Java Streams for Beginners: filter, map and collect Explained

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Since Java 8, Streams let you describe what to do with a collection instead of writing loops. The code reads like a pipeline.

The data

record Sale(String region, String product, int qty, double price) {
    double amount() { return qty * price; }
}

List<Sale> sales = List.of(
    new Sale("North", "Keyboard", 12, 1299),
    new Sale("South", "Mouse", 30, 599),
    new Sale("North", "Monitor", 4, 11499),
    new Sale("East", "Webcam", 6, 2799));

record (Java 16+) is a compact way to define a data class.

filter + map + collect

List<String> northProducts = sales.stream()
    .filter(s -> s.region().equals("North"))   // keep North rows
    .map(Sale::product)                        // take the product name
    .toList();                                 // [Keyboard, Monitor]

Totals and sorting

double total = sales.stream().mapToDouble(Sale::amount).sum();

List<Sale> biggest = sales.stream()
    .sorted(Comparator.comparingDouble(Sale::amount).reversed())
    .limit(2)
    .toList();

Group by — Java’s pivot table

Map<String, Double> byRegion = sales.stream()
    .collect(Collectors.groupingBy(Sale::region,
             Collectors.summingDouble(Sale::amount)));
// {North=61584.0, South=17970.0, East=16794.0}

toList() needs Java 16+; on older versions use .collect(Collectors.toList()). Reading Excel files in Java? See Apache POI.