Use Cases
This example is intended to demonstrate the extensibility differences between data structures and objects by extending them with
- an Area() function (extending behavior)
- a rectangle type (extending data type)
Data Structure Style
interface GeometricObject {}
class Square implements GeometricObject {
length float
}
class Circle implements GeometricObject {
radius float
}
function Circumference(g GeometricObject) float {
switch type(g):
case Square:
return 4 * g.length
case Circle:
return 2 * PI * g.radius
}
class Square implements GeometricObject {
length float
}
class Circle implements GeometricObject {
radius float
}
function Circumference(g GeometricObject) float {
switch type(g):
case Square:
return 4 * g.length
case Circle:
return 2 * PI * g.radius
}
- Adding an Area() function with a very similar anatomy to Circumference() is easy because it only requires adding new code.
- Adding a new datatype, Rectangle, is harder because it requires touching existing code, namely any functions like Circumference() or Area() that need to be enabled handle this datatype.
Object-Oriented Style
interface GeometricObject {
Circumference() float
}
class Square implements GeometricObject {
length float
constructor(length float) {
this.length = length
}
func Circumference() float {
return 4 * this.length
}
}
class Circle implements GeometricObject {
radius float
constructor(radius float) {
this.radius = radius
}
func Circumference() float {
return 2 * PI * this.radius
}
}
Circumference() float
}
class Square implements GeometricObject {
length float
constructor(length float) {
this.length = length
}
func Circumference() float {
return 4 * this.length
}
}
class Circle implements GeometricObject {
radius float
constructor(radius float) {
this.radius = radius
}
func Circumference() float {
return 2 * PI * this.radius
}
}
- Adding a function Area() to the GeometricObject interface is hard because it requires touching existing code, namely extending all Square and Circle classes with the new function.
- Adding a new datatype, Rectangle, is easy because only new code is added, namely a Rectangle class.
Conclusion
- The lesson learned here is that data structures are easy to extend with functions, and objects are easy to extend with data types.
- Although the difference between the two styles may seem insignificant in this simplified example, it has serious implications in complex production codebases with multiple data types and behaviors.