State Pattern

The State Pattern lets an object change its behaviour when its internal state changes, instead of checking a status field in every method.

A subscription moves through trialing, active, past due, and cancelled. Every method starts the same way:

public void pay() {
    if (status == TRIALING)       { /* take first payment, become active */ }
    else if (status == ACTIVE)    { /* renew */ }
    else if (status == PAST_DUE)  { /* catch up, become active */ }
    else if (status == CANCELLED) { /* reject */ }
}

The same four branches appear again in `cancel()