# Functional Interface

An interface with one abstract method that can be implemented by a lambda.

## What is **Functional Interface** in Java?

**Functional Interface** — An interface with one abstract method that can be implemented by a lambda.

```java
import java.util.function.Function;

class LabelFormatter {
    Function<String, String> uppercase = String::toUpperCase;
}
```

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Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/functional-interface