Java Android位置距离为空

我很难让这个代码正常工作。我希望能够计算两个位置之间的距离。传递给onLocationChanged方法的初始位置正在工作（这证明我有正确的条件），因为当我执行

LocationLabel.setText（location.toString（））时可以正常工作，而不会出现任何空指针异常。但是，当在第49行中调用
calculatedInstance
方法时，
a.distanceTo（b）第54行失败。知道为什么distanceTo方法一直失败吗

错误消息：

package com.competitivegamingaudio;
import android.app.Activity;
import android.content.Context;
import android.location.*;
import android.os.Bundle;
import android.util.Log;
import android.widget.*;
import com.google.android.maps.*;;

public class FindMyFriendsActivity extends Activity {


private static final String TAG = "FindMyFriendsActivity";
public TextView LocationLabel;
public TextView DistanceLabel;
public MapView myMapView;
public Location oldlocation;
public Location currentlocation;

@Override
public void onResume()
    {
    super.onResume();
    LocationManager locationManager=(LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
     locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER,0,0,locationListener);
     locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER,0,0,locationListener);
    }

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    LocationLabel=(TextView) findViewById(R.id.LocationLabel);
    DistanceLabel=(TextView)findViewById(R.id.DistanceLabel);



}

LocationListener locationListener =new LocationListener()
{
public void onLocationChanged (Location location)
    {
    currentlocation=new Location(location);
    LocationLabel.setText(location.toString());
    oldlocation=new Location(location);
    DistanceLabel.setText("Distance: "+calculatedistance(currentlocation,oldlocation));

    }
public String calculatedistance(Location a, Location b)
    {
    a.distanceTo(b);
    return ""+a;
    }
public void onStatusChanged(String provider, int status, Bundle extras)
    {}
public void onProviderEnabled(String provider)
    {

    }
public void onProviderDisabled(String provider)
{}

};

}

main.xml：

01-09 20:11:58.085: ERROR/AndroidRuntime(9716): java.lang.NullPointerException
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at com.competitivegamingaudio.FindMyFriendsActivity$1.onLocationChanged(FindMyFriendsActivity.java:47)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.location.LocationManager$ListenerTransport._handleMessage(LocationManager.java:227)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.location.LocationManager$ListenerTransport.access$000(LocationManager.java:160)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.location.LocationManager$ListenerTransport$1.handleMessage(LocationManager.java:176)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.os.Handler.dispatchMessage(Handler.java:99)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.os.Looper.loop(Looper.java:130)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at android.app.ActivityThread.main(ActivityThread.java:3821)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at java.lang.reflect.Method.invokeNative(Native Method)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at java.lang.reflect.Method.invoke(Method.java:507)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-09 20:11:58.085: ERROR/AndroidRuntime(9716):     at dalvik.system.NativeStart.main(Native Method)

好的，关于你的代码，我有很多地方要修改，但这里不是地方。要尝试解决您的问题，请尝试以下操作

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView  
android:id="@+id/LocationLabel"
android:layout_width="fill_parent" 
android:layout_height="wrap_content" 
android:text="@string/hello"
/> 
<TextView  
    android:id="@+id/DistanceLabel"
android:layout_width="fill_parent" 
android:layout_height="wrap_content" 
android:text="@string/hello"
/>
</LinearLayout>

通过从我的Java代码中复制DistanceLabel并将其粘贴到main.xml中类似的DistanceLabel上，解决了这个问题。所有的字符看起来都一样，但它修复了它。也许在十六进制编辑器中查看它会有所帮助，但现在我很高兴它能工作。谢谢你的Quonk先生。
在你的代码中，旧位置和当前位置是相同的？？发布日志，显示异常01-09 18:42:06.342:ERROR/AndroidRuntime（7891）：致命异常：main 01-09 18:42:06.342:ERROR/AndroidRuntime（7891）：java.lang.NullPointerException 01-09 18:42:06.342:ERROR/AndroidRuntime（7891）：在com.competitivegamingaudio.FindMyFriendsActivity$1.onLocationChanged（FindMyFriendsActivity.java:49）01-09 18:42:06.342:ERROR/AndroidRuntime（7891）：在android.location.LocationManager$ListenerTransport._handleMessage（LocationManager.java:227）等处。旧位置和当前位置故意相同。最初我用白宫的经纬度作为旧位置，但我得到了同样的错误。不，那不起作用。问题不在于locationManager或locationListener为null（它们不是）。它告诉我oldlocation是空的。问题是当你发布logcat时，你只发布了几行，然后用“etc”结束，这真的没有帮助。如果我注释掉DistanceLabel.setText（“距离：+CalculatedInstance（currentlocation，oldlocation）），则张贴显示“原因”的位然后程序运行正常。但是仍然不能帮助我获得两个位置之间的距离。正如我上面所说的，发布完整的logcat异常，包括'Caused by'部分，作为对您的问题的编辑。没有'Caused by'子句。这很奇怪，因为我以前在遇到其他错误时也看到过。
// package and imports here

public class FindMyFriendsActivity extends Activity {

    // TAG, TextViews, MapView, Locations as before but add
    // LocationManager and LocationListener here as below...
    LocationManager locationManager = null;
    LocationListener locationListener = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        LocationLabel = (TextView)findViewById(R.id.LocationLabel);
        DistanceLabel = (TextView)findViewById(R.id.DistanceLabel);

        locationManager=(LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

        locationListener = new LocationListener() {
            // Put LocationListener methods here
        }
    }

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

        locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListener);
        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);
    }
}