委托类中的getApplicationContext（）的NullPointerException

Question

I have a second class from which I call some methods to handle app updating. There is a progress dialog in them so I have to pass the current application context to its constructor. The code works fine when called directly in my MainActivity onCreate(), but it breaks down when I delegate the code to an external class. What's going wrong?

Method Call in OnCreate():

private AppUpdateHelper appUpdateHelper = new AppUpdateHelper(getApplicationContext());

@Override
    protected void onCreate(Bundle savedInstanceState) {
    appUpdateHelper.handleAppUpdate();
    }

Delegate Class:

public class AppUpdateHelper {

    private Context mContext;

    public AppUpdateHelper(Context mContext) {
        this.mContext = mContext;
    }

    public void handleAppUpdate() {
        String versionCode = getVersionCode(); // Get app's current version code

        // Is app update to date?
        if (isAppCurrent(versionCode)) {
            promptAppUpdate();
        }
    }


    private String getVersionCode() {
        String versionCode = null;
        try {
            PackageInfo pInfo = mContext.getPackageManager().getPackageInfo(mContext.getPackageName(), 0);
            versionCode = pInfo.versionName;
            // Log.w(mContext.getClass().getSimpleName(), "Current Version: " + versionCode);
        } catch (PackageManager.NameNotFoundException e) {
            e.printStackTrace();
        }
        return versionCode;
    }


    private boolean isAppCurrent(String versionCode) {
        ParseInstallation installation = ParseInstallation.getCurrentInstallation();
        String userAppVersion = installation.getString("appVersion");
        return !userAppVersion.equals(versionCode);
    }


    private void promptAppUpdate() {
        SweetAlertDialog pDialog = new SweetAlertDialog(mContext, SweetAlertDialog.WARNING_TYPE);
        pDialog.setTitleText("Update Available!");
        pDialog.setContentText("You must update to continue using Yeet Club!");
        pDialog.setConfirmClickListener(sDialog -> {
            final String appPackageName = mContext.getPackageName(); // getPackageName() from Context or Activity object
            try {
                mContext.startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("market://details?id=" + appPackageName)));
            } catch (ActivityNotFoundException anfe) {
                mContext.startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("https://play.google.com/store/apps/details?id=" + appPackageName)));
            }

            sDialog.dismissWithAnimation();
        });
        pDialog.setCancelable(false);
        pDialog.showCancelButton(false);
        pDialog.show();
    }

}

Exception:

08 - 20 15: 43: 43.874 21456 - 21456 / com.app.android E / AndroidRuntime: FATAL EXCEPTION: main
Process: com.app.android, PID: 21456
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo {
 com.app.android / com.app.android.activity.MainActivity
}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()'
on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java: 2458)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java: 2613)
at android.app.ActivityThread.access$900(ActivityThread.java: 180)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java: 1473)
at android.os.Handler.dispatchMessage(Handler.java: 111)
at android.os.Looper.loop(Looper.java: 207)
at android.app.ActivityThread.main(ActivityThread.java: 5710)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java: 900)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 761)
Caused by: 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: 117)
at com.app.android.activity.MainActivity. < init > (MainActivity.java: 77)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java: 1072)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java: 2448)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java: 2613) 
at android.app.ActivityThread.access$900(ActivityThread.java: 180) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java: 1473) 
at android.os.Handler.dispatchMessage(Handler.java: 111) 
at android.os.Looper.loop(Looper.java: 207) 
at android.app.ActivityThread.main(ActivityThread.java: 5710) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java: 900) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 761) 

Answer 1

You can pass the activity context it self using this key word.

 @Override
 protected void onCreate(Bundle savedInstanceState) {
 appUpdateHelper = new AppUpdateHelper(this);
 appUpdateHelper.handleAppUpdate();
}

Answer 2

This is very simple.

You can get context instance after called OnCreate

private AppUpdateHelper appUpdateHelper;

@Override
     protected void onCreate(Bundle savedInstanceState) {
     appUpdateHelper = new AppUpdateHelper(getApplicationContext());
     appUpdateHelper.handleAppUpdate();
    }

Answer 3

You can't call getApplicationContext() before the class initialized

You can only call after the class initialized. You can get it on onCreate() or other methods called by android.

you are safe to call inside onCreate()

 @Override
 protected void onCreate(Bundle savedInstanceState) {
 appUpdateHelper = new AppUpdateHelper(getActivity());  // if it is Fragment
 appUpdateHelper = new AppUpdateHelper(this);  // if it is Activity
 appUpdateHelper.handleAppUpdate();
}

Application context won't work with Dialog. Because application context does not have theme related information. Use Activity context instead of Application Context