Functional Interfaces in Java

 "A functional interface is an interface which contains only one abstract method ". They can have only one functionality to exhibit/implement. 

Runnable, Comparable etc. are some of the examples of functional interfaces. Runnable has only method "run()", Comparable has "compareTo()" method.

Functional Interface is additionally recognized as Single Abstract Method i.e. "SAM" Interfaces. 

Functional interface can be written as the interface with an annotation called @FunctionalInterface.


interface Sample {
void show();

As described functional interfaces can contain only one abstract method. However, they can include any quantity of default and static methods. 

In Functional interfaces, its not mandatory to use the abstract keyword as by default, the method defined inside the interface is abstract only.

From Java 8 onwards, lambda expressions can be used to represent the instance of a functional interface.

Functional Interface Examples : -

Example 1:

Before Java 8, to create instance of interface, We had to create a class and implement these interfaces or we had to create anonymous inner class objects and implement these interfaces like below .

// Java program to demonstrate functional interface before java 8.

class SampleFunctionalInterface{
    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");

Output: -

New thread created

Example 2:

Java 8 onwards, we can assign lambda expression to its functional interface object like this: 

// Java program to demonstrate Implementation of functional interface using lambda expressions

class SampleFunctionalInterface {
    public static void main(String args[])
        // lambda expression to create the object
        new Thread(() -> {
            System.out.println("New thread created");

Output : -

New thread created

@FunctionalInterface Annotation : -

@FunctionalInterface annotation is used to ensure that the functional interface can’t have more than one abstract method. 

If we try to write more than one abstract methods, the compiler flags an ‘Unexpected @FunctionalInterface annotation’ message. However, it is not mandatory to use this annotation.

Below is the example of this :

  1. /* Java program to demonstrate lambda expressions to implement a user defined functional interface.*/

  2. @FunctionalInterface
  3. interface Square {
  4.     int calculateArea(int x);
  5.     int calculatePerimeter(int x);
  6. }

  7. class Test {
  8.     public static void main(String args[])
  9.     {
  10.         int a = 5;
  11.         // lambda expression to define the calculateArea method
  12.         Square s = (int x) -> x * x;
  13.         int ans = s.calculateArea(a);
  14.         System.out.println(ans);
  15.     }
  16. }
Output: - Compile time Error.

In the above example, at line number 6, the compiler will show an ‘Unexpected @FunctionalInterface annotation’ message.

Some Built-in Java Functional Interfaces : -

After Java 8 , 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.

Java 8 has included 4 major kinds of functional interfaces as mentioned below :

  1. Consumer
  2. Predicate
  3. Function 
  4. Supplier

Along with the previous four interfaces, the first three interfaces, i.e., Consumer, Predicate, and Function have additional interfaces that are provided beneath – 

Consumer -> Bi-Consumer
Predicate -> Bi-Predicate
Function -> Bi-Function, Unary Operator, Binary Operator 

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. 

Bi-Consumer – 
Bi-Consumer is the most exciting variant of the Consumer interface. The consumer interface takes only one argument, but on the other side, the Bi-Consumer interface takes two arguments. Both, Consumer and Bi-Consumer have no return value. It also returns nothing just like the Consumer interface. It is used in iterating through the entries of the map. 

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 

In scientific logic, a function that accepts an argument and, in return, generates a boolean value as an answer is known as a predicate. Similarly, in the Java programming language, a predicate functional interface of Java is a type of function that accepts a single value or argument and does some sort of processing on it, and returns a boolean (True/ False) answer. The implementation of the Predicate functional interface also encapsulates the logic of filtering (a process that is used to filter stream components on the base of a provided predicate) in Java.

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.  

Bi-Predicate – 
Bi-Predicate is also an extension of the Predicate functional interface, which, instead of one, takes two arguments, does some processing, and returns the boolean value.

Syntax of Predicate Functional Interface – 

public interface Predicate<T> {
    boolean test(T t);
The predicate functional interface can also be implemented using a class. The syntax for the implementation of predicate functional interface using a class is given below – 

public class CheckForNull implements Predicate {
    public boolean test(Object o) {
        return o != null;
The Java predicate functional interface can also be implemented using Lambda expressions. An example of the implementation of the Predicate functional interface is given below – 

Predicate predicate = (value) -> value != null;
This implementation of functional interfaces in Java using Java Lambda expressions is more manageable and effective than the one implemented using a class as both the implementations are doing the same work, i.e., returning the same output.

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. There are many versions of Function interfaces because a primitive type can’t imply a general type argument, so we need these versions of function interfaces. Many different versions of the function interfaces are instrumental and are commonly used in primitive types like double, int, long. The different sequences of these primitive types are also used in the argument.

These versions are:

The Bi-Function is substantially related to a Function. Besides, it takes two arguments, whereas Function accepts one argument. 

The prototype and syntax of Bi-Function is given below –

public interface BiFunction<T, U, R> 
   R apply(T t, U u);
In the above code of interface, T and U are the inputs, and there is only one output which is R. 

Unary Operator and Binary Operator
There are also two other functional interfaces which are named Unary Operator and Binary Operator. They both extend the Function and Bi-Function, respectively. In simple words, Unary Operator extends Function, and Binary Operator extends Bi-Function. 

The prototype of the Unary Operator and Binary Operator is mentioned below :

i. Unary Operator
public interface UnaryOperator<T> extends Function<T, U> 
 ii. Binary Operator
public interface BinaryOperator<T> extends BiFunction<T, U, R> 
We can understand front the above example that the Unary Operator accepts only one argument and returns a single argument only. Still, in Unary Operator both the input and output values must be identical and of the same type. 

On the other way, Binary Operator takes two values and returns one value comparable to Bi- Function but similar to a Unary Operator, the input and output value types must be identical and of the same type.

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. This type of functional interface is generally used in the lazy generation of values. Supplier functional interfaces are also used for defining the logic for the generation of any sequence. For example – The logic behind the Fibonacci Series can be generated with the help of the Stream. generate method, which is implemented by the Supplier functional Interface. 

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 / Prototype of Supplier Functional Interface is –

public interface Supplier<T>{
// gets a result
// returns the specific result
Below is the implementation of the above topic:

// A simple program to demonstrate the use
// of predicate interface
import java.util.*;
import java.util.function.Predicate;
class Test {
    public static void main(String args[])
        // create a list of strings
        List<String> names = Arrays.asList(
            "JavaHubPoint", "JavaHubPointQuiz", "g1", "QA", "JavaHubPoint2");
        // declare the predicate type as string and use
        // lambda expression to create object
        Predicate<String> p = (s) -> s.startsWith("J");
        // Iterate through the list
        for (String st : names) {
            // call the test method
            if (p.test(st))

Important Points/Observations:
Here are some significant points regarding Functional interfaces in Java:

  1. In functional interfaces, there is only one abstract method supported. If the annotation of a functional interface, i.e., @FunctionalInterface is not implemented or written with a function interface, more than one abstract method can be declared inside it. However, in this situation with more than one functions, that interface will not be called a functional interface. It is called a non-functional interface.
  2. There is no such need for the @FunctionalInterface annotation as it is voluntary only. This is written because it helps in checking the compiler level. Besides this, it is optional.
  3. An infinite number of methods (whether static or default) can be added to the functional interface. In simple words, there is no limit to a functional interface containing static and default methods.
  4. Overriding methods from the parent class do not violate the rules of a functional interface in Java.
  5. The java.util.function package contains many built-in functional interfaces in Java 8.