As in API 26 getDeviceId()
is depreciated so you can use following code to cater API 26 and earlier versions
TelephonyManager telephonyManager = (TelephonyManager)getSystemService(Context.TELEPHONY_SERVICE);
String imei="";
if (android.os.Build.VERSION.SDK_INT >= 26) {
imei=telephonyManager.getImei();
}
else
{
imei=telephonyManager.getDeviceId();
}
Don't forget to add permission requests for READ_PHONE_STATE
to use the above code.
UPDATE: From Android 10 its is restricted for user apps to get non-resettable hardware identifiers like IMEI.