Concept · Core

Composition over Inheritance

Inheritance locks behaviour in at compile time and explodes combinatorially. Composition — “has-a” — keeps it swappable. Prefer it.

Asked atAmazonGoogleMetaAtlassian

step 1 / 10

1/2The inheritance trap

Robot

+ move()

WalkingRobot

+ move(): walk

FlyingRobot

+ move(): fly

WalkingFlyingRobot

+ move(): ??

The inheritance trap

 1class Robot          { void move(){} }
 2class WalkingRobot   extends Robot { void move(){ walk(); } }
 3class FlyingRobot    extends Robot { void move(){ fly();  } }
 4// need walk + fly?  extends BOTH → impossible

✦

Start with a Robot base class that declares move(). Reasonable so far.