CODEWITHSUNDEEP
javajava-stream
Denis Stephanov
Denis Stephanov

Reputation: 5231

Multiply items in stream

Is it possible with Java stream API to duplicate items a few times?

For instance let's say we have a list of orders where each order has a product code and quantity. I want to get a list of product codes which contains n copies of the given code where n is the quantity.

When I got 2 orders ("product1" : 3x, "product2": 2x) I want in result a list like this: ("product1", "product1", "product1", "product2", "product2")

Is there a pretty way to do that with streams without the old for cycle?

Code looks like this:

@Data
public class OrderRow {
   private String productCode;

   private int quantity;
}

Upvotes: 2

Views: 554

Answers (2)

Samuel Philipp
Samuel Philipp

Reputation: 11042

You need to use Stream.flatMap() and create a new stream with the items for each row:

List<String> result = orderRows.stream()
        .flatMap(row -> Stream.generate(row::getProductCode).limit(row.getQuantity()))
        .collect(Collectors.toList());

You also can use this:

List<String> result = orderRows.stream()
        .flatMap(row -> IntStream.range(0, row.getQuantity()).mapToObj(i -> row.getProductCode()))
        .collect(Collectors.toList());

Upvotes: 0

Naman
Naman

Reputation: 31878

You can use flatMap with Collections.nCopies as :

public static List<String> products(List<OrderRow> orderRows) {
    return orderRows.stream()
            .flatMap(o -> Collections.nCopies(o.quantity, o.productCode).stream())
            .collect(Collectors.toList());
}

Upvotes: 6

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