Android开发（Java）-崩溃

Question

I'm trying to learn more about Java. I don't know much anything about it. People are saying that if I know PHP I know Java. I don't think so. Java is too much dot grinding and it doesn't allow any exceptions.

My application crashes immediately when started. I have added "FINE_LOCATION" and "COARSE_LOCATION" to permissions of application.

package either.no.me.myapplication2;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends AppCompatActivity {

    TextView textLat;
    TextView textLon;
    Timer timer1 = new Timer();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textLat = (TextView) findViewById(R.id.textLat);
        textLon = (TextView) findViewById(R.id.textLon);
        GPSFinder gpsFinder = new GPSFinder();
        gpsFinder.GetLocation();
    }
}

class GPSFinder extends MainActivity {
    public void GetLocation() {
        LocationManager locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

        // Define a listener that responds to location updates
        LocationListener locationListener = new LocationListener() {
            public void onLocationChanged(Location location) {
                // Called when a new location is found by the network location provider.
                textLat.setText(Double.toString(location.getLatitude()));
                textLon.setText(Double.toString(location.getLongitude()));
            }

            public void onStatusChanged(String provider, int status, Bundle extras) {
            }

            public void onProviderEnabled(String provider) {
            }

            public void onProviderDisabled(String provider) {
            }
        };

        // Register the listener with the Location Manager to receive location updates
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            // TODO: Consider calling
            //    ActivityCompat#requestPermissions
            // here to request the missing permissions, and then overriding
            //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
            //                                          int[] grantResults)
            // to handle the case where the user grants the permission. See the documentation
            // for ActivityCompat#requestPermissions for more details.
            return;
        }
        locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListener);
    }
}

The error I am getting in logcat:

07-31 01:36:45.611 1703-1703/either.no.me.myapplication2 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                           Process: either.no.me.myapplication2, PID: 1703
                                                                           java.lang.RuntimeException: Unable to start activity ComponentInfo{either.no.me.myapplication2/either.no.me.myapplication2.MainActivity}: java.lang.IllegalStateException: System services not available to Activities before onCreate()
                                                                               at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2219)
                                                                               at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269)
                                                                               at android.app.ActivityThread.access$800(ActivityThread.java:135)
                                                                               at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
                                                                               at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                               at android.os.Looper.loop(Looper.java:136)
                                                                               at android.app.ActivityThread.main(ActivityThread.java:5045)
                                                                               at java.lang.reflect.Method.invokeNative(Native Method)
                                                                               at java.lang.reflect.Method.invoke(Method.java:515)
                                                                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
                                                                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
                                                                               at dalvik.system.NativeStart.main(Native Method)
                                                                            Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate()
                                                                               at android.app.Activity.getSystemService(Activity.java:4532)
                                                                               at either.no.me.myapplication2.GPSFinder.GetLocation(MainActivity.java:37)
                                                                               at either.no.me.myapplication2.MainActivity.onCreate(MainActivity.java:31)
                                                                               at android.app.Activity.performCreate(Activity.java:5231)
                                                                               at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104)
                                                                               at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2163)
                                                                               at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2269) 
                                                                               at android.app.ActivityThread.access$800(ActivityThread.java:135) 
                                                                               at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
                                                                               at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                               at android.os.Looper.loop(Looper.java:136) 
                                                                               at android.app.ActivityThread.main(ActivityThread.java:5045) 
                                                                               at java.lang.reflect.Method.invokeNative(Native Method) 
                                                                               at java.lang.reflect.Method.invoke(Method.java:515) 
                                                                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
                                                                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
                                                                               at dalvik.system.NativeStart.main(Native Method) 
07-31 01:36:48.875 1703-1703/either.no.me.myapplication2 I/Process: Sending signal. PID: 1703 SIG: 9

Answer 1

Remove GPSFinder class and use code of GetLocation method inside MainActivity like below

<pre>
package either.no.me.myapplication2;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends AppCompatActivity {

    TextView textLat;
    TextView textLon;
    Timer timer1 = new Timer();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textLat = (TextView) findViewById(R.id.textLat);
        textLon = (TextView) findViewById(R.id.textLon);
        getLocation();
    }

public void getLocation() {
        LocationManager locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

        // Define a listener that responds to location updates
        LocationListener locationListener = new LocationListener() {
            public void onLocationChanged(Location location) {
                // Called when a new location is found by the network location provider.
                textLat.setText(Double.toString(location.getLatitude()));
                textLon.setText(Double.toString(location.getLongitude()));
            }

            public void onStatusChanged(String provider, int status, Bundle extras) {
            }

            public void onProviderEnabled(String provider) {
            }

            public void onProviderDisabled(String provider) {
            }
        };

        // Register the listener with the Location Manager to receive location updates
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            // TODO: Consider calling
            //    ActivityCompat#requestPermissions
            // here to request the missing permissions, and then overriding
            //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
            //                                          int[] grantResults)
            // to handle the case where the user grants the permission. See the documentation
            // for ActivityCompat#requestPermissions for more details.
            return;
        }
        locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListener);
    }  } 

Answer 2

GPSFinder shouldn't derive from MainActivity. It isn't an Activity.

Also, activities should never be created via new- they won't be fully initialized. They should be started (or the code shouldn't be an activity at all like here). Because it wasn't properly initialized when it called getSystemServices it failed. If you need a context or system service it should be passed in, you shouldn't just derive from a Context class.