Value object

In computer science, a value object is a small object that represents a simple entity whose equality is not based on identity: i.e. two value objects are equal when they have the same value, not necessarily being the same object.[1][2]

Examples of value objects are objects representing an amount of money or a date range.

Being small, one can have multiple copies of the same value object that represent the same entity: it is often simpler to create a new object rather than rely on a single instance and use references to it.[2]

Value objects should be immutable:[3] this is required for the implicit contract that two value objects created equal, should remain equal. It is also useful for value objects to be immutable, as client code cannot put the value object in an invalid state or introduce buggy behaviour after instantiation.[4]

Value objects are among the building blocks of DDD.

Implementation

Due to the nuances of various object-oriented programming languages, each has their own methods and patterns for implementing and using value objects.

C#

In C# a class is a reference type while a struct (concept derived from the struct in C language) is a value type.[5] Hence an instance derived from a class definition is an object while an instance derived from a struct definition is said to be a value object (to be precise a struct can be made immutable to represent a value object declaring attributes as readonly[6]).

The following procedure can be carried out to add value object properties to a C# class:

1. Override the Object.Equals method to ensure the object is compared using business logic
2. Operator overload the default behavior of == and != to use the Equals method.
3. Override the Object.GetHashCode method and ensure that the hash is same for the objects who have same equality.
4. Make the class immutable[7] by removing any property setters and only passing member values through the constructors.[8]

C++

In C++ a value object can be built by overloading the assignment operator and using appropriate constness constraints on the fields (that will be evaluated once by the initializer list of the constructor) and on the methods of the class.

However, if the fields themselves are declared const (rather than use non-const fields while only exposing "getter" accessors), then it won't be possible to fully overwrite such a value object with another ( object1 = object2).

Java

Unlike C# and C++, Java has no support for custom value types at the language level. Every custom type is a reference type, and therefore has identity and reference semantics,[9] though extending support for custom value types is being considered.[10]

Java programmers therefore emulate value objects by creating immutable objects,[11] because if the state of an object does not change, passing references is semantically equivalent to copying value objects.

A class can be made immutable by declaring all attributes blank final,[12] and declaring all attributes to be of immutable type (such as String, Integer, or any other type declared in accordance with these rules), not of mutable type such an ArrayList or even a Date. They should also define equals and hashCode to compare values rather than references.

The term "VALJO" (VALue Java Object) has been coined to refer to the stricter set of rules necessary for a correctly defined immutable value object.[13]

Value type semantics will be introduced in Java by the Project Valhalla.[14][15]

Example

public class StreetAddress
{
    public StreetAddress(string street, string city)
    {
        Street = street;
        City = city;
    }

    public string Street { get; }
    public string City { get; }
}

See also

• Data transfer object
• Domain-driven design
• Value semantics

References

1. Fowler, Martin (2003). "Value Object". Catalog of Patterns of Enterprise Application Architecture. Martin Fowler (martinfowler.com). Retrieved 17 July 2011.
2. "Value Object". Portland Pattern Repository's Wiki. Cunningham & Cunningham, Inc. (c2.com). Retrieved 6 September 2012.
3. "Value Object Should be Immutable". Portland Pattern Repository's Wiki. Cunningham & Cunningham, Inc. (c2.com). Retrieved 6 September 2012.
4. Burns, Sam. "The Value of a Value Object". sam-burns.co.uk.
5. "Classes and Structs (C# Programming Guide)". Microsoft Developer Network (msdn.microsoft.com). 2012. Retrieved 5 September 2012.
6. "Creating an immutable value object in C# - Part III - Using a struct". Luca Bolognese's WebLog. 2012. Retrieved 7 September 2012.
7. Koirala, Shivprasad. "Immutable objects in C# - CodeProject". www.codeproject.com. Retrieved 2017-12-26.
8. koirala, Shivprasad. "Value Object Design Pattern in C#". www.codeproject.com. Retrieved 2017-12-26.
9. "Java Language Specification, chapter 4. Types, Values, and Variables". Retrieved 7 October 2015.
10. "JEP 169: Value Objects". Retrieved 7 October 2015.
11. "Immutable objects". Collected Java Practices. 2012. Retrieved 5 September 2012.
12. hence assignable only in the constructors
13. "VALJOs - Value Java Objects". Retrieved 19 October 2014.
14. "Project Valhalla". Retrieved 7 August 2018.
15. "Evolving the Java Language with @BrianGoetz". Retrieved 7 August 2018.