Value and Record Types - Speaker Deck

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Starring With vs Special Appearances by

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Java old! C# ABAP PHP Python

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Kolleg:in gesucht (Deutschlandweit)

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Buy it on Amazon.com: https://hschwentner.io

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Photo: Henning Schwentner Let me tell you a story

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@hschwentner Example: A Banking Domain bank account customer teller computer transaction

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to deposit to withdraw bank account money amount IBAN to pay interest

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@hschwentner Example: A Banking Domain computer Banking 3000

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@hschwentner Software != End in itself

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@hschwentner ! ! " Software is build Tech people business people # " for for by

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Software Domain

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Foto: Rosie the Riveter/Wikimedia/CC-PD-Mark

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@hschwentner Programming is Model Building

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@hschwentner Rules of thumb:

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Objects from real world turn into objects in software

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Actions from real world turn into operations in software

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=> Object-orientation

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Object-Oriented Programming Ole-Johan Dahl Photo: Stein Krogdahl, University of Oslo Kristen Nygaard Photo: Jorge Stolfi/Wikipedia !

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Photo: P. Campbell/Wikipedia Alan Kay Everything is an object "

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Foto: P. Campbell/Wikipedia James Gosling Everything is an object #

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Anders Hejlsberg Everything is an object

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Are those objects?

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@hschwentner In the real world not everything is an object

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@hschwentner Domain-Driven Design Eric Evans " Entity Value Object Aggregate Service Factory Repository

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@hschwentner Entity vs. Value

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Foto: P. Campbell/Wikipedia James Gosling Everything is an object int short long byte char float double boolean … except:

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Anders Hejlsberg Everything is an object …except structs

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Values from real world turn into values in software

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@hschwentner What is an object/entity?

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Identity

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Foto: Th.omas G.Graf/Wikipedia oto: H. Schwentner

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Grafik: Pixabay

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Foto: Bernd Schwabe in Hannover/Wikipedia

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@hschwentner Object: *Identity *Lifecycle *(mutable) state

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@hschwentner What is a value?

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Grafik: Public Domain/Pixabay

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Foto: A. Markiewicz

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@hschwentner

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@hschwentner Value: *No identity *immutable

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@hschwentner Example

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% & '

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' '

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% & '

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' ' & ' & '

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Foto: Bibi Saint-Pol/Wikipedia

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@hschwentner Objects in Languages like Java or C#

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Foto: Tiia Monto/Wikipedia

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@hschwentner Identity of an Object

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3 3

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@hschwentner Values in Languages like Java or C#

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built-in value types primitive types simple types

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int boolean/bool double char

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User-defined value types

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LocalDate/DateTime String

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object types that behave like value types value-based class (class with) value equality

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@hschwentner Values in Java

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int boolean double char

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unsigned complex

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iban postcode amount

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Foto: Public Domain/Pixabay

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Grafik: Gemeinfrei/Wikipedia

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value-based class

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@hschwentner Only final fields

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@hschwentner No state-changing methods

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@hschwentner equals() does not depend on identity

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value-based class <= 13

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LocalDate String

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record >= 16

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record: all fields final field-based equal() [and hashCode()] getters for all fields >= 16

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value type >= ??

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@hschwentner Values in C#

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int bool double char

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struct <= 7.1

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@hschwentner Only readonly fields

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@hschwentner No state-changing methods

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readonly struct >= 7.2

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@hschwentner equals() ! == "

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record <= 9

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@hschwentner Value equality

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@hschwentner equals() ! == !

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@hschwentner Beware: Can be mutable

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@hschwentner On the heap

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record struct <= 10

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@hschwentner Beware: Is mutable

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readonly record struct <= 10 !

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@hschwentner Why aren’t int and string enough?

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@hschwentner Objects/Entities in Java — Example

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public class Account { }

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import org.jmolecules.ddd.annotation.Entity; @Entity public class Account { }

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@Entity public class Account { private int _balance; public int getBalance() { return _balance; } public void setBalance(int balance) { _balance = balance; } } ✘ Bad: The account balance can be set to any value

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@Entity public class Account { private int _balance; public int balance() { return _balance; } public void deposit(int amount) { _balance += amount; } public void withdraw(int amount) { _balance -= amount; } Better: Operations with domain- specific behavior and names

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@Entity public class Account { private int _balance; public int balance() { return _balance; } public void deposit(int amount) { _balance += amount; } public void withdraw(int amount) { if (amount > balance()) { throw new IllegalArgumentException("Amount too big"); Even better: Preconditions can be checked

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@Entity public class Account { // ... public void withdraw(int amount) { assert amount <= balance(); _balance -= amount; } } Assertion using keyword assert

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GREAT, BUT… @Entity public class Account { // ... public void withdraw(int amount) { assert amount <= balance(); _balance -= amount; } } Can I withdraw a negative amount? In EUR or GBP or…?

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@hschwentner Values in Java — Example

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public class Amount { }

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import org.jmolecules.ddd.annotation.ValueObject; @ValueObject public class Amount { }

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@ValueObject public class Amount { private int _amount; private Currency _currency; }

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; }

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; private Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } public static Amount of(int amount, Currency currency) { return new Amount(amount, currency); } }

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } @Override public boolean equals(Object other) { return _amount == ((Amount) other)._amount && _currency.equals(((Amount) other)._currency); }

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@ValueObject public record Amount(int amount, Currency currency) {}

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } public Amount add(Amount otherAmount) { return Amount.of(_amount + otherAmount._amount, _currency); } } Der neue Typ hat richtiges fachliches Verhalten

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@ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } public Amount add(Amount otherAmount) { assert hasSameCurrency(otherAmount); return Amount.of(_amount + otherAmount._amount, _currency); } … und Verträge, die vor falschen Währungen schützen

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@ValueObject public record Amount(int amount, Currency currency) { public Amount add(Amount otherAmount) { assert hasSameCurrency(otherAmount); return new Amount(amount + otherAmount.amount(), currency); } public boolean hasSameCurrency(Amount otherAmount) { return otherAmount.currency() == currency; } }

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@hschwentner Values in C# — Example

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public class Amount { }

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using NMolecules.DDD; [ValueObject] public class Amount { }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; }

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[ValueObject] public class Amount { private int _amount; private Currency _currency; }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public override bool Equals(object other) { return other is Amount && _amount == ((Amount) other)._amount && _currency.Equals(((Amount) other)._currency); } }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public override bool Equals(object other) => other is Amount && _amount == ((Amount) other)._amount && _currency.Equals(((Amount) other)._currency); }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public static bool operator ==(Amount a, Amount b) => a.Equals(b); }

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[ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public static bool operator ==(Amount a, SignDate b) => a.Equals(b); public static bool operator !=(Amount a, SignDate b) => !a.Equals(b); }

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[ValueObject] public struct Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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[ValueObject] public struct Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } // Equals() does not have to be overridden }

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@hschwentner equals() ! == "

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[ValueObject] public record Amount(int amount, Currency currency);

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[ValueObject] public record Amount(int amount, Currency currency); // automatically readonly // Equals(), GetHashCode(), ToString(), operators // do not have to be overloaded and work as expected

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[ValueObject] public record struct Amount(int amount, Currency currency);

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[ValueObject] public readonly record struct Amount(int amount, Currency currency);

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@hschwentner Values in Java — Example continued

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But

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@hschwentner equals() ! == "

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by value

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by reference

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Memory*2

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int[]

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3 5 10 5 0

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Object[]

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3 10 5 0

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@hschwentner A Concrete Example

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@hschwentner LeasingNinja https://leasingninja.io

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@hschwentner Conclusion

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@hschwentner Key take aways: *value/object are different *only records yet * value types are still to come (*domain model and performance)

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Bild: Gemeinfrei

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@hschwentner Further Reading

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project valhalla

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https://domainstorytelling.org

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Workshop Domain-Driven Design concrete wps.de/ddd

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Bibliography Beck, Kent et al. Manifesto for Agile Software Development. 2001. Evans, Eric. Domain-Driven Design: Tackling Complexity in the Heart of Software. Boston: Addison-Wesley, 2004. Hofer, Stefan and Henning Schwentner. Domain Storytelling: a Collaborative, Visual, and Agile Way to Develop Domain- Driven Software. Boston: Addison-Wesley, 2022.

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Henning Schwentner ⌂ https://hschwentner.io @hschwentner [email protected] Kolleg:in gesucht (Deutschlandweit)