How to group objects in Java 8 - java-8

Question

WalletCreditNoteVO a1 = new WalletCreditNoteVO(1L, 1L, "A", WalletCreditNoteStatus.EXPIRED, null, null, CreditNoteType.CAMPAIGN_VOUCHER, BigDecimal.ONE, BigDecimal.ONE, "GBP");
WalletCreditNoteVO a2 = new WalletCreditNoteVO(1L, 1L, "A", WalletCreditNoteStatus.EXPIRED, null, null, CreditNoteType.CAMPAIGN_VOUCHER, BigDecimal.ONE, BigDecimal.TEN, "GBP");
WalletCreditNoteVO a3 = new WalletCreditNoteVO(2L, 1L, "A", WalletCreditNoteStatus.EXPIRED, null, null, CreditNoteType.CAMPAIGN_VOUCHER, BigDecimal.ONE, BigDecimal.ONE, "GBP");
WalletCreditNoteVO a4 = new WalletCreditNoteVO(2L, 1L, "A", WalletCreditNoteStatus.EXPIRED, null, null, CreditNoteType.CAMPAIGN_VOUCHER, BigDecimal.ONE, BigDecimal.TEN, "GBP");
final List<WalletCreditNoteVO> walletCreditNoteVOs = Lists.newArrayList(a1, a2, a3, a4);
Map<WalletCreditNoteVO, BigDecimal> collect2 = walletCreditNoteVOs.stream().collect(
groupingBy(wr -> new WalletCreditNoteVO(wr.getCreditNoteId(), wr.getWalletCustomerId(), wr.getCreditNoteTitle(),
wr.getWalletCreditNoteStatus(), wr.getCreditNoteStartDate(), wr.getCreditNoteExpiryDate(), wr.getCreditNoteType(), wr.getCreditNoteValue(), wr.getCurrency()),
mapping(WalletCreditNoteVO::getAvailableBalance,
reducing(BigDecimal.ZERO, (sum, elem) -> sum.add(elem)))));
I want to introduce condition for final reducing to be either sum (as written above) or last value in the list of BigDecimal based on the status of getWalletCreditNoteStatus
Can someone please help.
Thanks #xiumeteo . Below is improved solution
Function<WalletCreditNoteVO, WalletCreditNoteVO> function = wr -> new WalletCreditNoteVO(wr.getCreditNoteId(), wr.getWalletCustomerId(), wr.getCreditNoteTitle(),
wr.getWalletCreditNoteStatus(), wr.getCreditNoteStartDate(), wr.getCreditNoteExpiryDate(), wr.getCreditNoteType(), wr.getCreditNoteValue(), wr.getCurrency());
final Map<WalletCreditNoteVO, BigDecimal> collectMap =
walletCreditNoteVOs.stream()
.collect(groupingBy(function, LinkedHashMap::new, Collectors.collectingAndThen(
toList(),
(list) -> {
final List<BigDecimal> availableBalances = list.stream().map(WalletCreditNoteVO::getAvailableBalance).collect(toList());
if (list.stream().allMatch(WalletCreditNoteVO::isStatusExpired)) {
return availableBalances.stream().filter(o -> o != null).reduce((a, b) -> b).orElse(null).abs();
} else {
return availableBalances.stream().filter(o -> o != null).reduce(BigDecimal.ZERO, BigDecimal::add);
}
})));
List<WalletCreditNoteVO> walletCreditNoteVOGrouped = new ArrayList<>();
for(Map.Entry<WalletCreditNoteVO, BigDecimal> entry : collectMap.entrySet()){
WalletCreditNoteVO key = entry.getKey();
key.setAvailableBalance(entry.getValue());
walletCreditNoteVOGrouped.add(key);
}
I now want to remove 'for loop' and stream logic should just give me one list of WalletCreditNoteVO instead of Map of WalletCreditNoteVO as key and BigDecimal as value, with value set directly in the WalletCreditNoteVO
Thanks all again (I can't add code in my comments so adding it here).

Answer 1

So I did a test for your case, I created a dummy class that resembles yours:
public static class Something{
private String name;
private Integer sum;
private boolean checker;
public Something(String name, Integer sum, boolean checker) {
this.name = name;
this.sum = sum;
this.checker = checker;
}
public String getName() {
return name;
}
public boolean isChecker() {
return checker;
}
public Integer getSum() {
return sum;
}
#Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (o == null || getClass() != o.getClass()) {
return false;
}
Something something = (Something) o;
return new EqualsBuilder().append(getName(), something.getName()).append(getSum(), something.getSum()).isEquals();
}
#Override
public int hashCode() {
return new HashCodeBuilder(17, 37).append(getName()).append(getSum()).toHashCode();
}
}
And then I did this little test
List<Something> items = Arrays.asList(new Something("name", 10, false), new Something("name", 14, true), new Something("name", 11, false),
new Something("name", 11, false), new Something("noName", 12, false));
final Map<Something, Integer> somethingToSumOrLastElement =
items.stream()
.collect(Collectors.groupingBy(Function.identity(),
Collectors.collectingAndThen(
Collectors.toList(), // first we collect all your related items into a list
(list) -> { //this collector allow us to have a finisher, Function<List<Something>, Object>, let's define it
final List<Integer> integerStream = list.stream().map(Something::getSum).collect(Collectors.toList());
if (list.stream().allMatch(Something::isChecker)) { // we check for the method you want to check
//you have to change this depending on required logic
//for this case if that's true for every element in the list, we do the reduce by summing
return integerStream.stream().reduce(0, (sum, next) -> sum + next);
}
//if not, we just get the last element of that list
return integerStream.stream().reduce(0, (sum, next) -> next);
})));
I think this is ok, but maybe someone has a better idea on how to handle your issue.
Ping me if you need clarification :)