fluent-interface/README.md
The primary goal of the Fluent Interface pattern is to provide an easily readable and flowing API by chaining method calls, often referred to as method chaining. This approach is ideal for building complex objects step-by-step and improving the overall developer experience.
Real-world example
Imagine you are at a coffee shop customizing your order step-by-step. This approach is similar to how the Fluent Interface design pattern works in Java, allowing you to chain method calls to build and configure objects sequentially. 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).
Sequence diagram
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.
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.
public class SimpleFluentIterable<E> implements FluentIterable<E> {
// ...
}
The LazyFluentIterable is evaluated on termination.
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.
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
Use the Fluent Interface Pattern in Java when
Benefits:
Trade-offs: