TL;DR: Null does not exist in the real world. Its creator regrets it, and programmers worldwide suffer from it. Avoid being a part of it.

Programmers often usenull to represent various conditions such as absence, undefined values, or errors. However, this leads to coupling and errors due to multiple interpretations.

Problems

• Coupling between callers and senders.

• Mismatch between callers and senders.

• Pollution withif/when statements.

• Null is not polymorphic with real objects, resulting in Null Pointer Exceptions.

• Null does not exist in the real world, violating theBijection Principle.

Solutions

• Avoid using nullable types whenever possible.

• Use theNullObject pattern to eliminate conditional statements.

Exceptions

• APIs, databases, and external systems wherenull does exist.

Sample Code

Wrong

classCartItem(valprice:Double)classDiscountCoupon(valrate:Double)classCart(privatevalitems:List<CartItem>,privatevaldiscountCoupon:DiscountCoupon?){funsubtotal()=items.fold(0.0){acc,next->acc+next.price}funtotal()=discountCoupon?.let{subtotal()*(1-discountCoupon.rate)}?:subtotal()}funmain(){valcartItems=listOf(CartItem(1.0),CartItem(2.0),CartItem(7.0))varcart=Cart(cartItems,DiscountCoupon(0.15))println(cart.total())// 10 - 1.5 = 8.5cart=Cart(cartItems,null);println(cart.total())// 10 - null  = 10}

Right

classCartItem(valprice:Double)interfaceCoupon{fundiscount(subtotal:Double):Double}classDiscountCoupon(privatevalrate:Double):Coupon{overridefundiscount(subtotal:Double)=subtotal*(1-rate)}classNullCoupon:Coupon{overridefundiscount(subtotal:Double)=subtotal}// Notice that we're not using a nullable type for Coupon anymore!classCart(privatevalitems:List<CartItem>,privatevaldiscountCoupon:Coupon){funsubtotal()=items.fold(0.0){acc,next->acc+next.price}funtotal()=discountCoupon.discount(subtotal())}funmain(){valcartItems=listOf(CartItem(1.0),CartItem(2.0),CartItem(7.0))varcart=Cart(cartItems,DiscountCoupon(0.15))println(cart.total())// 10 - 1.5 = 8.5cart=Cart(cartItems,NullCoupon())println(cart.total())// 10 - nullObject  = 10}

Conclusion

• null is often considered a billion-dollar mistake. Despite that, many programming languages support its usage, and libraries even encourage it. In Kotlin, it's recommended to avoid nullable types unless absolutely necessary, and if needed, utilize the NullObject pattern to represent the absence of a field.

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More info

• Null: The Billion-Dollar Mistake

• Null safety in Kotlin

Credits

• Code Smell 12 - Null by @Maxi Contieri⭐⭐⭐