---
title: Fluent Interface
category: Behavioral
language: en
tag: 
    - API design
    - Code simplification
    - Decoupling
    - Object composition
    - Reactive
---

## Also known as

* Fluent API
* Method Chaining

## Intent

To provide an easily readable, flowing API by chaining method calls.

## Explanation

Real-world example

> Imagine you are at a coffee shop and you want to customize your coffee order. Instead of telling the barista everything at once, you specify each customization step-by-step in a way that flows naturally. For instance, you might say, "I'd like a large coffee, add two shots of espresso, no sugar, and top it with almond milk." This approach is similar to the Fluent Interface design pattern, where you chain together method calls to configure an object in a readable and intuitive manner. Just as you specify each part of your coffee order sequentially, a Fluent Interface allows you to chain method calls to build and configure objects step-by-step in code.

In plain words

> Fluent Interface pattern provides easily readable flowing interface to code.

Wikipedia says

> In software engineering, a fluent interface is an object-oriented API whose design relies extensively on method chaining. Its goal is to increase code legibility by creating a  domain-specific language (DSL).

**Programmatic Example**

We need to select numbers based on different criteria from the list. It's a great chance to  utilize fluent interface pattern to provide readable easy-to-use developer experience.

In this example two implementations of a `FluentIterable` interface are given.

```java
public interface FluentIterable<E> extends Iterable<E> {

  FluentIterable<E> filter(Predicate<? super E> predicate);

  Optional<E> first();

  FluentIterable<E> first(int count);

  Optional<E> last();

  FluentIterable<E> last(int count);

  <T> FluentIterable<T> map(Function<? super E, T> function);

  List<E> asList();

  static <E> List<E> copyToList(Iterable<E> iterable) {
    var copy = new ArrayList<E>();
    iterable.forEach(copy::add);
    return copy;
  }
}
```

The `SimpleFluentIterable` evaluates eagerly and would be too costly for real world applications.

```java
public class SimpleFluentIterable<E> implements FluentIterable<E> {
  // ...
}
```

The `LazyFluentIterable` is evaluated on termination.

```java
public class LazyFluentIterable<E> implements FluentIterable<E> {
  // ...
}
```

Their usage is demonstrated with a simple number list that is filtered, transformed and collected. The result is printed afterward.

```java
public static void main(String[] args) {

    var integerList = List.of(1, -61, 14, -22, 18, -87, 6, 64, -82, 26, -98, 97, 45, 23, 2, -68);

    prettyPrint("The initial list contains: ", integerList);

    var firstFiveNegatives = SimpleFluentIterable
            .fromCopyOf(integerList)
            .filter(negatives())
            .first(3)
            .asList();
    prettyPrint("The first three negative values are: ", firstFiveNegatives);


    var lastTwoPositives = SimpleFluentIterable
            .fromCopyOf(integerList)
            .filter(positives())
            .last(2)
            .asList();
    prettyPrint("The last two positive values are: ", lastTwoPositives);

    SimpleFluentIterable
            .fromCopyOf(integerList)
            .filter(number -> number % 2 == 0)
            .first()
            .ifPresent(evenNumber -> LOGGER.info("The first even number is: {}", evenNumber));


    var transformedList = SimpleFluentIterable
            .fromCopyOf(integerList)
            .filter(negatives())
            .map(transformToString())
            .asList();
    prettyPrint("A string-mapped list of negative numbers contains: ", transformedList);


    var lastTwoOfFirstFourStringMapped = LazyFluentIterable
            .from(integerList)
            .filter(positives())
            .first(4)
            .last(2)
            .map(number -> "String[" + number + "]")
            .asList();
    prettyPrint("The lazy list contains the last two of the first four positive numbers "
            + "mapped to Strings: ", lastTwoOfFirstFourStringMapped);

    LazyFluentIterable
            .from(integerList)
            .filter(negatives())
            .first(2)
            .last()
            .ifPresent(number -> LOGGER.info("Last amongst first two negatives: {}", number));
}
```

Program output:

```
08:50:08.260 [main] INFO com.iluwatar.fluentinterface.app.App -- The initial list contains: 1, -61, 14, -22, 18, -87, 6, 64, -82, 26, -98, 97, 45, 23, 2, -68.
08:50:08.265 [main] INFO com.iluwatar.fluentinterface.app.App -- The first three negative values are: -61, -22, -87.
08:50:08.265 [main] INFO com.iluwatar.fluentinterface.app.App -- The last two positive values are: 23, 2.
08:50:08.266 [main] INFO com.iluwatar.fluentinterface.app.App -- The first even number is: 14
08:50:08.267 [main] INFO com.iluwatar.fluentinterface.app.App -- A string-mapped list of negative numbers contains: String[-61], String[-22], String[-87], String[-82], String[-98], String[-68].
08:50:08.270 [main] INFO com.iluwatar.fluentinterface.app.App -- The lazy list contains the last two of the first four positive numbers mapped to Strings: String[18], String[6].
08:50:08.270 [main] INFO com.iluwatar.fluentinterface.app.App -- Last amongst first two negatives: -22
```

## Class diagram

![Fluent Interface](./etc/fluentinterface.png "Fluent Interface")

## Applicability

Use the Fluent Interface pattern when

* Designing APIs that are heavily used and where readability of client code is of high importance.
* Building complex objects step-by-step, and there is a need to make the code more intuitive and less error-prone.
* Enhancing code clarity and reducing the boilerplate code, especially in configurations and object-building scenarios.

## Tutorials

* [An Approach to Internal Domain-Specific Languages in Java (InfoQ)](http://www.infoq.com/articles/internal-dsls-java)

## Known uses

* [Java 8 Stream API](http://www.oracle.com/technetwork/articles/java/ma14-java-se-8-streams-2177646.html)
* [Google Guava FluentIterable](https://github.com/google/guava/wiki/FunctionalExplained)
* [JOOQ](http://www.jooq.org/doc/3.0/manual/getting-started/use-cases/jooq-as-a-standalone-sql-builder/)
* [Mockito](http://mockito.org/)
* [Java Hamcrest](http://code.google.com/p/hamcrest/wiki/Tutorial)
* Builders in libraries like Apache Camel for integration workflows.

## Consequences

Benefits:

* Improved code readability and maintainability.
* Encourages building immutable objects since methods typically return new instances.
* Reduces the need for variables as the context is maintained in the chain.

Trade-offs:

* Can lead to less intuitive code for those unfamiliar with the pattern.
* Debugging can be challenging due to the chaining of method calls.
* Overuse can lead to complex and hard-to-maintain code structures.

## Related Patterns

* [Builder](https://java-design-patterns.com/patterns/builder/): Often implemented using a Fluent Interface to construct objects step-by-step. The Builder Pattern focuses on constructing complex objects, while Fluent Interface emphasizes the method chaining mechanism.
* [Chain of Responsibility](https://java-design-patterns.com/patterns/chain-of-responsibility/): Fluent Interfaces can be seen as a specific utilization of the Chain of Responsibility, where each method in the chain handles a part of the task and then delegates to the next method.

## Credits

* [Domain-Driven Design: Tackling Complexity in the Heart of Software](https://amzn.to/3UrXkh2)
* [Domain Specific Languages](https://amzn.to/3R1UYDA)
* [Effective Java](https://amzn.to/4d4azvL)
* [Java Design Pattern Essentials](https://amzn.to/44bs6hG)
* [Fluent Interface (Martin Fowler)](http://www.martinfowler.com/bliki/FluentInterface.html)