Android Fragment Add/replace and Backstack

No matter how fragment is joined, the Backstack,backstack is determined by the Addtobackstack function, and the fragment is added to the return stack only if the function is called.

So what's the difference between add and replace?

Add fragment, the underlying fragment is overwritten, when the return key is pressed, the current fragment POPs back to the stack, the underlying fragment directly displayed, without rebuilding.

Replace to add fragment, the underlying fragment is replaced (replace it), when the return key is pressed, the current fragment pop-up return stack, the underlying fragment needs to be rebuilt. And then show it again.

Android Fragment Add/replace and Backstack