Coupling & Design

A marker interface has zero methods. It marks a capability. The JVM checks instanceof at runtime. Serializable allows byte stream conversion. Cloneable allows clone(). Remote allows RMI. Modern designs prefer annotations.

Tight coupling means depending on a concrete class (new X() inside). Loose coupling means depending on an interface plus injection (DI). Loose enables swapping, testing with mocks, and changing without breaking. OOP plus DI always strives for loose coupling.