Java 8 stream - merge collections of objects sharing the same Id

jaudo :

I have a collection of invoices :

class Invoice {
  int month;
  BigDecimal amount
}

I'd like to merge these invoices, so I get one invoice per month, and the amount is the sum of the invoices amount for this month.

For example:

invoice 1 : {month:1,amount:1000}
invoice 2 : {month:1,amount:300}
invoice 3 : {month:2,amount:2000}

Output:

invoice 1 : {month:1,amount:1300}
invoice 2 : {month:2,amount:2000}

How can I do this with java 8 streams?

EDIT : as my Invoice class was mutable and it was not a problem to modify them, I choosed Eugene's solution

Collection<Invoice>  invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            })).values();

Eugene :

If you are OK returning a Collection it would look like this:

Collection<Invoice>  invoices = list.collect(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            })).values();

If you really need a List:

 list.stream().collect(Collectors.collectingAndThen(Collectors.toMap(Invoice::getMonth, Function.identity(), (left, right) -> {
                left.setAmount(left.getAmount().add(right.getAmount()));
                return left;
            }), m -> new ArrayList<>(m.values())));

Both obviously assume that Invoice is mutable...

Guess you like

Origin http://10.200.1.11:23101/article/api/json?id=434803&siteId=1

Java 8 stream - merge collections of objects sharing the same Id

java8 stream grouping by merge objects or aggregate

Java 8 - Stream with Objects and primitive wrappers

Java8: Stream map two properties in the same stream

Java collections --Stream

[js] Merge array objects with the same key name

Java streams: merge/map collections

Java 8 stream handles the same part (intersection) and deduplication of the List collection

Comparing elements at same position of two list using Java 8 stream

Java uses reducing in stream() to accumulate (merge) the order quantities of breakfast, lunch, and dinner (group calculation) with the same name

How to filter objects with two lists using java 8 stream?

And the stream of objects in Java Serialization

The same id may get different view objects

js delete objects with the same id in two arrays

[Java Basics] Java8 uses stream().filter() to filter List objects (find a collection of eligible objects)

Vue array object, multiple objects in the array will merge the same colorName value

Initialise Java 8 Collections

Java 8 Streams - how to merge elements from the list with the same fields to one element and sum up

Java Stream Reduce of array of objects

java stream groupingBy embedded objects

How to correctly copy collections with objects in Java

Use the stream to filter out the same data according to the conditions of the objects in the list

Check whether list of custom objects have same value for a property in Java 8

java 8 stream in another stream _java 8 stream – java stream

java 8 stream in another stream _java 8 stream – java stream

Java 8 Stream how to merge 2 non-equal-size lists with custom logic to pick duplicates

How to merge two list of POJOS using stream JAVA 8 modifying some values when necessary

js merges array objects (merge objects with the same attributes in the array together to form a new array)

Java 8 Stream API: how to use GroupingBy to have the same object in multiple groups?

Java - Merge objects of list given a condition