How to do if statements

I have a project . I want a class DrawLetter.class but the back button is an error .

Example

Click A-Z > (Clicked B) > ( how to write letter B will show in xml form w/ a button WriteLetter ) > (Clicked the button WriteLetter) > the WriteLetter.xml will show > if I click Back button on WriteLetter.xml it will show the A.xml because it is my default back function and dont know how to use if statement.

How to use if statement base on the user Clicked letter ?

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