Question

我是java和android studio的新手。我正在尝试在用户位于地图上指定点的特定距离内时启动新活动。

以下课程确定用户距离每个点的距离，当用户在一定距离（10米）内时，我希望启动新活动，用户将能够根据位置回答问题。

然而，当我尝试开始新活动时，我收到一个错误：

E/AndroidRuntime: FATAL EXCEPTION: main

Process: uk.ac.ucl.cege.cegeg077.ucfapwh.firstapp, PID: 6616
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
    at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:106)
    at uk.ac.ucl.cege.cegeg077.ucfapwh.firstapp.CustomLocationListener.onLocationChanged(CustomLocationListener.java:67)
    at android.location.LocationManager$ListenerTransport._handleMessage(LocationManager.java:281)
    at android.location.LocationManager$ListenerTransport.access$000(LocationManager.java:210)
    at android.location.LocationManager$ListenerTransport$1.handleMessage(LocationManager.java:226)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5343)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)

这是我到目前为止的代码：

import android.app.Activity;
import android.content.Intent;
import android.location.Location;
import android.location.LocationListener;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;
import java.lang.reflect.Array;
import java.util.ArrayList;

public class CustomLocationListener extends Map implements LocationListener {
public Map parentActivity ;
public ArrayList<GeoPoint> coordList;
// this method is called every time the user moves around - i.e. changes     location
// it pops up a toast message with the new coordinates
public void onLocationChanged(Location location) {

    // now measure distance from all the pre-set proximity alerts
    for (int i = 0; i < coordList.size(); i++) {

        // create GeoPoint object for each pair of coordinates
        GeoPoint gp = coordList.get(i);

        // create Location object
        Location fixedLoc = new Location("one");

        // get lat and lng values from GeoPoint object
        Float lat = Float.valueOf(String.valueOf(gp.getLatitude()));
        Float lng = Float.valueOf(String.valueOf(gp.getLongitude()));

        // set location for proximity alert
        fixedLoc.setLatitude(lat);
        fixedLoc.setLongitude(lng);

        // use Android distanceTo function to calculate the distances
        float distance = location.distanceTo(fixedLoc);

        Log.i("****DISTANCE****",String.valueOf(distance));
        Log.i("*****FIXEDLOC****", fixedLoc.toString());

        for (int j = 0; j < coordList.size(); j++ ) {

            if (i == j && distance < 120) {

                GeoPoint geoPoint = coordList.get(j);

                Log.i("****PROXI LAT*****", geoPoint.getLatitude().toString());
                Log.i("****PROXI LNG****", geoPoint.getLongitude().toString());

                Intent quizIntent = new Intent(getApplicationContext(),Questions.class);
                startActivity(quizIntent);

            }
        }

    }

}

// these methods are called when the GPS is switched on or off
// and will allow the App to warn the user and then
// shut down without an error
public void onProviderDisabled(String s) {
}
public void onProviderEnabled(String s) {
}

// this method is required by the LocationListener
// we do not need to do anything here
// but in a full implementation this could be used to react when the GPS signal is not available
public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
    // TODO Auto-generated method stub

}

}

抱歉格式不佳。任何帮助将不胜感激。

Answer 1

解决：

要从非活动类中检索上下文，需要以下代码：

private Context context;

public NonActivityClass(Context context) {
    this.context = context.getApplicationContext();
}

然后触发Intent并开始新活动：

                Intent i = new Intent(context, Activity.class);
                i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                context.startActivity(i);

感谢您的帮助。

get Context in non-Activity class

Answer 2

您无法使用Application上下文来启动应用程序。您应该使用活动上下文。如果（猜测）地图是您的活动，则更新开始活动代码

Intent quizIntent = new Intent(this,Questions.class);
                startActivity(quizIntent);

有关应用程序上下文无法用于启动活动https://possiblemobile.com/2013/06/context/

的更多详细信息

Answer 3

从我的观点来看，你有两个选择：

将CustomLocationListener声称为私有，我要说NameOfYourActivity。

接下来，将Intent构造函数从getApplicationContext()更改为NameOfYourActivity.this。

将Context的{{1}}设置为NameOfYourActivity中的属性，然后从CustomLocationListener更改Intent构造函数到上下文属性。

希望有所帮助

Answer 4

你得到null因为你的类没有扩展android活动。您可以在非活动类中获取应用程序上下文，如解释here。

尝试启动新活动时出现空指针异常

4 个答案: