forEach and count
- Java course
- Intermediate
- Lesson 109/116
- 5 min
Learn processing with forEach and counting with count.
The forEach terminal op
forEach(Consumer) is a terminal operation; it runs an action for each element and closes the stream. names.stream().forEach(s -> System.out.println(s)); prints each name.
forEach returns no value; it is used for side effects. On a sequential stream it walks elements in source order. Usually result collectors like Collectors.toList() are preferred over forEach; forEach is fine when a side effect is required.
count vs sum
count() returns the number of elements in the stream as long; sum() works only on numeric streams (IntStream etc.) and returns the total of values. Both are terminal.
For example names.stream().count() gives the name count; names.stream().mapToInt(String::length).sum() gives the total length. The reduce terminal can also produce a sum iteratively: .reduce(0, (a, b) -> a + b). All three suit different goals.
Practice Print and count with a stream
Declare List<String> names = new ArrayList<>();; add "Ayşe" and "Alex". First print each name with names.stream().forEach(s -> System.out.println(s));. Then print the count with System.out.println(names.stream().count());.
Starter code
import java.util.List;
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
// Declare List<String>, add two names, print with forEach, print count
}
}Sample solution
import java.util.List;
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
List<String> names = new ArrayList<>();
names.add("Ayşe");
names.add("Alex");
names.stream().forEach(s -> System.out.println(s));
System.out.println(names.stream().count());
}
}Expected output
Ayşe Alex 2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the count call
long n = names.stream().filter(s -> s.length() > 3).____();long n = names.stream().filter(s -> s.length() > 3).count();Explanation: count() returns the number of elements.
2 What does count() return on a Stream?
- int
- void
- long Correct answer
- Optional<Long>
Explanation: count() returns a long; the number of elements in the stream.
3 Match the terminal operations
| Term | Meaning |
|---|---|
forEach(...) |
Produces side effect |
count() |
Element count |
sum() |
Numeric total |
reduce(...) |
Iterative reduction |