Functional Interface in Java – Complete Blog (From Scratch)

Introduction

Java 8 introduced a very important concept called Functional Interface. Functional interfaces are mainly used with Lambda Expressions to write shorter and cleaner code. They are widely used in modern Java, especially in Stream API, Collections, and multithreading.

In this blog, we will learn:

• What is a Functional Interface

• Why we use it

• How to create it

• Different ways to implement it

• Lambda Expression

• Real-life examples

• Interview questions

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What is a Functional Interface?

A Functional Interface is an interface that contains only one abstract method but it can have multiple default and static methods.

Definition

A Functional Interface is an interface that contains exactly one abstract method and can be implemented using a lambda expression.

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Why Do We Use Functional Interface?

Before Java 8, we had to write a lot of code using anonymous classes. After Java 8, lambda expressions made the code short and easy, and lambda works only with functional interfaces.

So the main purpose of a functional interface is:

• To use Lambda Expression

• To reduce code

• To improve readability

• To support functional programming in Java

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How to Create a Functional Interface

There are two ways:

1. Without annotation

2. With @FunctionalInterface annotation

Example 1: Without Annotation

interface A {
    void show();
}

Example 2: With Annotation

@FunctionalInterface
interface A {
    void show();
}

If we add more than one abstract method, it will give an error.

@FunctionalInterface
interface A {
    void show();
    void display(); // Error: Not a functional interface
}

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Ways to Implement Functional Interface

There are 3 ways to implement a functional interface:

1. Using Normal Class

interface A {
    void show();
}

class B implements A {
    public void show() {
        System.out.println("Hello from Normal Class");
    }
}

class Test {
    public static void main(String[] args) {
        A obj = new B();
        obj.show();
    }
}

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2. Using Anonymous Class

interface A {
    void show();
}

class Test {
    public static void main(String[] args) {
        A obj = new A() {
            public void show() {
                System.out.println("Hello from Anonymous Class");
            }
        };
        obj.show();
    }
}

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3. Using Lambda Expression

interface A {
    void show();
}

class Test {
    public static void main(String[] args) {
        A obj = () -> System.out.println("Hello from Lambda");
        obj.show();
    }
}

Lambda expression reduces code and is the most commonly used method.

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Lambda Expression Syntax

(parameters) -> { body }

Examples:

() -> System.out.println("Hello");

(x) -> x * x;

(a, b) -> a + b;

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Functional Interface with Parameters Example

@FunctionalInterface
interface Add {
    int sum(int a, int b);
}

class Test {
    public static void main(String[] args) {
        Add obj = (a, b) -> a + b;
        System.out.println(obj.sum(10, 20));
    }
}

Output:

30

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Inbuilt Functional Interfaces in Java

Java already provides many functional interfaces in the package java.util.function.

Interface	Method	Work
Predicate	test()	Condition check
Function	apply()	Input → Output
Consumer	accept()	Takes input, no return
Supplier	get()	Returns value
Runnable	run()	Multithreading
Comparator	compare()	Sorting

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Difference: Normal Interface vs Functional Interface

Normal Interface	Functional Interface
Multiple abstract methods	Only one abstract method
Cannot use lambda	Can use lambda
More code	Less code
Used before Java 8	Used after Java 8

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Real-Life Use Case

Functional interfaces are used in:

• Sorting using Comparator

• Filtering using Predicate

• Stream API

• Thread creation using Runnable

• Event handling

Example:

Runnable r = () -> System.out.println("Thread running");
new Thread(r).start();

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Interview Questions

Q1: What is a functional interface? An interface with only one abstract method.

Q2: Can a functional interface have multiple methods? Yes, but only one abstract method. It can have static and default methods.

Q3: Which annotation is used for functional interface? @FunctionalInterface

Q4: Can we use lambda without functional interface? No.

Q5: Give examples of functional interfaces. Runnable, Comparator, Predicate, Function, Consumer, Supplier.

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Conclusion

Functional interfaces are one of the most important features introduced in Java 8. They help in writing short, clean, and readable code using lambda expressions. They are heavily used in modern Java development, especially in Stream API and multithreading.

Final One-Line Definition

Functional Interface = Interface with one abstract method + used with Lambda Expression.

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Practice Questions

1. Create a functional interface to find square of a number.

2. Create a functional interface to check if a number is prime.

3. Create a functional interface to find maximum of two numbers.

4. Implement all using lambda expression.

If you practice these, your concept will be very strong.