Functional Interfaces
Learn about functional interfaces like Runnable, Supplier, Consumer, and custom ones.
Functional Interfaces is a free Java Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Intro
Functional interfaces are the foundation of lambdas. They define exactly one abstract method.
Examples
Examples: Runnable, Callable, Supplier, Consumer, Function.
Code: Runnable
Runnable has one abstract method: run(). We can assign a lambda to it.
public class Main {
public static void main(String[] args) {
Runnable r = () -> System.out.println("Hello again");
r.run();
}
}
Custom interface
You can create your own functional interface with one abstract method.
Code: custom interface
This defines a custom functional interface and uses a lambda to implement it.
public class Main {
@FunctionalInterface
interface Greeter {
void greet(String name);
}
public static void main(String[] args) {
Greeter g = (n) -> System.out.println("Hi " + n);
g.greet("Ada");
}
}
Built-in
Supplier<T> returns a value, Consumer<T> takes a value, Function<T,R> transforms a value.
Functional interface check
Quick check: What is a functional interface?
Recap
Recap: Functional interfaces enable lambdas. Use built-in ones or define your own with @FunctionalInterface.
Frequently asked questions
Is the “Functional Interfaces” lesson free?
Yes — the full text of “Functional Interfaces” is free to read here on the web, and the Java Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO.
What will I learn in “Functional Interfaces”?
Learn about functional interfaces like Runnable, Supplier, Consumer, and custom ones. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Java Academy?
No prior experience is required. Java Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Functional Interfaces” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Java Academy lesson?
Yes. Every Java Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Lambda Syntax & Scope
- Functional Interfaces
- Method References & Optional