optimize nested for loops within nested if statements

Question

I am trying to optimize this code. The only optimization I can think of is to ?¿? a return or break statement after applyOfferChanges(...) inside the second if condition which does not work either because there could be multiple matches. I cannot either use a map for favoriteMerchantsList because getFavoriteMerchantsList() return type is a list (of favorited merchants). Any ideas?

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());
 }
 }
 }
 }
}

• Null: Welcome to Code Review! Please provide additional code which forms a complete program so that reviewers can run and test your code.
• Roland Illig: Please edit the question title to say what your code achieves instead of what you want to be done to your code.
• Roland Illig: Please add some details about how many favorite and store merchants there are usually. And what ahould happen if there are multiple matches.
• RoToRa: Interesting information would be also: What is a "Merchant" and what is the relationship between a "store merchant" and a "favorite merchant"? Why do they use the same class, but need to matched like that?
• mtj: Furthermore, please define "optimize". In terms of runtime? (Is there a concrete problem?) In terms of readability? In terms of less lines of code?

Answer 1

You are implementing an inner join as a nested loop join. There are several other ways to implement joins, such as hash-based or sort-merge sort. Which one is better depends on the length of the lists.

I will sketch a hash join as that usually works most of the time.

void applyFavoriteChangesToMerchantStore(){
 List<Merchant> favoriteMerchantsList = FavoriteMerchantStore.getInstance().getFavoriteMerchantsList();
 if(favoriteMerchantsList != null && !favoriteMerchantsList.isEmpty()) {
 // build the hash table
 Map<String, Merchant> favoriteMerchantById = favoriteMerchantsList.stream().collect(Collectors.toMap(Merchant::getId, Function.identity()));
 List<Merchant> storeMerchantList = MerchantStore.getInstance().getMerchantList();
 for (Merchant storeMerchant : storeMerchantList) {
 // probe the hash table
 Merchant favoriteMerchant = favoriteMerchantById.get(storeMerchant.getId())
 if(favoriteMerchant != null){
 //merchant match found
 //set merchant favorite status
 storeMerchant.setFavoriteMerchant(favoriteMerchant.getFavoriteMerchant());
 //set offer favorite status
 applyOfferChanges(favoriteMerchant.getOffferList(),
 storeMerchant.getOffferList());
 }
 }
 }
}

I assumed that TextUtils.equals is equal to String.equals. If it's more lenient (e.g., case-insensitive), you need to normalize the lookup key (e.g., String.toLowerCase()).