Functional Interface : Function

Bishal Saha

2 min read

·

Aug 15, 2026

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What is a Functional Interface?

A functional interface is an interface that has only one abstract method.

Java provides some built-in functional interfaces. One of them is:

Function<T, R>

It means:

Take one input and return one output.

For example:

Function<Integer, Integer> square = x -> x * x;

Here:

• Integer → input type
• Integer → return type
• x -> x * x → the function

So:

Input → Function → Output

5   →  x * x   → 25

Functional Interface in JWT Example

This code has:

private <T> T extractClaim(String token, Function<Claims, T> claimsResolver) {

    final Claims claims = extractAllClaims(token);

    return claimsResolver.apply(claims);
}

The important part is:

Function<Claims, T> claimsResolver

This means:

“Give me a function that takes Claims as input and returns some type T."

For example, when we call:

extractClaim(token, Claims::getSubject);

Claims::getSubject is the function.

It can be understood as:

claims -> claims.getSubject()

So the flow is:

JWT Token
    ↓
extractAllClaims()
    ↓
Claims object
    ↓
claimsResolver
    ↓
getSubject()
    ↓
Username

Example 1: Getting Username

We have:

public String extractUsername(String token) {
    return extractClaim(token, Claims::getSubject);
}

Here:

Claims::getSubject

means:

claims -> claims.getSubject()

So the Function receives:

Claims

and returns:

String

Therefore:

Function<Claims, String>

Why did we create extractClaim()?

Without a functional interface, wecould write separate logic for every claim.

But with:

Function<Claims, T>

we can make one reusable method:

extractClaim(token, Claims::getSubject);

The method doesn’t need to know which claim you want.

You simply give it a function that tells it what to extract.

Easy way to remember

Think of Function as a worker:

             Claims
                ↓
       ┌─────────────────┐
       │     Function    │
       │ "What to get?"  │
       └─────────────────┘
                ↓
             Result

For username:

Claims → getSubject() → String

In one line

Function<Claims, T> is a functional interface that allows your extractClaim() method to accept different functions for extracting different values from JWT claims.