Modify the talker message on android_core (android_tutorial_pubsub)

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Hi, I am a electronic engineer student and i am using your pakage android_core for an android app; I want to change the message that publish the talker ("Hello world!") with a custom message; but when I look the main activity of android_tutorial_pubsub I couldn`t found the line or the method that allows me to change the message.

I really appreciate your attention and any info that you can provide me to solve my problem.