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...