Stream layer
- Java course
- Intermediate
- Lesson 114/116
- 6 min
Set up data processing with Stream and lambda.
Find large orders with a Stream
We'll add a largeOrders method to OrderBook. This method gathers orders with amount 100 or more into a new list using the chain orders.stream().filter(o -> o.amount >= 100).collect(Collectors.toList()).
filter expects a Predicate; the lambda o -> o.amount >= 100 tests each order. The terminal collect(Collectors.toList()) returns the matching orders as a new List<Order>. The original list is unchanged.
collect(Collectors.toList())
The collect(Collectors.toList()) terminal op gathers matching stream elements into a new List. The result is a mutable list; the stream source is unaffected. Since Java 16 the .toList() shortcut also exists.
The largeOrders method returns a List<Order>; you can run further operations on this list. For example book.largeOrders().size() gives the count of large orders. Each call produces a new list.
Practice Add the largeOrders method
Add a largeOrders method to the OrderBook class: List<Order> largeOrders() { return orders.stream().filter(o -> o.amount >= 100).collect(Collectors.toList()); }. In main, create an OrderBook book = new OrderBook();, add three orders: "1", "Ayşe", 50, "2", "Alex", 150, "3", "Mehmet", 200. Print the count of large orders with System.out.println(book.largeOrders().size());.
Starter code
import java.util.List;
import java.util.ArrayList;
import java.util.stream.Collectors;
class Order {
String id;
String customer;
int amount;
Order(String id, String customer, int amount) {
this.id = id;
this.customer = customer;
this.amount = amount;
}
}
class OrderBook {
List<Order> orders = new ArrayList<>();
void add(Order o) { orders.add(o); }
void listAll() {
orders.forEach(o -> System.out.println(o.id + " " + o.customer + " " + o.amount));
}
}
// Add the largeOrders method to OrderBook
// main: create an OrderBook, add three orders, print largeOrders sizeSample solution
class Order {
String id;
String customer;
int amount;
Order(String id, String customer, int amount) {
this.id = id;
this.customer = customer;
this.amount = amount;
}
}
class OrderBook {
List<Order> orders = new ArrayList<>();
void add(Order o) { orders.add(o); }
void listAll() {
orders.forEach(o -> System.out.println(o.id + " " + o.customer + " " + o.amount));
}
List<Order> largeOrders() {
return orders.stream().filter(o -> o.amount >= 100).collect(Collectors.toList());
}
}
public class Main {
public static void main(String[] args) {
OrderBook book = new OrderBook();
book.add(new Order("1", "Ayşe", 50));
book.add(new Order("2", "Alex", 150));
book.add(new Order("3", "Mehmet", 200));
System.out.println(book.largeOrders().size());
}
}Expected output
2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does filter expect in the largeOrders method?
- Runnable
- Consumer
- Function
- Predicate Correct answer
Explanation: filter expects a Predicate<Order> that returns boolean.
2 Complete the filter lambda
orders.stream().filter(____ -> o.amount >= 100)orders.stream().filter(o -> o.amount >= 100)Explanation: A single filter lambda parameter can be one letter.
3 Match the Stream chain parts
| Term | Meaning |
|---|---|
stream() |
Starts the stream |
filter(...) |
Keeps matches |
collect(...) |
Gathers a List |
Collectors.toList() |
Produces a List |