class Plant {
    enum LifeCycle { ANNUAL, PERENNIAL, BIENNIAL }
    final String name;
    final LifeCycle lifeCycle;
    Plant(String name, LifeCycle lifeCycle) {
        this.name = name;
        this.lifeCycle = lifeCycle;
    }
    @Override public String toString() {
        return name;
    }
}

// 잘못된 예시!!
Set<Plant>[] plantsByLifeCycle = (Set<Plant>[])new Set[Plant.LifeCycle.values().length];
for (int i = 0; i < plantsByLifeCycle.length; i++) {
    plantsByLifeCycle[i] = new HashSet<>();
}
for (Plant p : garden) {
    plantsByLifeCycle[p.lifeCycle.ordinal()].add(p);
}
// 결과 출력
for (int i = 0; i < plantsByLifeCycle.length; i++) {
    System.out.printf("%s: %s%n", Plant.LifeCycle.values()[i], plantsByLifeCycle[i]);
}

Map<Plant.LifeCycle, Set<Plant>> plantsByLifeCycle = new EnumMap<>(Plant.LifeCycle.class);
for (Plant.LifeCycle lc : Plant.LifeCycle.values()) {
    plantsByLifeCycle.put(lc, new HashSet<>());
}
for (Plant p : garden) {
    plantsByLifeCycle.get(p.lifeCycle).add(p);
}
System.out.println(plantsByLifeCycle);

//초안 
Arrays.stream(garden).collect(groupingBy(p -> p.lifeCycle));

Arrays.stream(garden).collect(groupingBy(p -> p.lifeCycle, () -> new EnumMap<>(LifeCycle.class), toSet()))