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.

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.

Value objects should be immutable: 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.

Value objects work better if they have native support for copy-by-value semantics, i.e. the expression

valueObject1 = valueObject2

assigns the value of the valueObject1 by creating a copy of the valueObject2, instead of assigning a reference to the second object, as happens in most object oriented languages for assignments among objects.