JAVA List GroupBy multiple fields and Sum

Question

I have list having fields Name, Currency and Amount. I want to group it by Name, Currency and Sum of Amount.

List<myPojo> rows= ................

public class myPojo
{

    private String name;
    private String currency;
    private BigDecimal amount;

......................
}

I want result/output in list

To me it seems to be just sorted, first by Name, then by whatever. I don't see any grouping in the output. Anyway, what have you tried so far? — Amongalen, Jul 21 '20 at 11:16
@ShivprsadSammbhare the question stands: what have you tried and where did you get stuck? — Federico klez Culloca, Jul 21 '20 at 11:18
Possibly related: [Group by multiple field names in java 8](https://stackoverflow.com/q/28342814), [Group by and sum objects like in SQL with Java lambdas?](https://stackoverflow.com/q/26340688) — Pshemo, Jul 21 '20 at 11:18

Eklavya · Accepted Answer · 2020-07-21T12:17:09.050

You can use Stream API. Use Collectors.toMap and use AbstractMap.SimpleEntry as key of map. Then define merge function for multiple values of the same key.

List<myPojo> res = new ArrayList<>(rows.stream()
                .collect(Collectors.toMap(
                        e -> new AbstractMap.SimpleEntry<>(e.getName(), e.getCurrency()),
                        Function.identity(),
                        (a, b) -> new myPojo(a.getName(), a.getCurrency(), a.getAmount().add(b.getAmount()))))
                .values());

Demo:

List<myPojo> list = new ArrayList<>();
list.add(new myPojo("A", "USD", new BigDecimal(1.0)));
list.add(new myPojo("A", "USD", new BigDecimal(2.0)));
list.add(new myPojo("A", "USD", new BigDecimal(3.0)));
list.add(new myPojo("B", "USD", new BigDecimal(1.0)));
list.add(new myPojo("B", "USD", new BigDecimal(2.0)));
list.add(new myPojo("B", "USD", new BigDecimal(3.0)));
list.add(new myPojo("A", "US", new BigDecimal(1.0)));
list.add(new myPojo("A", "US", new BigDecimal(2.0)));
list.add(new myPojo("A", "US", new BigDecimal(3.0)));
List<myPojo> res = new ArrayList<>(list.stream()
            .collect(Collectors.toMap(
                    e -> new AbstractMap.SimpleEntry<>(e.getName(), e.getCurrency()),
                    Function.identity(),
                    (a, b) -> new myPojo(a.getName(), a.getCurrency(), a.getAmount().add(b.getAmount()))))
            .values());
System.out.println(res.toString());

Output:

[myPojo [name=B, currency=USD, amount=6], 
 myPojo [name=A, currency=USD, amount=6], 
 myPojo [name=A, currency=US, amount=6]]

Note: Try to capitalize the name of the class like MyPojo for better convention

What if myPojo don't have constructor. Then how to get these values — Shivprsad Sammbhare, Jul 21 '20 at 12:29
Create a constructor then, without constructor how you set values ? Without constructor, you need to set each value using setter. like `(a,b) -> {myPojo p = new myPojo(); p.setName(a.getName());..... return p;` — Eklavya, Jul 21 '20 at 12:33
Another way is `(a, b) -> { a.setAmount(a.getAmount().add(b.getAmount())); return a;}))` but it will mulate your current list — Eklavya, Jul 21 '20 at 12:43
@ShivprsadSammbhare Is your problem solved? feel free to ask for help — Eklavya, Jul 21 '20 at 13:58

score 1 · Answer 2 · edited Oct 30 '20 at 21:10

You could use groupingBy to generate a Map<String,Map<String,BigDecimal>> corresponding to Map<Name,Map<Currency,sum of Amount>>

List<myPojo> rows = List.of(new myPojo("ABCD", "USD", new BigDecimal(20)),
                            new myPojo("XYZ",  "GBP", new BigDecimal(60)),
                            new myPojo("XYZ",  "THB", new BigDecimal(35)),
                            new myPojo("ABCD", "INR", new BigDecimal(90)),
                            new myPojo("ABCD", "USD", new BigDecimal(80)),
                            new myPojo("XYZ",  "THB", new BigDecimal(45)));
    
Map<String,Map<String,BigDecimal>> map = 
            rows.stream()
            .collect(Collectors.groupingBy(myPojo::getName,
                    Collectors.groupingBy(myPojo::getCurrency,
                            Collectors.reducing(BigDecimal.ZERO, myPojo::getAmount, BigDecimal::add))));
    System.out.println(map);

 // output: {XYZ={GBP=60, THB=80}, ABCD={USD=100, INR=90}}

JAVA List GroupBy multiple fields and Sum

2 Answers2