Sum of int from list of object [duplicate]

Question

I have a class as following

class Account{
    String Name;
    int amount;
}

If I have a list as List<Account> then how can I get the total amount from this list without using any loop like for or foreach?

Answer 1

You can create a Stream of your accounts using stream(), map that stream to the amount of each account using mapToInt and sum the resulting IntStream using IntStream.sum().

List<Account> accounts = new ArrayList<>();
int totalAmount = accounts.stream().mapToInt(Account::getAmount).sum();

This code assumes that there is a getter getAmount for amount.

Answer 2

If you don't mind using a third party library, there is sumOfInt available in Eclipse Collections. It returns a long instead of an int, so the possibility of an overflow is much less likely.

You can either use a static utility method on the Iterate class which works with any java.lang.Iterable type as follows:

List<Account> accounts = new ArrayList<>();
long sum = Iterate.sumOfInt(accounts, Account::getAmount);

Or you if your list is a MutableList, you can use the method directly available on the list.

MutableList<Account> accounts = Lists.mutable.empty();
long sum = accounts.sumOfInt(Account::getAmount);

Note: I am a committer for Eclipse Collections