Module 13: Functional Programming and Streams

Functional Interfaces You Will Actually Use

Use core functional interfaces that appear often in streams and callbacks.

Author

Java Learner Editorial Team

Reviewer

Technical review by Java Learner

Last reviewed

2026-04-17

Java version

Java 25 LTS

How this lesson was prepared: AI-assisted draft, manually edited for clarity, and checked against current Java documentation and runnable examples.

Learning goals

Identify the most common functional interfaces
Match each interface to its role
Use `@FunctionalInterface` correctly

A functional interface has one abstract method: That single method is what the lambda implements.

Common built-ins are worth learning early: Predicate<T> tests, Function<T,R> transforms, Consumer<T> uses a value, and Supplier<T> creates or provides one.

These names describe data flow: Does the code test, transform, consume, or produce?

@FunctionalInterface is a helpful signal: It tells readers the interface is meant for lambda use and gives compiler protection.

Runnable examples

Predicate checks a condition

import java.util.function.Predicate;

Predicate<Integer> isEven = number -> number % 2 == 0;
System.out.println(isEven.test(8));

Expected output

true

Mini exercise

Create a `Function<String, Integer>` that returns the length of a string.

Summary

Functional interfaces are lambda targets.
Built-in interfaces cover the most common behavior shapes.
Their names usually tell you exactly what role they play.

Next step

Next, start building stream pipelines from collections and arrays.

Sources used

Functional Interfaces You Will Actually Use

Predicate checks a condition

Which built-in interface is best for yes/no tests?