Baeldung
baeldung.com › home › java › core java › functional interfaces in java
Functional Interfaces in Java | Baeldung
March 26, 2025 - As an example, there is no out-of-the-box functional interface for a function that takes a short and returns a byte, but nothing stops us from writing our own:
GeeksforGeeks
geeksforgeeks.org › java › java-functional-interfaces
Java Functional Interfaces - GeeksforGeeks
November 20, 2025 - A functional interface in Java is an interface that has only one abstract method, making it suitable for use with lambda expressions and method references (introduced in Java 8). Use @FunctionalInterface to ensure only one abstract method (annotation is optional). Enable clean, concise code using lambdas and method references. Example: Using a Functional Interface with a Lambda Expression
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
Guide to Functional Interfaces in Java
When I don't remember which to use I always refer to this list . I've always felt this functional interface business is just bad design when compared with C# simple Action/Function. Another victim of type erasure. More on reddit.com
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March 7, 2017(ELI5) What exactly is the use of "functional Interface"?
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October 23, 2024What is the purpose of an Interface?
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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: Learn Functional programming fast
If you want to master functional programming, the best place to start is with the Java Function interface. This example will show you four different ways to implement this functional interface in your code — starting with how to use an actual class, and how to create very concise code with a lambda function.
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 create a `Function` called `stringLengthFunction` that converts a string to its length using the `length()` method. A `Predicate` is a functional interface that represents an operation that takes an input of type `T` and returns a boolean result.
DigitalOcean
digitalocean.com › community › tutorials › java-8-functional-interfaces
Java 8 Functional Interfaces | DigitalOcean
August 3, 2022 - java.lang.Runnable is a great example of functional interface with single abstract method run().
Javatpoint
javatpoint.com › java-8-functional-interfaces
Functional Interfaces - javatpoint
Java 8 Functional Interfaces with examples and topics on functional interface, anonymous class, lambda for list, lambda for comparable, default methods, method reference, java date and time, java nashorn, java optional, stream, filter etc.
DZone
dzone.com › coding › java › functional interface explained in detail introduced from java 8
Functional Interface Explained in Detail - Java
March 29, 2022 - In the above example, the Function is accepting one string and also returns one string. The BiFunction is similar to Function except it accepts two inputs, whereas Function accepts one argument. The sample code for the BiFunction interface is given below.
Oracle
docs.oracle.com › javase › 8 › docs › api › java › lang › FunctionalInterface.html
FunctionalInterface (Java Platform SE 8 )
2 weeks ago - 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.
Scaler
scaler.com › home › topics › functional interfaces in java
Functional Interfaces in Java - Scaler Topics
April 20, 2024 - For instance, let there be a BookStore interface with few methods and a Library interface as a Functional Interface. The compiler throws an exception, as the BookStore interface has 3 abstract methods.
Jenkov
jenkov.com › tutorials › java-functional-programming › functional-interfaces.html
Java Functional Interfaces
March 8, 2021 - First this example creates a new AddThree instance and assigns it to a Function variable. Second, the example calls the apply() method on the AddThree instance. Third, the example prints out the result (which is 7). You can also implement the ...
Oracle
docs.oracle.com › javase › 8 › docs › api › java › util › function › package-summary.html
java.util.function (Java Platform SE 8 )
2 weeks ago - 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 ...
Blogger
javarevisited.blogspot.com › 2018 › 01 › what-is-functional-interface-in-java-8.html
What is a Functional interface in Java 8? @FunctionalInterface Annotation Examples Tutorial
A BiFunction is a functional interface ... of BiFunction functional interface. A rather simpler example is Predicate which accepts a type T and returns a boolean....
Top answer 1 of 5
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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.
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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.
TechVidvan
techvidvan.com › tutorials › functional-interface-in-java
Functional Interface in Java - TechVidvan
June 30, 2020 - There can be any number of default and static methods in a functional interface that have an implementation. Some of the examples of functional interfaces in Java are Runnable, ActionListener, Comparable interfaces.
Tutorialspoint
tutorialspoint.com › java › java_functional_interfaces.htm
Java - Functional Interfaces
Predicate predicate = (value) -> ... passed. In this example, were using a predicate functional interface to filter odd numbers from a list of integers with the help of a lambda expression....
Kotlin
kotlinlang.org › docs › fun-interfaces.html
Functional (SAM) interfaces | Kotlin Documentation
2 weeks ago - With callable references to functional interface constructors enabled, this code can be replaced with just a functional interface declaration: ... Its constructor will be created implicitly, and any code using the ::Printer function reference will compile. For example:
Reflectoring
reflectoring.io › one-stop-guide-to-java-functional-interfaces
One Stop Guide to Java Functional Interfaces
June 11, 2024 - Functional Interfaces: These are interfaces with a single abstract method, making them perfect for lambda expressions and method references. Common examples include Predicate, Function, Consumer, Supplier, and Operator.
Hero Vired
herovired.com › learning-hub › blogs › functional-interface-in-java
Functional Interface in Java: Uses, Types, Examples
June 29, 2024 - 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:
GeeksforGeeks
geeksforgeeks.org › java › function-interface-in-java
Function Interface in Java - GeeksforGeeks
July 11, 2025 - Explanation: In the above example, two functions are declared with the help of Function interface. The first function takes an integer and add 5 to it and the second function takes an integer and then multiply it with 2. These two functions ...
BeginnersBook
beginnersbook.com › 2017 › 10 › java-functional-interfaces
Java Functional Interfaces
September 11, 2022 - The functional interface should have Only one abstract method. Along with the one abstract method, they can have any number of default and static methods. @FunctionalInterface interface MyFunctionalInterface { public int addMethod(int a, int b); } public class BeginnersBookClass { public static void main(String args[]) { // lambda expression MyFunctionalInterface sum = (a, b) -> a + b; System.out.println("Result: "+sum.addMethod(12, 100)); } }