如何从片段类访问片段的Arraylist

Question

I have 2 fragments called "Home fragment" and "Code Fragment". I have an arraylist of fragments in the main activity. I have set the "home fragment" on the main View and I have a button on the home fragment. I want to add the "Code fragment" to the arraylist and and show it in place of Home fragment. But when I try to do that it throws null point exception that arraylist not initialized. Please help I am new to android studio as well as in Java.

Thanks and regards