# Code Smell 12 - Null

*Programmers use Null as different flags. It can hint an absence, an undefined value, en error etc. 
Multiple semantics lead to coupling and errors.*

> TL;DR: Null is schizofrenic and does not exist in real world. His creator regreted and programmers around the world suffer it. Don't be a part of it.

# Problems

- Coupling among the callers and the senders.

- Mismatch among the callers and the senders. 

- If/Switch/Case Polluting.

- Null is not polymorphic with real objects. Hence, *Null Pointer Exception*

- Null does not exist on real world. Thus, it violates [Bijection Principle](https://maximilianocontieri.com/the-one-and-only-software-design-principle)

# Solutions

- Avoid Null. 

- Use [NullObject pattern](https://en.wikipedia.org/wiki/Null_object_pattern) to avoid ifs.

- Use [Optionals](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html).

%[https://maximilianocontieri.com/null-the-billion-dollar-mistake]

# Exceptions

- APIs, Databases and external systems where NULL does exist.

# Sample Code

## Wrong

[Gist Url]: # (https://gist.github.com/mcsee/2f7aee362e99fbe447d8c1e8aaeb9f8e)
```javascript
class CartItem{
  constructor(price) {
     this.price = price;
  }
}

class DiscountCoupon {
  constructor(rate){
    this.rate = rate;
  } 
}

class Cart{        
    constructor(selecteditems, discountCoupon){
        this.items = selecteditems;
        this.discountCoupon = discountCoupon;
    }     
    subtotal(){    
        return this.items.reduce((previous, current) => previous + current.price, 0);      
    }  
    total(){    
        if (this.discountCoupon == null)
          return this.subtotal();
        else
          return this.subtotal() * (1 - this.discountCoupon.rate);      
    }
}

cart = new Cart([new CartItem(1), new CartItem(2), new CartItem(7)], new DiscountCoupon(0.15));
//10 - 1.5 = 8.5

cart = new Cart([new CartItem(1), new CartItem(2), new CartItem(7)], null);
//10 - null  = 10
```

## Right

[Gist Url]: # (https://gist.github.com/mcsee/a7571cf0ce9e7666bd6a393520f9a968)
```javascript
class CartItem{
  constructor(price) {
     this.price = price;
  }
}

class DiscountCoupon {
  constructor(rate){
    this.rate = rate;
  } 
  discount(subtotal){
    return subtotal * (1 - this.rate);    
  }
}

class NullCoupon {    
  discount(subtotal){
    return subtotal;    
  }
}

class Cart{        
    constructor(selecteditems, discountCoupon){
        this.items = selecteditems;
        this.discountCoupon = discountCoupon;
    }     
    subtotal(){    
        return this.items.reduce((previous, current) => previous + current.price, 0);      
    }  
    total(){    
        return this.discountCoupon.discount(this.subtotal());      
    }
}

cart = new Cart([new CartItem(1), new CartItem(2), new CartItem(7)], new DiscountCoupon(0.15));
//10 - 1.5 = 8.5

cart = new Cart([new CartItem(1), new CartItem(2), new CartItem(7)], new NullCoupon());
//10 - nullObject  = 10
```

# Detection

Most Linters can show null usages and warn us.

# Tags

- Null

# Conclusion

- Null is the billion-dollar mistake. Yet, most program languages support them and libraries suggest its usage.

# More info

- [Null: The Billion-Dollar Mistake](https://maximilianocontieri.com/null-the-billion-dollar-mistake)

# Credits

Photo by <a href="https://unsplash.com/@kydroon">Kurt Cotoaga</a> on <a href="https://unsplash.com/s/photos/black-hole">Unsplash</a>

* * *

> I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

_Tony Hoare_

%[https://maximilianocontieri.com/software-engineering-great-quotes]

* * *

This article is part of the CodeSmell Series.

%[https://maximilianocontieri.com/how-to-find-the-stinky-parts-of-your-code]

*Last update: 2021/06/16*