alt Hacker News> We now know that we prefer composition over inheritance
When people say "composition over inheritance" in Java discussions, they usually mean the trivial modeling rule: prefer has-a over is-a.
But that’s not what composition is really about.
The deeper idea is interface composition -- building types by composing multiple behavioral contracts behind a single cohesive surface.
Java provides inheritance and interfaces, but it doesn’t provide first-class delegation or traits. So most developers never really practice interface composition. They either subclass, or they wire objects together and expose the wiring.
The slogan survived. The concept mostly didn’t.
The manifold project, for example, experiments with language-level delegation to make interface composition practical with Java.
https://github.com/manifold-systems/manifold/blob/master/man...
Replies
Well, what mainstream language has better tools for composition?
> Java provides inheritance and interfaces, but it doesn’t provide first-class delegation or traits.
I'm not sure I am missing first class delegation much (not a lot of UI projects in Java these days).
But interfaces with default (and static) method implementations are actually quite usable as traits / mixins. Since Java 8 IIRC.
You can also pass around functions / lambdas (coincidentally also since Java 8) to compose functionality together. A bit harder to follow and/or understand, but another perfectly legitimate and very powerful tool nevertheless.