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I have a list:

List<FaAccount> accounts = someMethod();

this list contains accountid, currency, balance. Now I want to aggregate balance by currency (sum of balance for each currency). I've created new class with currency and balance variables. This, only copies info from FaAccount into FaAccountBalance

List<FaAccountBalance> aggr = new ArrayList<>();
for(FaAccount account : accounts){
            FaAccountBalance ag = new FaAccountBalance();
            ag.setCurrency(account.getCurrency());
            ag.setBalance(account.getBalance());
            aggr.add(ag);
}

How can I put there (into aggr List) one more for loop or some other technique, so that to represent sum of balance for each currency?

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  • Where do you want to put the loop exactly and what do you want to sum up ? I dont get your question... Commented Aug 1, 2015 at 13:05
  • @Kami, I want to sum up balance, grouped by currency. For instance, in accounts there may be ['USD', 500], ['USD',200]. In aggr, I want ['USD',700]. The question is where and how I can use for loop for this purpose Commented Aug 1, 2015 at 13:07

3 Answers 3

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Instead of using a different for loop, you can do it in the same for loop using a HashMap

List<FaAccountBalance> aggr = new ArrayList<>();
HashMap<String,Integer> map = new HashMap<>();
for(FaAccount account : accounts){
        String currency = account.getCurrency();
        int balance =account.getBalance();
        FaAccountBalance ag = new FaAccountBalance();
        ag.setCurrency(currency);
        ag.setBalance(balance);
        aggr.add(ag);        
        map.put(currency,map.getOrDefault(currency,0) + balance);
}

After this for loop, you will have aggregated balance for each currency, just print it like:

for(String currency: map.keySet()){
    System.out.println(currency + " : " + map.get(currency));
}

UPDATE If you want a BigDecimal, you can't just add it with +. You have to use add

map.put(currency,map.getOrDefault(currency, BigDecimal.ZERO).add(new BigDecimal(balance)));

If you want to Store it in List, instead of printing the hashmap contents, add them to the list.

List< FaAcountBalance> result  = new LinkedList<>();
for(String currency: map.keySet()){
    result.add(new FaAcountBalance(currency,map.get(currency)));
}
return result;
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9 Comments

Thanks. what about map.put(currency,map.getOrDefault(currency,BigDecimal.ZERO) + balance); what is wrong? i've changed HashMap to BigDecimal, and is it going to return me desired list ?
There is nothing wrong, it just depends on what type you want to use for aggregated balance.
as BigDecimal, it says getOrDefault(Object,BigDecimal) in Map cannot be applied to BifDecimal
@Sher oh you are talking about the syntax error, I updated my answer. You can't use + directly.
Almost done. One last question, how to put what is in Map into List?
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An other option would consist of using Collectors.groupingBy then simply map each Map.Entry into a FaAccountBalance and collect them as list :

final List<FaAccountBalance> accountBalances = 
    accounts
        .stream()
        .collect(Collectors.groupingBy(FaAccount::getCurrency, Collectors.summingInt(FaAccount::getBalance)))
        .entrySet()
        .stream()
        .map(entry -> {
            final FaAccountBalance f = new FaAccountBalance();
            f.setCurrency(entry.getKey());
            f.setBalance(entry.getValue());
            return f;
        }).collect(Collectors.toList());
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3 Comments

Java 8 streams are so unreadable and uneasy to use... One reason is Java compiler's terrible inference type, and the other reason is that the API is just not easily discoverable. I did not know about summingInt, which makes your answer helpful. However, I now consider Java streams as a pain unless you just want to perform simple operations. Scala has a much better API.
@Dici Seems like we have divergent opinions here, I've always found the stream API very intuitive and don't find it hard to read at all. In fact, also doing scala, I find the Java API more readable. However, I must say that I find the Scala one more complete.
One thing I don't like is the collect approach. It leads you to have deeply nested expressions if you want to do complex stuff with multiple collectingAndThen. I prefer list.groupBy(...).mapValues(...) over list.collect(groupingBy(...), summingInt(...)). Also, stream() clutters the code, and it's even worse for a Map on which you must first call either entrySet, values, keySet. You have 3 lines of these in your code, which add nothing to the logic. I'm not criticizing your answer, which is ok, but the Java API.
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First, your design is not ideal, you have some redundancy. Indeed, you are explicitly copying information, this suggests it is not clear for you who should own this data. I would suggest this better approach :

public class FaAccount {
    private final FaAccountBalance balance;
    private final int accountId;
}

public class FaAccountBalance {
    private float amount;
    private Currency currency;
}

The aggregation is then pretty easy : 

Map<Currency, Double> aggregation = new HashMap<>();
for (FaAccount account : accounts) {
   FaAccountBalance balance = account.getBalance();
   aggregation.put(balance.getCurrency(), aggregation.getOrDefault(0) + balance.getAmount());
}

Finally, 

List<FaAccountBalance> aggregatedBalances = 
    aggregation.entrySet()
               .map(entry -> new FaAccountBalance(entry.getKey(), entry.getValue()))
               .collect(Collectors.toList());
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6 Comments

and how to put it into List?
To put what into which list
Why I can't put BigDecimal.ZERO into getOrDefaultBalance? I've changed Double to BigDecimal
You don't give enough details for me to answer. Did you update it everywhere you need ? Do you have an compile error message ?
karthik gave you the answer : you cannot sum BigDecimals with +. I would add that your question was not complete enough for us to give a complete answer. Think about it next time, you could have included a small snippet of each of your classes to show us the data types
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