Java Functional Interfaces
Last Updated :
02 Aug, 2025
A functional interface in Java is an interface that contains only one abstract method. Functional interfaces can have multiple default or static methods, but only one abstract method.
From Java 8 onwards, lambda expressions and method references can be used to represent the instance of a functional interface.
Example: Using a Functional Interface with Lambda Expression
Java
public class Geeks {
public static void main(String[] args) {
// Using lambda expression to implement Runnable
new Thread(() -> System.out.println("New thread created")).start();
}
}
Explanation:
- Above program demonstrates use of lambda expression with the Runnable functional interface.
- Runnable has one abstract method run(), so it qualifies as a functional interface.
- Lambda ()-> System.out.println("New thread created") defines the run() method.
- new Thread().start() starts a new thread that executes the lambda body
Note: A functional interface can also extend another functional interface.
@FunctionalInterface Annotation
@FunctionalInterface annotation is used to ensure that the functional interface cannot have more than one abstract method. In case more than one abstract methods are present, the compiler flags an "Unexpected @FunctionalInterface annotation" message. However, it is not mandatory to use this annotation.
Note: @FunctionalInterface annotation is optional but it is a good practice to use. It helps catching the error in early stage by making sure that the interface has only one abstract method.
Example: Defining a Functional Interface with @FunctionalInterface Annotation
Java
@FunctionalInterface
interface Square {
int calculate(int x);
}
class Geeks {
public static void main(String args[]) {
int a = 5;
// lambda expression to define the calculate method
Square s = (int x) -> x * x;
// parameter passed and return type must be same as defined in the prototype
int ans = s.calculate(a);
System.out.println(ans);
}
}
Explanation:
- Square is a functional interface with a single method calculate(int x).
- A lambda expression (int x) -> x * x is used to implement the calculate method.
- Lambda takes x as input and returns x * x.
Java Functional Interfaces Before Java 8
Before Java 8, we had to create anonymous inner class objects or implement these interfaces. Below is an example of how the Runnable interface was implemented prior to the introduction of lambda expressions.
Example: Java program to demonstrate functional interface
Java
class Geeks {
public static void main(String args[]) {
// create anonymous inner class object
new Thread(new Runnable() {
@Override public void run()
{
System.out.println("New thread created");
}
}).start();
}
}
Built-In Java Functional Interfaces
Since Java SE 1.8 onwards, there are many interfaces that are converted into functional interfaces. All these interfaces are annotated with @FunctionalInterface. These interfaces are as follows:
- Runnable: This interface only contains the run() method.
- Comparable: This interface only contains the compareTo() method.
- ActionListener: This interface only contains the actionPerformed() method.
- Callable: This interface only contains the call() method.
Types of Functional Interfaces in Java
Java SE 8 included four main kinds of functional interfaces which can be applied in multiple situations as mentioned below:
- Consumer
- Predicate
- Function
- Supplier
1. Consumer
The consumer interface of the functional interface is the one that accepts only one argument or a gentrified argument. The consumer interface has no return value. It returns nothing. There are also functional variants of the Consumer DoubleConsumer, IntConsumer and LongConsumer. These variants accept primitive values as arguments.
Other than these variants, there is also one more variant of the Consumer interface known as Bi-Consumer.
Syntax / Prototype of Consumer Functional Interface:
Consumer<Integer> consumer = (value) -> System.out.println(value);
This implementation of the Java Consumer functional interface prints the value passed as a parameter to the print statement. This implementation uses the Lambda function of Java.
2. Predicate
The Predicate interface represents a boolean-valued function of one argument. It is commonly used for filtering operations in streams.
Just like the Consumer functional interface, Predicate functional interface also has some extensions. These are IntPredicate, DoublePredicate and LongPredicate. These types of predicate functional interfaces accept only primitive data types or values as arguments.
Syntax:
public interface Predicate<T> {
boolean test(T t);
}
The Java predicate functional interface can also be implemented using Lambda expressions.
Predicate predicate = (value) -> value != null;
3. Function
A function is a type of functional interface in Java that receives only a single argument and returns a value after the required processing. Many different versions of the function interfaces are instrumental and are commonly used in primitive types like double, int, long.
Syntax:
Function<Integer, Integer> function = (value) -> value * value;
- Bi-Function: The Bi-Function is substantially related to a Function. Besides, it takes two arguments, whereas Function accepts one argument.
- Unary Operator and Binary Operator: There are also two other functional interfaces which are named as Unary Operator and Binary Operator. They both extend the Function and Bi-Function respectively, where both the input type and the output type are same.
4. Supplier
The Supplier functional interface is also a type of functional interface that does not take any input or argument and yet returns a single output.
The different extensions of the Supplier functional interface hold many other suppliers functions like BooleanSupplier, DoubleSupplier, LongSupplier and IntSupplier. The return type of all these further specializations is their corresponding primitives only.
Syntax:
Supplier<String> supplier = () -> "Hello, World!";
Example: Using Predicate Interface to Filter Strings
Java
import java.util.*;
import java.util.function.Predicate;
class Geeks {
public static void main(String args[]) {
// create a list of strings
List<String> n = Arrays.asList(
"Geek", "GeeksQuiz", "g1", "QA", "Geek2");
// declare the predicate type as string and use lambda expression to create object
Predicate<String> p = (s) -> s.startsWith("G");
// Iterate through the list
for (String st : n) {
// call the test method
if (p.test(st))
System.out.println(st);
}
}
}
OutputGeek
GeeksQuiz
Geek2
Functional Interfaces Table
Functional Interfaces | Description | Method |
---|
Runnable | It represents a task that can be executed by a thread. | void run() |
---|
Comparable | It compares two objects for ordering. | int compareTo(T o) |
---|
ActionListener | It handles an action event in event-driven programming. | void actionPerformed(ActionEvent e) |
---|
Callable | It represents a task that can return a result or throw an exception. | V call() throws Exception |
---|
Consumer | It accepts a single input argument and returns no result. | void accept(T t) |
---|
Predicate | It accepts a single argument and returns a boolean result. | boolean test(T t) |
---|
Function | It accepts a single argument and returns a result. | R apply(T t) |
---|
Supplier | It does not take any arguments but provides a result. | T get() |
---|
BiConsumer | It accepts two arguments and returns no result. | void accept(T t, U u) |
---|
BiPredicate | It accepts two arguments and returns a boolean result. | boolean test(T t, U u) |
---|
BiFunction | It accepts two arguments and returns a result. | R apply(T t, U u) |
---|
UnaryOperator | This is a special case of Function, where input and output types are the same. | T apply(T t) |
---|
BinaryOperator | This is a special case of BiFunction, where input and output types are the same. | T apply(T t1, T t2) |
---|
Similar Reads
Java Tutorial Java is a high-level, object-oriented programming language used to build web apps, mobile applications, and enterprise software systems. It is known for its Write Once, Run Anywhere capability, which means code written in Java can run on any device that supports the Java Virtual Machine (JVM).Java s
10 min read
Basics
Introduction to JavaJava is a high-level, object-oriented programming language developed by Sun Microsystems in 1995. It is platform-independent, which means we can write code once and run it anywhere using the Java Virtual Machine (JVM). Java is mostly used for building desktop applications, web applications, Android
4 min read
Java Programming BasicsJava is one of the most popular and widely used programming language and platform. A platform is an environment that helps to develop and run programs written in any programming language. Java is fast, reliable and secure. From desktop to web applications, scientific supercomputers to gaming console
4 min read
Java MethodsJava Methods are blocks of code that perform a specific task. A method allows us to reuse code, improving both efficiency and organization. All methods in Java must belong to a class. Methods are similar to functions and expose the behavior of objects.Example: Java program to demonstrate how to crea
7 min read
Access Modifiers in JavaIn Java, access modifiers are essential tools that define how the members of a class, like variables, methods, and even the class itself, can be accessed from other parts of our program. They are an important part of building secure and modular code when designing large applications. In this article
6 min read
Arrays in JavaIn Java, an array is an important linear data structure that allows us to store multiple values of the same type. Arrays in Java are objects, like all other objects in Java, arrays implicitly inherit from the java.lang.Object class. This allows you to invoke methods defined in Object (such as toStri
9 min read
Java StringsIn Java, a String is the type of object that can store a sequence of characters enclosed by double quotes and every character is stored in 16 bits, i.e., using UTF 16-bit encoding. A string acts the same as an array of characters. Java provides a robust and flexible API for handling strings, allowin
8 min read
Regular Expressions in JavaIn Java, Regular Expressions or Regex (in short) in Java is an API for defining String patterns that can be used for searching, manipulating, and editing a string in Java. Email validation and passwords are a few areas of strings where Regex is widely used to define the constraints. Regular Expressi
7 min read
OOPs & Interfaces
Classes and Objects in JavaIn Java, classes and objects are basic concepts of Object Oriented Programming (OOPs) that are used to represent real-world concepts and entities. A class is a template to create objects having similar properties and behavior, or in other words, we can say that a class is a blueprint for objects.An
10 min read
Java ConstructorsIn Java, constructors play an important role in object creation. A constructor is a special block of code that is called when an object is created. Its main job is to initialize the object, to set up its internal state, or to assign default values to its attributes. This process happens automaticall
10 min read
Java OOP(Object Oriented Programming) ConceptsBefore Object-Oriented Programming (OOPs), most programs used a procedural approach, where the focus was on writing step-by-step functions. This made it harder to manage and reuse code in large applications.To overcome these limitations, Object-Oriented Programming was introduced. Java is built arou
10 min read
Java PackagesPackages in Java are a mechanism that encapsulates a group of classes, sub-packages and interfaces. Packages are used for: Prevent naming conflicts by allowing classes with the same name to exist in different packages, like college.staff.cse.Employee and college.staff.ee.Employee.They make it easier
8 min read
Java InterfaceAn Interface in Java programming language is defined as an abstract type used to specify the behaviour of a class. An interface in Java is a blueprint of a behaviour. A Java interface contains static constants and abstract methods. Key Properties of Interface:The interface in Java is a mechanism to
11 min read
Collections
Exception Handling
Java Exception HandlingException handling in Java is an effective mechanism for managing runtime errors to ensure the application's regular flow is maintained. Some Common examples of exceptions include ClassNotFoundException, IOException, SQLException, RemoteException, etc. By handling these exceptions, Java enables deve
8 min read
Java Try Catch BlockA try-catch block in Java is a mechanism to handle exceptions. This make sure that the application continues to run even if an error occurs. The code inside the try block is executed, and if any exception occurs, it is then caught by the catch block.Example: Here, we are going to handle the Arithmet
4 min read
Java final, finally and finalizeIn Java, the keywords "final", "finally" and "finalize" have distinct roles. final enforces immutability and prevents changes to variables, methods, or classes. finally ensures a block of code runs after a try-catch, regardless of exceptions. finalize is a method used for cleanup before an object is
4 min read
Chained Exceptions in JavaChained Exceptions in Java allow associating one exception with another, i.e. one exception describes the cause of another exception. For example, consider a situation in which a method throws an ArithmeticException because of an attempt to divide by zero.But the root cause of the error was an I/O f
3 min read
Null Pointer Exception in JavaA NullPointerException in Java is a RuntimeException. It occurs when a program attempts to use an object reference that has the null value. In Java, "null" is a special value that can be assigned to object references to indicate the absence of a value.Reasons for Null Pointer ExceptionA NullPointerE
5 min read
Exception Handling with Method Overriding in JavaException handling with method overriding in Java refers to the rules and behavior that apply when a subclass overrides a method from its superclass and both methods involve exceptions. It ensures that the overridden method in the subclass does not declare broader or new checked exceptions than thos
4 min read
Java Advanced
Java Multithreading TutorialThreads are the backbone of multithreading. We are living in the real world which in itself is caught on the web surrounded by lots of applications. With the advancement in technologies, we cannot achieve the speed required to run them simultaneously unless we introduce the concept of multi-tasking
15+ min read
Synchronization in JavaIn multithreading, synchronization is important to make sure multiple threads safely work on shared resources. Without synchronization, data can become inconsistent or corrupted if multiple threads access and modify shared variables at the same time. In Java, it is a mechanism that ensures that only
10 min read
File Handling in JavaIn Java, with the help of File Class, we can work with files. This File Class is inside the java.io package. The File class can be used to create an object of the class and then specifying the name of the file.Why File Handling is Required?File Handling is an integral part of any programming languag
6 min read
Java Method ReferencesIn Java, a method is a collection of statements that perform some specific task and return the result to the caller. A method reference is the shorthand syntax for a lambda expression that contains just one method call. In general, one does not have to pass arguments to method references.Why Use Met
9 min read
Java 8 Stream TutorialJava 8 introduces Stream, which is a new abstract layer, and some new additional packages in Java 8 called java.util.stream. A Stream is a sequence of components that can be processed sequentially. These packages include classes, interfaces, and enum to allow functional-style operations on the eleme
15+ min read
Java NetworkingWhen computing devices such as laptops, desktops, servers, smartphones, and tablets and an eternally-expanding arrangement of IoT gadgets such as cameras, door locks, doorbells, refrigerators, audio/visual systems, thermostats, and various sensors are sharing information and data with each other is
15+ min read
JDBC TutorialJDBC stands for Java Database Connectivity. JDBC is a Java API or tool used in Java applications to interact with the database. It is a specification from Sun Microsystems that provides APIs for Java applications to communicate with different databases. Interfaces and Classes for JDBC API comes unde
12 min read
Java Memory ManagementJava memory management is the process by which the Java Virtual Machine (JVM) automatically handles the allocation and deallocation of memory. It uses a garbage collector to reclaim memory by removing unused objects, eliminating the need for manual memory managementJVM Memory StructureJVM defines va
4 min read
Garbage Collection in JavaGarbage collection in Java is an automatic memory management process that helps Java programs run efficiently. Java programs compile to bytecode that can be run on a Java Virtual Machine (JVM). When Java programs run on the JVM, objects in the heap are created, which is a portion of memory dedicated
7 min read
Memory Leaks in JavaIn programming, a memory leak happens when a program keeps using memory but does not give it back when it's done. It simply means the program slowly uses more and more memory, which can make things slow and even stop working. Working of Memory Management in JavaJava has automatic garbage collection,
3 min read
Practice Java
Java Interview Questions and AnswersJava is one of the most popular programming languages in the world, known for its versatility, portability, and wide range of applications. Java is the most used language in top companies such as Uber, Airbnb, Google, Netflix, Instagram, Spotify, Amazon, and many more because of its features and per
15+ min read
Java Programs - Java Programming ExamplesIn this article, we will learn and prepare for Interviews using Java Programming Examples. From basic Java programs like the Fibonacci series, Prime numbers, Factorial numbers, and Palindrome numbers to advanced Java programs.Java is one of the most popular programming languages today because of its
8 min read
Java Exercises - Basic to Advanced Java Practice Programs with SolutionsLooking for Java exercises to test your Java skills, then explore our topic-wise Java practice exercises? Here you will get 25 plus practice problems that help to upscale your Java skills. As we know Java is one of the most popular languages because of its robust and secure nature. But, programmers
7 min read
Java Quiz | Level Up Your Java SkillsThe best way to scale up your coding skills is by practicing the exercise. And if you are a Java programmer looking to test your Java skills and knowledge? Then, this Java quiz is designed to challenge your understanding of Java programming concepts and assess your excellence in the language. In thi
1 min read
Top 50 Java Project Ideas For Beginners and Advanced [Update 2025]Java is one of the most popular and versatile programming languages, known for its reliability, security, and platform independence. Developed by James Gosling in 1982, Java is widely used across industries like big data, mobile development, finance, and e-commerce.Building Java projects is an excel
15+ min read