Android : Continuous Click on image button without lifting finger in android

So in my app i have this 4x4 imagebutton. I don't have any experience in android programming since this is my very first app. As in the picture, I am forming a word "BIT" (displayed in texview). My question is how can i click these imagebuttons to form words WITHOUT lifting the finger? Any suggestions will be highly appreciated. I would love if you'll also help me with the codes.