Code reuse is good. But subclassing generates a static coupling.
TL;DR: Favor composition over inheritance. Always. Period.
Problems
Coupling
Maintainability
Solutions
- Favor composition.
Exceptions
- If hierarchy follows the principle behaves like then it is safe.
Sample Code
Wrong
public class Rectangle {
int length;
int width;
public Rectangle(int length, int width){
length = length;
width = width;
}
public int area(){
return length * width;
}
}
public class Square extends Rectangle {
public Square(int size){
super(size, size);
}
public int area(){
return length * length;
}
}
public class Box extends Rectangle{
}
Right
abstract public class Shape{
abstract public int area();
}
public final class Rectangle extends Shape {
int length;
int width;
public Rectangle(int length, int width){
length = length;
width = width;
}
public int area(){
return length * width;
}
}
public final class Square extends Shape {
int size;
public Square(int size){
size = size;
}
public int area(){
return size * size;
}
}
public final class Box {
Square shape;
public Box(int size){
shape = new Square(size);
}
public int area(){
return shape.area();
}
}
Detection
- Overriding can issue warnings when subclassing concrete methods.
- Deep Hierarchies (more than 3 levels) are also a clue of bad subclassing.
Tags
- Composition
Conclusion
In legacy systems is very common to have Deep Hierarchies and method overriding, we need to refactor them and subclass by essential reasons and not implementative ones.
Relations
More info
Credits
Photo by Brandon Green on Unsplash
This article is part of the CodeSmell Series.
Last update: 2021/06/15
Top comments (1)
I don't know well Java but isn't there interfaces in this language? Because extending abstract classes is still inheritance IMO, even if you can use them as interfaces. In Ruby we only have abstract classes too, so I can understand ahah.
In all cases, your Box class is an anti composition pattern. It should encapsulate an instance of Square instead of creating it in its constructor. It makes the code less testable ;)