# Code Smell 271 - The Hollywood Principle

> TL;DR: The Hollywood Principle promotes loose coupling by inverting control. High-level components decide when and how to use low-level components.

# Problems

- Tight [coupling](https://maximilianocontieri.com/coupling-the-one-and-only-software-design-problem)
- Difficult to extend
- Reduced flexibility
- Increased complexity
- Violation of SOLID principles
- Lack of Testability

# Solutions

1. Apply Inversion of control
2. Use Dependency injection
3. Depend on abstractions

# Context

The [Hollywood Principle](https://en.wiktionary.org/wiki/Hollywood_principle) is a software design principle emphasizing loose coupling between components. 

High-level components *should not* directly control the execution flow in low-level components. 

Low-level components should register themselves with high-level components, and high-level components should decide *when* and *how* to use them.

This is also known as *Inversion of Control*.

# Sample Code

## Wrong

[Gist Url]: # (https://gist.github.com/mcsee/d925b228b08c4772a5ac7aac291221af)

```typescript
class TicketCart {
    private paymentMethod: PaymentMethod;

    constructor(paymentMethodType: string) {
        // TicketCart is tightly coupled
        // to specific payment method classes 
        // like CreditCardProcessor and CryptoService.  
        if (paymentMethodType === 'creditCard') {
            this.paymentMethod = new CreditCardProcessor();
        } else if (paymentMethodType === 'Crypto') {
            this.paymentMethod = new CryptoService();
        } else {
            throw new Error('Invalid payment method');
        }
    }

    checkout(money: Money): void {
        this.paymentMethod.pay(money);
    }
}

const cart = new TicketCart('creditCard');
const money = new Money(126, 'USD');
cart.checkout(money);
```

## Right

[Gist Url]: # (https://gist.github.com/mcsee/8b20655d72060014443b93b8a4578ca8)

```typescript
interface PaymentMethod {
    pay(total: Money): void;
}

class TicketCart {
    private paymentMethod: PaymentMethod;

    constructor(paymentMethod: PaymentMethod) {
        // This solution is more open and less coupled
        // because it relies on abstractions
        this.paymentMethod = paymentMethod;
    }

    checkout(total: Money): void {
        this.paymentMethod.pay(total);
    }
}

class CreditCardProcessor implements PaymentMethod {
    pay(total: Money): void {
        console.log(`Processing payment of ${total.Amount()} 
        ${total.currency()} using credit card.`);
    }
}

const creditCardProcessor = new CreditCardProcessor();
const cart = new TicketCart(creditCardProcessor);
const total = new Money(126, 'USD');
cart.checkout(total);
```

# Detection

[X] Manual

This is a design smell
 
# Tags

- Coupling

# Level

[X ] Intermediate

# AI Generation

AI generators can sometimes create code that violates the Hollywood Principle if you don't explicitly instruct them to follow [inversion of control](https://en.wikipedia.org/wiki/Inversion_of_control) patterns. 

They often generate straightforward, tightly coupled code by default.

# AI Detection

AI tools can effectively detect violations of the Hollywood Principle by analyzing code dependencies and identifying tight coupling with proper instructions. (see below).

## Try Them!

*Remember: AI Assistants make lots of mistakes*

| Without Proper Instructions    | With Specific Instructions |
| -------- | ------- |
| [ChatGPT](https://chat.openai.com/?q=Correct+and+explain+this+code%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | [ChatGPT](https://chat.openai.com/?q=Remove+the+direct+dependency+and+use+dependency+injection%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) |
| [Claude](https://claude.ai/new?q=Correct+and+explain+this+code%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | [Claude](https://claude.ai/new?q=Remove+the+direct+dependency+and+use+dependency+injection%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) |
| [Perplexity](https://perplexity.ai/?q=Correct+and+explain+this+code%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | [Perplexity](https://perplexity.ai/?q=Remove+the+direct+dependency+and+use+dependency+injection%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) |
| [Copilot](https://www.bing.com/chat?showconv=1&sendquery=1&q=Correct+and+explain+this+code%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | [Copilot](https://www.bing.com/chat?showconv=1&sendquery=1&q=Remove+the+direct+dependency+and+use+dependency+injection%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) |
| [Gemini](https://gemini.google.com/?q=Correct+and+explain+this+code%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | [Gemini](https://gemini.google.com/?q=Remove+the+direct+dependency+and+use+dependency+injection%3A+%60%60%60typescript%0D%0Aclass+TicketCart+%7B%0D%0A++++private+paymentMethod%3A+PaymentMethod%3B%0D%0A%0D%0A++++constructor%28paymentMethodType%3A+string%29+%7B%0D%0A++++++++%2F%2F+TicketCart+is+tightly+coupled%0D%0A++++++++%2F%2F+to+specific+payment+method+classes+%0D%0A++++++++%2F%2F+like+CreditCardProcessor+and+CryptoService.++%0D%0A++++++++if+%28paymentMethodType+%3D%3D%3D+%27creditCard%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CreditCardProcessor%28%29%3B%0D%0A++++++++%7D+else+if+%28paymentMethodType+%3D%3D%3D+%27Crypto%27%29+%7B%0D%0A++++++++++++this.paymentMethod+%3D+new+CryptoService%28%29%3B%0D%0A++++++++%7D+else+%7B%0D%0A++++++++++++throw+new+Error%28%27Invalid+payment+method%27%29%3B%0D%0A++++++++%7D%0D%0A++++%7D%0D%0A%0D%0A++++checkout%28money%3A+Money%29%3A+void+%7B%0D%0A++++++++this.paymentMethod.pay%28money%29%3B%0D%0A++++%7D%0D%0A%7D%0D%0A%0D%0Aconst+cart+%3D+new+TicketCart%28%27creditCard%27%29%3B%0D%0Aconst+money+%3D+new+Money%28126%2C+%27USD%27%29%3B%0D%0Acart.checkout%28money%29%3B%0D%0A%60%60%60) | 

# Conclusion

This principle can improve your code quality, reduce complexity, and enhance testability.

# Relations

%[https://maximilianocontieri.com/code-smell-198-hidden-assumptions]

%[https://maximilianocontieri.com/code-smell-32-singletons]

%[https://maximilianocontieri.com/code-smell-63-feature-envy]

%[https://maximilianocontieri.com/code-smell-16-ripple-effect]

%[https://maximilianocontieri.com/code-smell-61-coupling-to-classes]

%[https://maximilianocontieri.com/code-smell-64-inappropriate-intimacy]

# More Info

%[https://maximilianocontieri.com/coupling-the-one-and-only-software-design-problem]

%[https://en.wikipedia.org/wiki/Inversion_of_control]

%[https://en.wiktionary.org/wiki/Hollywood_principle]

# Disclaimer

Code Smells are my [opinion](https://maximilianocontieri.com/i-wrote-more-than-90-articles-on-2021-here-is-what-i-learned).
  
* * *

> "Dependency Injection is fundamentally about passing dependencies to objects, rather than having objects create or find them." 

_Martin Fowler_
 
%[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]