Java Functional Interface

Bishal Saha

2 min read

·

Jul 22, 2026

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What is a Functional Interface?

A Functional Interface is simply an interface that has only one abstract method.

interface Greeting {
    void sayHello();
}

It has only one method. Java calls this a Functional Interface.

You can even write

@FunctionalInterface
interface Greeting {
    void sayHello();
}

The annotation (@FunctionalInterface) is optional, but it tells Java:

“This interface must always have exactly one abstract method.”

If you accidentally add another abstract method

@FunctionalInterface
interface Greeting {
    void sayHello();
    void bye();
}

Java gives an error.

Why were Functional Interfaces created?

Before Java 8, if you wanted to pass some behavior (code) to another method, you had to create a class.

class GreetingImpl implements Greeting {

    public void sayHello() {
        System.out.println("Hello");
    }

}

Then

Greeting g = new GreetingImpl();
g.sayHello();

Lots of code.

Java 8 introduced Lambda Expressions, but lambdas need something to fit into.

That “something” is a Functional Interface.

Think of it like this

Lambda
   ↓
must fit into
   ↓
Functional Interface

Without Functional Interfaces, Lambda Expressions wouldn’t exist.

Real-life analogy

Imagine a TV remote.

Power Button

The remote has one button When you press it, something happens. You don’t care how it happens. Same idea.

A Functional Interface is like

Button

↓

One action

Whenever someone presses it, Java executes whatever code you supplied.

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Type Functional Interfaces

1. Consumer

Consumer Blog

Input

Yes

Output

Nothing

Consumer<String> c = name -> System.out.println(name);

Think

Take

↓

Use

↓

Done

2. Supplier

Input

Nothing

Output

Something

Supplier<Integer> s = () -> 10;

Think

Give me something

3. Function

Function Blog

Input

Yes

Output

Yes

Function<String,Integer> f = name -> name.length();

Think

Input

↓

Process

↓

Return result

4. Predicate

Input

Yes

Output

true/false

Predicate<Integer> p = x -> x > 10;

Think

Question

↓

Answer

↓

True or False