Why wild arguments don't work?

Question

In forEvery method definition,
If i remove
Function<Integer, Integer> by,
Function<?,? > or Function<? extends Number,? extends Number>
it shows error.
why?
https://code.sololearn.com/cBr6EHBfu1v4/?ref=app

Answer 1

The first type parameter must be Integer since you call it on an Integer value in your code. However you can replace the second one with ? extends Number