Java
collect groupingBy
Learn grouping with Collectors.groupingBy.
Grouping a stream
To terminate a stream you use the collect method. The Collectors class offers ready-made collectors for this. The most common one is groupingBy: it splits elements into groups by a classifier function.
Example: products.stream().collect(Collectors.groupingBy(Product::category)). Each product lands in a group based on its category; the result is a Map<String, List<Product>>. The keys are categories, the values are the lists of products in each category.
groupingBy output and ordered map
By default groupingBy returns a HashMap; the order of keys is undefined. To make the output deterministic you use the three-argument overload: classifier, map supplier and a downstream collector.
Example: Collectors.groupingBy(Product::category, TreeMap::new, Collectors.toList()). TreeMap::new keeps keys in natural order; the forEach output is then identical on every run. If you swap the downstream for Collectors.counting() you get a count per group.
Exercise Group products by category
Create a list of three products (record Product(String name, String category, double price)). Group the stream by category with Collectors.groupingBy; pass TreeMap::new and Collectors.toList() for ordered output. Then use forEach to print each category's count with System.out.println.
Show example solution
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;
record Product(String name, String category, double price) {}
public class GroupByCategory {
public static void main(String[] args) {
List<Product> products = List.of(
new Product("Pen", "Office", 12.5),
new Product("Notebook", "Office", 25.0),
new Product("Bread", "Grocery", 5.0)
);
Map<String, List<Product>> byCategory = products.stream()
.collect(Collectors.groupingBy(Product::category, TreeMap::new, Collectors.toList()));
byCategory.forEach((k, v) -> System.out.println(k + ": " + v.size()));
}
}
Test yourself
-
Which collector splits a stream into groups by a classifier function?
- Collectors.groupingBy(...)
- Collectors.toList(...)
- Stream.of(...)
- Optional.of(...)
Show answer
Collectors.groupingBy(...) — groupingBy groups elements by the classifier.
-
Complete the groupingBy call
Map<String, List<Product>> byCat = products.stream()
.collect(Collectors.groupingBy(____));
Show answer
Map<String, List<Product>> byCat = products.stream()
.collect(Collectors.groupingBy(Product::category));
-
Match the groupingBy concepts
- groupingBy
- Groups by a key
- classifier
- Classifier function
- collect
- Terminal method
- TreeMap::new
- Ordered map supplier
Next lesson: partitioningBy
