Java : @FunctionalInterface (annotation) with Examples
August 26, 2025
September 29, 2023
FunctionalInterface (Java SE 24 & JDK 24) in Java with Examples.
You will find code samples for most of the FunctionalInterface methods.
Summary
An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface as defined by the Java Language Specification. Conceptually, a functional interface has exactly one abstract method.
Instances of functional interfaces can be created with lambda expressions.
@FunctionalInterface
public interface Foo {
void func();
}
public class Main {
public static void main(String[] args) {
// A lambda expression.
final Foo foo = () -> System.out.println("Foo!");
foo.func();
}
}
// --- PowerShell ---
//PS R:\java-work> ls -Name
//Foo.java
//Main.java
//
//PS R:\java-work> javac *.java
//
//PS R:\java-work> java Main
//Foo!
An example of a compile error :
@FunctionalInterface
public interface Foo {
void func1();
void func2();
}
// --- PowerShell ---
//PS R:\java-work> javac *.java
//Foo.java:2: error: Unexpected @FunctionalInterface annotation
//@FunctionalInterface
//^
// Foo is not a functional interface
// multiple non-overriding abstract methods found in interface Foo
//1 error
Please see also Java Language Specification :
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