src/main/java/com/github/oxo42/stateless4j/StateMachine.java
public void onUnhandledTrigger(final Action2<S, T> unhandledTriggerAction) {
        if (unhandledTriggerAction == null) {
            throw new IllegalStateException("unhandledTriggerAction");
        }
        this.unhandledTriggerAction = new Action3<S, T, Object[]>() {
            @Override
            public void doIt(S state, T trigger, Object[] arg3) {
                unhandledTriggerAction.doIt(state, trigger);
            }
        };
    }
public void onUnhandledTrigger(final Action2<S, T> unhandledTriggerAction) {
        if (unhandledTriggerAction == null) {
            throw new IllegalStateException("unhandledTriggerAction");
        }
        this.unhandledTriggerAction = new Action3<S, T, Object[]>() {
            @Override
            public void doIt(S state, T trigger, Object[] arg3) {
                unhandledTriggerAction.doIt(state, trigger);
            }
        };
    }
Override the default behaviour of throwing an exception when an unhandled trigger is fired

@param unhandledTriggerAction An action to call when an unhandled trigger is fired