Java8 Stream Collectors - Splitting a list based on sum of values - java-8

Question

I am trying partition a list into multiple sublists based on a condition that sum of a particular field should be less than 'x'. Below is sameple code:
public class TestGrouping {
public static class Transaction{
String txnId;
String comment;
Amount amount;
public Transaction(String txnId, String comment, Amount amount) {
this.txnId = txnId;
this.comment = comment;
this.amount = amount;
}
}
public static class Amount{
String amountValue;
public Amount(String amountValue) {
this.amountValue = amountValue;
}
}
public static void main(String[] args) {
List<Transaction> transactionList = new ArrayList<>();
Transaction txn1 = new Transaction("T1","comment1",new Amount("81"));
Transaction txn2 = new Transaction("T2","comment2",new Amount("5"));
Transaction txn3 = new Transaction("T3","comment3",new Amount("12"));
Transaction txn4 = new Transaction("T4","comment4",new Amount("28"));
transactionList.add(txn1);
transactionList.add(txn2);
transactionList.add(txn3);
transactionList.add(txn4);
//below is what i thought might work
// transactionList.stream().collect(groupingBy (r->Collectors.summingInt(Integer.valueOf(r.amount.amountValue)),Collectors.mapping(t -> t, toList())));
}
The goal is to split the transactionList into 2 (or more) sublists - where the sum of 'amount' is less than 100. So i could have a sublist have only txn1 - having amount as 81; and the other sublist have txn2, txn3, txn4 (as sum of these is less 100). Other possibility is - have sublist1 having txn1, txn2, txn3; and another sublist with just txn4. Not trying to create the most 'optimal' lists basically, just that sum of amounts should be less than 100.
Any clues?

Answer 1

The Idea is to use a custom collector to generate a list of pair(amountSum, transactions), the list should initialy be sorted. The accumulator method (here Accumulator.accept) do the grouping logic, I didn't implement combine because there is no need for a combiner in non parallel stream.
Bellow the code snippet, hope it helps.
public class TestStream {
public class Transaction {
String txnId;
String comment;
Amount amount;
public Transaction(String txnId, String comment, Amount amount) {
this.txnId = txnId;
this.comment = comment;
this.amount = amount;
}
}
public class Amount {
String amountValue;
public Amount(String amountValue) {
this.amountValue = amountValue;
}
}
#Test
public void test() {
List<Transaction> transactionList = new ArrayList<>();
Transaction txn1 = new Transaction("T1", "comment1", new Amount("81"));
Transaction txn2 = new Transaction("T2", "comment2", new Amount("5"));
Transaction txn3 = new Transaction("T3", "comment3", new Amount("12"));
Transaction txn4 = new Transaction("T4", "comment4", new Amount("28"));
transactionList.add(txn1);
transactionList.add(txn2);
transactionList.add(txn3);
transactionList.add(txn4);
transactionList.stream()
.sorted(Comparator.comparing(tr -> Integer.valueOf(tr.amount.amountValue)))
.collect(ArrayList<Pair<Integer, List<Transaction>>>::new, Accumulator::accept, (x, y) -> {
})
.forEach(t -> {
System.out.println(t.left);
});
}
static class Accumulator {
public static void accept(List<Pair<Integer, List<Transaction>>> lPair, Transaction tr) {
Pair<Integer, List<Transaction>> lastPair = lPair.isEmpty() ? null : lPair.get(lPair.size() - 1);
Integer amount = Integer.valueOf(tr.amount.amountValue);
if (Objects.isNull(lastPair) || lastPair.left + amount > 100) {
lPair.add(
new TestStream().new Pair<Integer, List<Transaction>>(amount,
Arrays.asList(tr)));
} else {
List<Transaction> newList = new ArrayList<>();
newList.addAll(lastPair.getRight());
newList.add(tr);
lastPair.setLeft(lastPair.getLeft() + amount);
lastPair.setRight(newList);
}
}
}
class Pair<T, V> {
private T left;
private V right;
/**
*
*/
public Pair(T left, V right) {
this.left = left;
this.right = right;
}
public V getRight() {
return right;
}
public T getLeft() {
return left;
}
public void setLeft(T left) {
this.left = left;
}
public void setRight(V right) {
this.right = right;
}
}
}