collect groupingBy
- Java course
- Advanced
- Lesson 1/82
- 5 min
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.
Practice 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.
Starter code
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) {}
// Group the products by category
// Use Collectors.groupingBy with a TreeMap supplier
// Print each category's count with forEachSample 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()));
}
}Expected output
Office: 2 Grocery: 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which collector splits a stream into groups by a classifier function?
- Collectors.groupingBy(...) Correct answer
- Collectors.toList(...)
- Stream.of(...)
- Optional.of(...)
Explanation: groupingBy groups elements by the classifier.
2 Complete the groupingBy call
Map<String, List<Product>> byCat = products.stream()
.collect(Collectors.groupingBy(____));Map<String, List<Product>> byCat = products.stream()
.collect(Collectors.groupingBy(Product::category));Explanation: Product::category is a method reference and works as classifier.
3 Match the groupingBy concepts
| Term | Meaning |
|---|---|
groupingBy |
Groups by a key |
classifier |
Classifier function |
collect |
Terminal method |
TreeMap::new |
Ordered map supplier |