Value Objects in an OO setting

1. Value Objects
Dhaval Dalal
https://dhavaldalal.wordpress.com
@softwareartisan

2. Value Object
=
Value
+
Object
i.e., Object havingValue Semantics

3. Example ofValues
• 5
• INR5
• 5%
• 5MB

4. Properties of Values
• Values model abstractions (concepts) from
problem domain.
• Values have no life-cycle (they don’t exist in
time, they are eternal)
• Values can only be interpreted as they
remain unaltered.
• Values are referentially transparent.

5. Properties of Objects
• Objects are representations of phenomena from a
problem domain.
• Objects have a life cycle (i.e., they exist in time, can
be instantiated, changed, and deleted).
• Objects have identity that unambiguously denotes
them (thus, they can be referenced).
• Objects can be shared, which is a consequence of
that objects can be referenced.

6. Combining these properties
• We get Immutability on Objects.
• Why to cherish immutability?
• As there is no modification of internal state, they are
intrinsically thread-safe, and hence don’t require any
synchronisation. Thus, they are painless to work in a
concurrent environment.
• As they are immutable, instances can be shared and
reused across entire application (using Flyweight Design
Pattern).
• Easy to verify correctness of the operations as no internal
state is altered (they behave like pure functions)

7. A word of warning!
• In OO languages like Java, C#, Groovy, Scala etc…
• Resist all temptation to mutate object’s internal state!
• For example, removing ‘final’ or ‘sealed’ attributes on fields. In Scala avoid changing
a ‘val’ to a ‘var’. In Javascript, use ‘const’ instead of ‘var’.
• Any operation on an Immutable object must produce a new
Immutable object with the new state. Don’t make it a mutable object
any cost!
• In purely FP languages, you don’t have that scope, e.g in
Haskell or Erlang, value once assigned can never change.
• However, in languages that are embracing the FP route or
embrace both paradigms (OO and FP), mutability and
immutability co-exist.
• In Scala, implementValue Object using ‘case’ classes.

8. References
• Values in Object Systems
• Dirk Baumer, Dirk Riehele, et al.
• The CHECKS pattern language of Information
Integrity.
• Ward Cunningham, James O. Coplien and Doug Schmidt.
• Programming Concurrency on the JVM
• Venkat Subramaniam