How to filter duplicates by unique key with HashMap in Scala?

Question

I've implemented a simple filtering method in Java by creating a composite String key (SaleType + SaleDate). Then I just put each sale by its unique key to a HashMap. As a result I have a HashMap with sales filtered by the SaleType + SaleDate key.

I've recently started learning Scala and want to try the same filtration logic with it. How may this be implemented using Scala opportunities? I suppose a filter method may be used. But how can I construct a String key from the Sale object and then put it to the uniqueSalesMap?

private static List<Sale> getUniqueSales(List<Sale> sales) {
    Map<String, Sale> uniqueSalesMap = Maps.newHashMap();
    for (Sale sale : sales) {
        String saleKey = sale.getSaleType() + sale.getSaleDate();
        uniqueSalesMap.put(saleKey, sale);
    }
    return new ArrayList<Sale>(uniqueSalesMap.values());
}

Possible duplicate of Scala: Remove duplicates in list of objects — Alvaro Carrasco
– Alvaro Carrasco, Commented Jul 4, 2018 at 14:08

Arnon Rotem-Gal-Oz · Accepted Answer · 2018-07-04 18:43:32Z

2

One way would using groupBy

sales.groupBy(s=>s.salesType +s.saleDate).values.map(_.head)

answered Jul 4, 2018 at 18:43

Arnon Rotem-Gal-Oz

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Comments

Andrey Tyukin · Accepted Answer · 2018-07-04 14:21:36Z

1

Here is a relatively concise way that avoids constructing any intermediate Maps:

import collection.breakOut

val m: Map[String, Sale] = 
  (for (s <- sales) yield (s.getSaleType + s.getSaleDate, s))(breakOut)

m.values.toList

but to be honest, I don't see much of an advantage when compared to the original Java code. You could have written

val m = collection.mutable.HashMap.empty[String, Sale]
for (s <- sales) {
  m(s.getSaleType + s.getSaleDate) = s
}
m.values.toList

It doesn't seem any less clear to me.

edited Jul 4, 2018 at 14:21

answered Jul 4, 2018 at 14:16

Andrey Tyukin

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Comments

Tim · Accepted Answer · 2018-07-04 16:23:48Z

1

This does what you want, but not in the way that you wanted to do it, so I'm not sure if it counts as an answer or not.

sales.map(s => s.saleType + s.saleDate).distinct

answered Jul 4, 2018 at 16:23

Tim

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