Android Java NullPointerException when trying to see if GPS is enabled

Question

I'm a noob to Android (and not too hot on Java either).

I am trying to make sure GPS & internet are enabled before launching the map activity in my app. I'm getting a null pointer exception when calling CheckGPSStatus() in the code below and cannot resolve it.

(I've been pulling my hair out for the last 2 days trying to solve this; I've read loads of questions similar to this already & still can't solve it. I've also cut and paste a different programs code and while it works in its own project, when i practically copy it into my project it doesn't work in mine).

Any and all help gratefully received!

Error:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at com.example.racewatch.MainActivity.CheckGPSStatus(MainActivity.java:64)
at com.example.racewatch.MainActivity$1.onClick(MainActivity.java:46)

MainActivity.java

import android.app.ActivityManager;
import android.content.Context;
import android.content.Intent;
import android.location.LocationManager;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    
    Button launch_button;
    boolean gps_enabled = false;
    boolean internet_enabled = false;
    Context context;
    LocationManager lm;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        launch_button = findViewById(R.id.launch_button);

        launch_button.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick (View view){
                CheckGPSStatus();
                CheckLocationStatus();

                if (gps_enabled && internet_enabled) {
                    Intent i = new Intent(getApplicationContext(), Route.class);
                    startActivity(i);
                } else {
                    Toast.makeText(MainActivity.this, "Please enable Date & GPS Location", Toast.LENGTH_SHORT).show();
                }
            }   // end of OnClick

        // TODO: need to check for older build versions

        });  // end of onClickListener
    } // end of onCreate method


    public void CheckGPSStatus() {
        lm = (LocationManager)context.getSystemService(Context.LOCATION_SERVICE);
        gps_enabled = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
    }

    public void CheckLocationStatus() {
        lm = (LocationManager)context.getSystemService(Context.LOCATION_SERVICE);
        internet_enabled = lm.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
    }



} // end of class```


        

Answer 1

In this case, the problem is that the context field has not been initialized. However, you are in an Activity, which is itself a Context, so you don't need that field in the first place. Just call getSystemService() directly:

public void CheckGPSStatus() {
    lm = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
    gps_enabled = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
}