First of all annotation @FunctionalInterface is used by Java's built-in functional interfaces Predicate,Function,Consumer, etc...
From the other hand you may want to create your custom one like the following:
@FunctionalInterface
public interface ThrowingConsumer<T> {
void accept(T t) throws CustomException;
}
Then you can use it as a method parameter:
public <T, R> void doSomething(T value, ThrowingConsumer<T, R> consumer) {
// ...
}
And then call it like this:
doSomething(someValue, this::customConsumerMethodThrowingAnException);
It is worth to mention that @FunctionalInterface is not required. The compiler will be perfectly fine with any interface meeting the requirements.
The compiler treats it in a way similar to dealing with @Override annotation. The code compiles even without it. But once added it makes the code clearer and safer for the ones who will maintain the code in the future.
GeeksforGeeks
geeksforgeeks.org › java › java-functional-interfaces
Java Functional Interfaces - GeeksforGeeks
September 1, 2026 - Java 8 introduced the ... references. Among these, four commonly used functional interfaces are Predicate, Function, Consumer, and Supplier....
Baeldung
baeldung.com › home › java › core java › functional interfaces in java
Functional Interfaces in Java | Baeldung
March 26, 2025 - Not all functional interfaces appeared in Java 8. Many interfaces from previous versions of Java conform to the constraints of a FunctionalInterface, and we can use them as lambdas. Prominent examples include the Runnable and Callable interfaces that are used in concurrency APIs.
Real world example of using a functional interface in Java - Stack Overflow
I know a functional interface means you can have exactly/only 1 abstract method with more than 1 default method(s) but I am wondering how to relate to it with a real-world example/situation of usin... More on stackoverflow.com
Struggling to understand the idea behind the Supplier<T> functional interface. Real world examples?
Having the Supplier as a middle-man means you can change the implementation dynamically. Of course it is an extra step that is not always needed. But it allows you to define higher-order functions that are highly customizable. I often treat Supplier as a factory interface that produces some object and I don't care how. For example, consider Optional. It has a method orElseThrows that accepts a Supplier that produces a Throwable. That way I can easily throw an exception of any kind whenever a value is missing, e.g. public Foo getFooByBar(Bar bar) { return fooRepository.findFooByBar(bar) .orElseThrow(() -> new FooNotFoundException(bar)); } More on reddit.com
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December 1, 2023What is the purpose of an Interface?
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February 26, 2023Handling Checked Exceptions in Java Functional Interfaces (Lambdas)
Usually I either have ugly inline try catch blocks, a private method doing the same, or I make sure the method I call only uses unchecked exceptions - but it’s not ideal and and unfortunately a posterbook example of Java's design. More on reddit.com
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September 23, 2024Functional Interface | Lambda Expression in Java
Functional Interface in Java 8 Explained | Lambda Expressions ...
03:34
#73 Functional Interface New in Java - YouTube
13:23
Functional Interface in Java (With Real-World Examples) | Java ...
01:00
What is Functional Interface in Java? #javaprogramming - YouTube
Medium
medium.com › @nagarjun_nagesh › functional-interfaces-in-java-fe5ebf5bafed
Functional Interfaces in Java. In the previous article, we introduced… | by Nagarjun (Arjun) Nagesh | Medium
February 23, 2024 - In this example, we use a static method reference `StringUtils::printUpperCase` to print each element of the `names` list in uppercase. Functional interfaces are a fundamental concept in Java that enables the use of lambda expressions and method references. They provide a way to represent anonymous functions and leverage the benefits of functional programming in Java.
Oracle
docs.oracle.com › javase › 8 › docs › api › java › lang › FunctionalInterface.html
FunctionalInterface (Java Platform SE 8 )
July 21, 2026 - If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface's abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere. Note that instances of functional interfaces can be created with lambda expressions, method references, or constructor references.
TheServerSide
theserverside.com › blog › Coffee-Talk-Java-News-Stories-and-Opinions › Get-the-most-from-Java-Function-interface-with-this-example
A simple Java Function interface example
The code works, but most modern Java developers would use a lambda expression for such a simple function. Because Function is a functional interface, it can be implemented with a lambda expression.
Published: December 5, 2025
Medium
medium.com › @mshubham2002 › functional-interfaces-in-java-with-real-world-examples-fbbd56042116
💡Functional Interfaces in Java (With Real-World Examples) | by shubham mishra | Medium
June 5, 2025 - @FunctionalInterface public interface Calculator { int operation(int a,int b); } public static void main(String[] args){ Calculator sum =(a,b)->a+b; Calculator multiply =(a,b)->a*b; System.out.println(sum.operation(2,3)); System.out.println...
CodeSignal
codesignal.com › learn › courses › mastering-functional-interfaces › lessons › introduction-to-functional-interfaces
Introduction to Functional Interfaces
We’ll explore these methods with examples to help you understand how they can be applied in your code. Let's start by defining a class that implements the Function interface using the apply method. import java.util.function.Function; public class SquareFunction implements Function<Integer, Integer> { @Override public Integer apply(Integer x) { return x * x; } } public static void main(String[] args) { Function<Integer, Integer> square = new SquareFunction(); System.out.println(square.apply(5)); // Outputs 25 }
Top answer 1 of 5
6
First of all annotation @FunctionalInterface is used by Java's built-in functional interfaces Predicate,Function,Consumer, etc...
From the other hand you may want to create your custom one like the following:
@FunctionalInterface
public interface ThrowingConsumer<T> {
void accept(T t) throws CustomException;
}
Then you can use it as a method parameter:
public <T, R> void doSomething(T value, ThrowingConsumer<T, R> consumer) {
// ...
}
And then call it like this:
doSomething(someValue, this::customConsumerMethodThrowingAnException);
It is worth to mention that @FunctionalInterface is not required. The compiler will be perfectly fine with any interface meeting the requirements.
The compiler treats it in a way similar to dealing with @Override annotation. The code compiles even without it. But once added it makes the code clearer and safer for the ones who will maintain the code in the future.
2 of 5
5
We've always had functional interfaces before JDK8 but no lambdas, method references etc.
As of JDK8, they provide a target type for lambda expressions, method references and in turn, have better readability and more compact code.
Example, prior to Java-8 if you wanted to provide some logic that will be executed each time a Button component is clicked you'd do:
btn.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("Hello World!");
}
});
This is bulky, hard to read and not compact enough. because EventHandler is by definition a functional interface i.e. it has a SAM as of jdk8 you can now do:
btn.setOnAction(event -> System.out.println("Hello World!"));
You only see the part of the code you care about i.e. the logic to be executed when the button is clicked.
Further, due to the fact that we can use functional interfaces as target types for lambda expressions & methods references, this would be useful when:
- passing a comparator to a sort method e.g.
List.sort,Stream.sorted,Collections.sortetc. - passing a block of code to run a task in a separate thread
etc...
while keeping the code readable, compact and concise.
Functional interfaces are used extensively in the Java-stream API.
There's no reason for you to create your own functional interface except there's not one that meets your requirements from java.util.function or the name of the functional interface is not as readable so thus you may create your own.
There's also a @FunctionalInterface annotation recommended to be used but not required whenever you're creating a functional interface (the standard library uses this a lot).
This enables the compiler to check that the annotated entity is an interface with a single abstract method otherwise gives an error.
This is also quite helpful in being able to catch errors when refactoring your code.
DigitalOcean
digitalocean.com › community › tutorials › java-8-functional-interfaces
Java 8 Functional Interfaces | DigitalOcean
Technical tutorials, Q&A, events — This is an inclusive place where developers can find or lend support and discover new ways to contribute to the community.
DZone
dzone.com › data engineering › data › functional interfaces in java
Functional Interfaces in Java - DZone
November 30, 2022 - To use the lambda expression in the functional interface, we will declare a reference to MyInterfaceDemo and then assign the lambda expression to the reference. Finally, we will print out the value of the Golden Ratio value using the reference interface. ... import java.lang.FunctionalInterface; // Creating and Marking Functional Interface @FunctionalInterface interface MyInterface { // This is the abstract method double getGoldenRatioValue(); } public class Main { public static void main( String[] args ) { //declaring reference to the functional interface MyInterface ref; //using Lambda Expression ref = () -> 1.61803; System.out.println("Value of Golden Ratio = " + ref.getGoldenRatioValue()); } }
Oracle
docs.oracle.com › javase › 8 › docs › api › java › util › function › package-summary.html
java.util.function (Java Platform SE 8 )
July 21, 2026 - Functional interfaces often represent abstract concepts like functions, actions, or predicates. In documenting functional interfaces, or referring to variables typed as functional interfaces, it is common to refer directly to those abstract concepts, for example using "this function" instead of "the function represented by this object".
Hero Vired
herovired.com › learning-hub › blogs › functional-interface-in-java
Functional Interface in Java: Uses, Types, Examples
Lambda expressions are a concise way to represent a single-method interface using a lightweight syntax. They facilitate the implementation of functional interfaces in java, enabling us to write more elegant and readable code. Here’s a simple example of a functional interface in java and a corresponding lambda expression:
Scaler
scaler.com › home › topics › functional interfaces in java
Functional Interfaces in Java - Scaler Topics
April 20, 2024 - The BookStore interface can extend Library as BookStore is a normal Interface. There is no compilation error. ... In the same example as before, let BookStore and Library both have the abstract method but with the same name. In case, BookStore has only one method same as Library issue_book or no method, then BookStore can extend Library. ... Scaler learners achieved 2.5x salary growth with average post-Scaler CTC reaching ₹23L. ... Java has pre-defined or built-in functional interfaces for commonly occurring cases.
GitHub
github.com › neiljbrown › java-functional-interface-examples
GitHub - neiljbrown/java-functional-interface-examples: A set of Java code examples illustrating the set of functional interfaces that were added to the JDK's java.util.function package in Java 8+. · GitHub
This project provides a set of Java code examples illustrating the set of functional interfaces that were added to the JDK’s java.util.function package in Java 8+, their general use cases and usage in the standard JDK library (Stream and Collection APIs).
Author: neiljbrown
JavaTechOnline
javatechonline.com › home › functional interface in java examples
Functional Interface In Java Examples - JavaTechOnline
March 1, 2024 - We use it to provide a type to its sub-classes for executing some logic from other classes. Additionally, Functional Interface is an interface which has only one abstract method. Further, it can have any number of static, default methods and even public methods of java.lang.Object class.
Jenkov
jenkov.com › tutorials › java-functional-programming › functional-interfaces.html
Java Functional Interfaces
March 8, 2021 - Here is an example of implementing the Predicate interface using a Java lambda expression: ... This lambda implementation of the Predicate interface effectively does the same as the implementation above that uses a class. The Java UnaryOperator interface is a functional interface that represents an operation which takes a single parameter and returns a parameter of the same type.
Javatpoint
javatpoint.com › java-8-functional-interfaces
Java 8 Functional Interfaces
Java provides improved version of type inference in Java 8. the following example explains, how we can use type... ... Java A lambda expression is a new and important feature of Java that was included in Java SE 8. It provides a clear and concise way to represent one method interface using an expression.
GeeksforGeeks
geeksforgeeks.org › function-interface-in-java-with-examples
Function Interface in Java with Examples | GeeksforGeeks
August 7, 2024 - Lambda expressions basically express instances of functional interfaces (An interface with a single abstract method is called functional interface. An example is java.lang.Runnable). lambda expressions implement the only abstract function and therefore implement functional interfaces.
Java Code Geeks
examples.javacodegeeks.com › home › functional interface
Java Functional Interface Example - Java Code Geeks
July 6, 2022 - Lambda expression is used for functional interface instance. A java functional interface has many default methods. Runnable, Callable, ActionListener, Comparable, and FileFilter are good examples of Java functional interfaces.
DZone
dzone.com › coding › java › functional interface explained in detail introduced from java 8
Functional Interface Explained in Detail Introduced From Java 8
March 29, 2022 - Functional interfaces are introduced as part of Java 8. It is implemented using the annotation called @FunctionalInterface. It ensures that the interface should have only one abstract method. The usage of the abstract keyword is optional as the method defined inside the interface is by default abstract.