What the difference between not and!

Kotlin has not operator and ! which both will reverse the value of the boolean, but I want to know what is the differences between them and when to use not or !?

They’re the same thing, and ! is basically always preferred. not() is an operator fun, which means it can be called with !.

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Kotlin operators are just a special syntax to call special functions.You can define them by adding a function to any class

operator fun MyClass.not() = TODO()

When you call !someValue kotlin automatically translates this to someValue.not() (if there is an operator function defined).
This is useful if you write your custom maths library, eg. for complex numbers but can be used in other situations as well. It’s aslso used to make using BigDecimal easier.

Generally you should use the operator and not the function. So prefer !someValue over someValue.not().

If you want to know more about operator overloading you can look here: