Use "this" in static method in android

Question

This is my first android app so maybe this is a stupid question. So, please consider if I am doing any mistake.

I am building a app related to bluetooth.

I have a method in my MainActivity.java:

public void showDeviceListDialog() {
    BluetoothDeviceListDialog dialog = new BluetoothDeviceListDialog(this);
    dialog.setOnDeviceSelectedListener(this);
    dialog.setTitle("Paired Devices");
    dialog.setDevices(bluetoothSerial.getPairedDevices());
    dialog.showAddress(true);
    dialog.show();
}

I need to call this method from another fragment. So, when I call this method like this: MainActivity.showDeviceListDialog(); it is asking for making the method Static. But when I am making it Static I am getting errors on "this" [ dialog.setOnDeviceSelectedListener(this); ] on my method.

I have already read some posts like this and this but I didn't got help about my problem.

I have tried this from my fragment:

 MainActivity mc = new MainActivity();
 mc.showDeviceListDialog();

but this is showing NullPointerException.

So, Please tell me how to call it from my fragment without this errors. Thank you.

You can use interface as a callback to the activity. Never do `MainActivity mc = new MainActivity();` — Raghunandan, Oct 06 '16 at 07:47
you can't use `this` with `static`. You shouldn't use `new` on classes that extend `Activity` as well — Blackbelt, Oct 06 '16 at 07:48
@Blackbelt obviously you can do it, but that doesn't mean that you should — OneCricketeer, Oct 06 '16 at 07:49
I suggest stepping back from your current activity and working through some basic Java tutorials. — T.J. Crowder, Oct 06 '16 at 07:49
guys, this is NOT a DUPLICATE. although the question seem about the usage of `static` method, the solution he is asking does not really involve creating a static method. this is a matter of **accessing an activity from Fragment**. — user1506104, Oct 06 '16 at 08:04
@Blackbelt I was asking for a solution to call a non-static method of a activity from a fragment. Not asking what is a static method and why I cant use 'this' in static method. So, my question is not a duplicate of this questions. — Bishwajyoti Roy, Oct 06 '16 at 14:02

ΦXocę 웃 Пepeúpa ツ · Answer 1 · 2016-10-06T07:49:20.557

1

this is a reference pointing to the current instance, therefore makes no sence in static methods ... and doing something like

MainActivity mc = new MainActivity();

is not the way android wants you to create an activity... you need to get the Activity like calling the getActivity(); method

edited Oct 06 '16 at 07:49

answered Oct 06 '16 at 07:47

ΦXocę 웃 Пepeúpa ツ

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thanks for the comment – ΦXocę 웃 Пepeúpa ツ Oct 06 '16 at 07:49
1

Doesn't really address the solution of the initial problem, which was calling the activity method. OP simply allowed the IDE to make the method static – OneCricketeer Oct 06 '16 at 07:51

Eran · Accepted Answer · 2016-10-06T08:00:23.823

1

A Fragment has a getActivity() method which returns the Activity associated with the Fragment.

Therefore, you can call this method from a Fragment without making it static :

getActivity ().showDeviceListDialog ();

EDIT (thanks to user1506104's comment) :

Actually, you'll have to cast the Activity returned by getActivity() to MainActivity in order to call that method :

((MainActivity) getActivity ()).showDeviceListDialog ();

This is assuming the Fragment is associated with an Activity of that type.

edited Oct 06 '16 at 08:00

answered Oct 06 '16 at 07:49

Eran

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this should work given that the fragment in question is a child of the Activity that has `showDeviceListDialog()`. – user1506104 Oct 06 '16 at 07:57
Thanks for your answer @Eran. It just worked! XD – Bishwajyoti Roy Oct 06 '16 at 08:12

score 0 · Answer 3 · answered Oct 06 '16 at 07:49

0

You cannot create activity like that : MainActivity mc = new MainActivity(); . Android is responsible for this. Use Intent instead. See this tutorial https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html about static methods.

answered Oct 06 '16 at 07:49

Alex Shutov

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we should not be inlcuding URLs in answers section – user1506104 Oct 06 '16 at 08:06

Use "this" in static method in android

3 Answers3