Multi-stream: left join keeps unmatched rows with Optional

A left join must not silently drop rows whose key is missing on the right. Optional.ofNullable around the lookup turns a null into a visible default instead of a NullPointerException three lines later.

Code

record Product(String sku, String name) {}
record Line(String sku, int qty) {}

var products = List.of(new Product("S1", "Pen"), new Product("S2", "Book"));
var lines = List.of(new Line("S1", 2), new Line("S9", 5), new Line("S2", 1));

Map<String, Product> bySku = products.stream()
        .collect(Collectors.toMap(Product::sku, p -> p));

lines.stream()
     .map(l -> l.sku() + " x" + l.qty() + " -> "
             + Optional.ofNullable(bySku.get(l.sku()))
                       .map(Product::name)
                       .orElse("<unknown sku>"))
     .forEach(System.out::println);

Output

S1 x2 -> Pen
S9 x5 -> <unknown sku>
S2 x1 -> Book