filter
- Java course
- Intermediate
- Lesson 107/116
- 5 min
Learn to filter elements with filter.
Filtering with filter
filter(Predicate) is an intermediate operation that keeps elements matching a condition in a new stream. Predicate<T> is a functional interface with a single test(T) method that returns boolean.
Example: names.stream().filter(s -> s.length() > 3) keeps names longer than three. filter is lazy, so it does nothing on its own; a terminal operation must follow. You can also chain multiple filter calls.
What is a Predicate?
Predicate<T> is a condition function; its test(T) method returns boolean. s -> s.isEmpty() is a Predicate lambda. filter calls the Predicate for each element; if it returns true the element is kept.
You can combine multiple conditions with &&: s -> s.length() > 3 && s.startsWith("A"). Or chain two filter calls; the result is the same. For composite conditions you can also use combiners like Predicate.and.
Practice Count long names
Declare List<String> names = new ArrayList<>();; add "Ayşe", "Alexander", "Alex". Count names longer than five with long count = names.stream().filter(s -> s.length() > 5).count();. Print it with System.out.println(count);.
Starter code
import java.util.List;
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
// Declare List<String>, add 3 names, count names longer than 5, print
}
}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("Alexander");
names.add("Alex");
long count = names.stream().filter(s -> s.length() > 5).count();
System.out.println(count);
}
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the filter call
names.stream().____(s -> s.length() > 3)names.stream().filter(s -> s.length() > 3)Explanation: The filter method expects a Predicate.
2 Which functional interface does filter expect?
- Predicate Correct answer
- Consumer
- Supplier
- Runnable
Explanation: filter expects a Predicate<T>; the test(T) method returns boolean.
3 Match the filter concepts
| Term | Meaning |
|---|---|
filter(...) |
Keeps matches |
Predicate<T> |
Condition function |
test(T) |
Returns boolean |
s -> s.length() > 3 |
Lambda Predicate |