为只有在满足条件mMap时才设置变量num == 0。
将您的代码更改为：
@Override
public void onMapReady(GoogleMap googleMap) {
mMap = googleMap;
Intent intent = getIntent();
int num = intent.getIntExtra("placeName", 0);
if (num == 0) {
//....
} else {
//....
}
}