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Something unique on android you can't change

I'm doing an application, for the trial version I have to recognize phone from something like IMEI or MAC address, but if you are good you can spoof them, so is there something on Android that doesn't change at all? Thank you

there is something called UDID which is generated on boot and it only cleared when device reset to its factory defaults. So you can use it if it matches your requirement. Here is the link to which tells how to generate it.

How to get unique id for clients without registration? (Jodel app for e.g)

Imagine an application that consists of loads of clients and one server. The clients should be able to use the application without the need to register a username for e.g. (Therefore getting the feeling of anonymously using the app)
What unique data in a smartphone could be use to identify "anonymous" users and seperate them uniquely, so that each user would have its own personal data.
Could one use the IMEI id?
Is this possible?

At I/O 15 Google announced the 'instance ID' which uniquely identifies an app installation, and can be used for a veriety of other purposes as well.
I give more detail in an answer to an existing, similar question here:
https://stackoverflow.com/a/30948111/150016

Yes You can use the IMEI, but then you need a permission...or you could give every User a Unique ID on the server side
I think a long type variable is enough so the first one that connects you give him the ID 1 the app gets the ID and so on and so forth

You can use a built in feature here:
import android.provider.Settings.Secure;
private String uniqueIdentifier = Secure.getString(getContext().getContentResolver(),
Secure.ANDROID_ID);
where Secure.ANDROID_ID is presented as "A 64-bit number (as a hex string) that is randomly generated when the user first sets up the device and should remain constant for the lifetime of the user's device."

This is exactly the use case for the new Instance ID: getting a unique ID that you can use to identify even a not logged in user to your server.
This works on both phones and tablets (something IMEI does not) and avoid issues with ANDROID_ID (such as a few manufacturers always using the same ANDROID_ID on all devices).

You don't want to use the IMEI, as it will only apply to devices that have cell phones in them. There are plenty of wifi only devices out there in the wild.
Also, to get the IMEI, you need to ask for extra permission in your manifest.
<uses-permission android:name="android.permission.READ_PHONE_STATE" />
You'd be better off either generating your own random GUID and storing it in your app's SQLite database, or use the Android ID field.
Android ID will give you extra benefits like providing a different identifier for different user profiles within a device.
Another answer here mentioned using Instance ID, but the big caveat there is it requires Google Play Services, so your app wouldn't work on AOSP / forked devices (i.e. Amazon's tablets). I'd have just inserted this as a comment to that answer, but my rep won't let me comment yet.

How do mobile games store and retrieve unique user data

I'd like to know how some games such as LINE Bubble or whatever store user statistics and game data, ect.. I'm guessing it must be with php and sql or something like that, but can someone be a bit more specific? Since some of these games don't require direct input of usernames and don't use passwords how do they know the correct data to retrieve? Do they use the phone number or some sort of device id that the mobile os provides?
Thanks!

Each OS provides a unique identifier to the user, one way or the other.
To get a unique identifier for iPhone, see this answer https://stackoverflow.com/a/22937460/1891327
For android you can see this
https://stackoverflow.com/a/2785493/1891327

How to uniquely identify android device to track devices

Hello I am working on an application in which we are tracking our app installation using device unique ID.
We are doing right now like this as https://stackoverflow.com/a/2785493/2455259
private String android_id = Secure.getString(getContext().getContentResolver(),
Secure.ANDROID_ID);
I got to know that from android 4.2 it can have different for different users https://stackoverflow.com/a/27013749/2455259.
Now I can't take mac address of wifi or bluetooth because it requires to make it turn on then fetch. Even I can't take IMEI because some tables don't have call feature so they don't have IMEI.
Now what are the option to uniquely identify android device ?

Now what are the option to uniquely identify android device ?
I am afraid assigning own unique number is most effective approach, because ANDROID_ID is broken (the answer you link to is incorrect - see comments there too), relying on it is quite futile as it can sometimes be the same for many devices or null, so anything but unique, therefore using it is pointless as it adds more troubles than helps. There's post on Android blog about this topic "Identifying App Installations" that you may want to get thru as well.

I know this is old and the OP might have already moved on but with the introduction of Instance ID, we can use it to identify a unique installation.
The associated code is pretty straightforward too.
String iid = InstanceID.getInstance(context).getId();
The benefits include not having to specify permissions for Android Marshmallow and above. This seems to be a plus for developers who would need a unique id at the outset and cannot rely on user provided permission model for the same.

Unique ID for each user in Android

I want to allocate unique ID to each user as soon as he installs the application so that whenever the app contacts the server I know who is contacting.
For this purpose, I thought that on first time installation, the app contacts the server and gets unique ID. But I don't know where to store it permanently so that next time when app is started, it knows what its ID is rather than contacting server.
Sorry if that is some obvious question as I am newbie.

This question has been asked many times on Stack Overflow.
In short: Android has always supported a unique ID. However, prior to Android 2.2, the ID was not always identical on certain kinds of phones. Since 2.2 is pretty ubiquitous by now, I would use that ID.
The Android Developer Blog has a good article about this.
And as Joachim said - you may want to consider a different approach altogether. Android's unique ID is good and persistent across factory resets, but not across a device upgrade. Also keep in mind that many people have several devices (like a phone and a tablet). You may want to use the Google account instead, the AccountManager can help you there.

Use SharedPreferences to store the unique id.
Here is an example:
Android SharedPreferences
For more complex data, you can use SQlite.

For unique id, you can use IMEI of device on which application is going to install. Refer this link for how to get IMEI number. Then stored that IMEI number in shared preference. Refer Guillermo lobar's link for that. You need to check for that unique id in preference when you application starts. At very first time, save that in preference. So when next time it checks for that id, app find it in preference and hence no need to connecting server. :)

You could get the IMEI of the device. As of API 26, getDeviceId() is deprecated. If you need to get the IMEI of the device, use the following:
String deviceId = "";
if (Build.VERSION.SDK_INT >= 26) {
deviceId = getSystemService(TelephonyManager.class).getImei();
}else{
deviceId = getSystemService(TelephonyManager.class).getDeviceId();
}