java - 优化嵌套 if 语句中的嵌套 for 循环

Question

我正在尝试优化这段代码。我能想到的唯一优化是在第二个 if 条件内的 applyOfferChanges(...) 之后返回或break语句。 有什么想法吗？

void applyFavoriteChangesToMerchantStore(){

    List<Merchant> favoriteMerchantsList = FavoriteMerchantStore.getInstance().getFavoriteMerchantsList();
    if(favoriteMerchantsList != null && !favoriteMerchantsList.isEmpty()) {
        List<Merchant> storeMerchantList = MerchantStore.getInstance().getMerchantList();
        for (Merchant storeMerchant : storeMerchantList) {
            for (Merchant favoriteMerchant: favoriteMerchantsList){
                if(TextUtils.equals(storeMerchant.getId(), favoriteMerchant.getId())){
                    //merchant match found
                    //set merchant favorite status

                   storeMerchant.setFavoriteMerchant(favoriteMerchant.getFavoriteMerchant());
                    //set offer favorite status
                    applyOfferChanges(favoriteMerchant.getOffferList(),
                            storeMerchant.getOffferList());
                }
            }
        }
    }
}

Answer 1

这完全取决于您尚未向我们展示的内容。列表有多大，TextUtils.equals 到底有什么作用？假设它是 android.text.TextUtils，答案很明确:不要使用两个嵌套循环(复杂度 O(m*n))，而是使用 HashMap(复杂度 O(m+n))。

而不是

for (Merchant storeMerchant : storeMerchantList) {
    for (Merchant favoriteMerchant: favoriteMerchantsList) {
         if(TextUtils.equals(storeMerchant.getId(), favoriteMerchant.getId())) {
             ....
         }

做

Map<String, Merchant> favoriteMerchantMap = new HashMap<>();
for (Merchant favoriteMerchant : favoriteMerchantsList) {
    favoriteMerchantMap.put(favoriteMerchant.getId(), favoriteMerchant));
}
for (Merchant storeMerchant : storeMerchantList) {
    Merchant favoriteMerchant = favoriteMerchantMap.get(storeMerchant.getId());
    if (favoriteMerchant != null) {
             ....
    }