Using Java Streams to process a list with multiple fields that impact the result

Question

Java 8 here. I have the following POJO:

@Getter
@Setter
public class Orderline {
    private Integer qty;
    private BigDecimal price;
    private String productName; 
}

I'm looking for a Stream-y way of iterating through a List<Orderlines> and coming up with a subtotal based on their individual quantities and prices. The "old" (pre-Stream) way of doing this would look like:

List<Orderline> orderlines = order.getOrderlines();
double sub = 0.0;
for (Orderline ol : orderlines) {
    sub += ol.getQty() * ol.getPrice();
}

BigDecimal subtotal = BigDecimal.valueOf(sub);

My best attempt at using Streams to accomplish this is:

BigDecimal subtotal = order.getOrderlines().stream()
    .map(Orderline::getPrice)
    .reduce(BigDecimal.ZERO, BigDecimal::add);

However this doesn't take their quantities into consideration. Any ideas how I can accomplish this?

Can you add an amount() method to the POJO which calculates this value and then use .reduce on that? — Nicko
– Nicko, Commented Sep 11, 2019 at 9:02

g_bor · Accepted Answer · 2019-09-10 20:01:15Z

2

You could use

map(x->x.getPrice().multiply(BigDecimal.valueOf(x.getQty())))

in your second line.

edited Sep 10, 2019 at 20:01

answered Sep 10, 2019 at 19:22

g_bor

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8 Comments

hotmeatballsoup Over a year ago

Thanks @g_bor (+1) but this adds a compiler error: "Operator '*' cannot be applied to 'java.lang.Integer', 'java.math.BigDecimal"

MC Emperor Over a year ago

@hotmeatballsoup Then simply replace x.getPrice() * x.getQty() with x.getQty.multiply(BigInteger.valueOf(x.getPrice())).

g_bor Over a year ago

This is an independent error. It says you can't simply * an Integer and a Bigdecimal. This should cause an error in the old approach also.

MC Emperor Over a year ago

@g_bor You could, however, edit your post so this problem is fixed.

g_bor Over a year ago

Thanks for taking care. I believe it is correct now.

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