[java] Functional style of Java 8's Optional.ifPresent and if-not-Present?

Another solution could be following:

This is how you use it:

    final Opt<String> opt = Opt.of("I'm a cool text");
    opt.ifPresent()
        .apply(s -> System.out.printf("Text is: %s\n", s))
        .elseApply(() -> System.out.println("no text available"));

Or in case you in case of the opposite use case is true:

    final Opt<String> opt = Opt.of("This is the text");
    opt.ifNotPresent()
        .apply(() -> System.out.println("Not present"))
        .elseApply(t -> /*do something here*/);

This are the ingredients:

  1. Little modified Function interface, just for the "elseApply" method
  2. Optional enhancement
  3. A little bit of curring :-)

The "cosmetically" enhanced Function interface.

@FunctionalInterface
public interface Fkt<T, R> extends Function<T, R> {

    default R elseApply(final T t) {
        return this.apply(t);
    }

}

And the Optional wrapper class for enhancement:

public class Opt<T> {

    private final Optional<T> optional;

    private Opt(final Optional<T> theOptional) {
        this.optional = theOptional;
    }

    public static <T> Opt<T> of(final T value) {
        return new Opt<>(Optional.of(value));
    }

    public static <T> Opt<T> of(final Optional<T> optional) {
        return new Opt<>(optional);
    }

    public static <T> Opt<T> ofNullable(final T value) {
        return new Opt<>(Optional.ofNullable(value));
    }

    public static <T> Opt<T> empty() {
        return new Opt<>(Optional.empty());
    }

    private final BiFunction<Consumer<T>, Runnable, Void> ifPresent = (present, notPresent) -> {
        if (this.optional.isPresent()) {
            present.accept(this.optional.get());
        } else {
            notPresent.run();
        }
        return null;
    };

   private final BiFunction<Runnable, Consumer<T>, Void> ifNotPresent = (notPresent, present) -> {
        if (!this.optional.isPresent()) {
            notPresent.run();
        } else {
            present.accept(this.optional.get());
        }
        return null;
    };

    public Fkt<Consumer<T>, Fkt<Runnable, Void>> ifPresent() {
        return Opt.curry(this.ifPresent);
    }

    public Fkt<Runnable, Fkt<Consumer<T>, Void>> ifNotPresent() {
        return Opt.curry(this.ifNotPresent);
    }

    private static <X, Y, Z> Fkt<X, Fkt<Y, Z>> curry(final BiFunction<X, Y, Z> function) {
        return (final X x) -> (final Y y) -> function.apply(x, y);
    }
}

This should do the trick and could serve as a basic template how to deal with such requirements.

The basic idea here is following. In a non functional style programming world you would probably implement a method taking two parameter where the first is a kind of runnable code which should be executed in case the value is available and the other parameter is the runnable code which should be run in case the value is not available. For the sake of better readability, you can use curring to split the function of two parameter in two functions of one parameter each. This is what I basically did here.

Hint: Opt also provides the other use case where you want to execute a piece of code just in case the value is not available. This could be done also via Optional.filter.stuff but I found this much more readable.

Hope that helps!

Good programming :-)

Examples related to java

Under what circumstances can I call findViewById with an Options Menu / Action Bar item? How much should a function trust another function How to implement a simple scenario the OO way Two constructors How do I get some variable from another class in Java? this in equals method How to split a string in two and store it in a field How to do perspective fixing? String index out of range: 4 My eclipse won't open, i download the bundle pack it keeps saying error log

Examples related to functional-programming

Dart: mapping a list (list.map) Index inside map() function functional way to iterate over range (ES6/7) How can I count occurrences with groupBy? How do I use the includes method in lodash to check if an object is in the collection? Does Java SE 8 have Pairs or Tuples? Functional style of Java 8's Optional.ifPresent and if-not-Present? What is difference between functional and imperative programming languages? How does functools partial do what it does? map function for objects (instead of arrays)

Examples related to java-8

Default interface methods are only supported starting with Android N Class has been compiled by a more recent version of the Java Environment Why is ZoneOffset.UTC != ZoneId.of("UTC")? Modify property value of the objects in list using Java 8 streams How to use if-else logic in Java 8 stream forEach Android Studio Error: Error:CreateProcess error=216, This version of %1 is not compatible with the version of Windows you're running Error:could not create the Java Virtual Machine Error:A fatal exception has occured.Program will exit What are functional interfaces used for in Java 8? java.time.format.DateTimeParseException: Text could not be parsed at index 21 Java 8 lambda get and remove element from list

Examples related to optional

Java 8 optional: ifPresent return object orElseThrow exception Default optional parameter in Swift function Difference between `Optional.orElse()` and `Optional.orElseGet()` Why should Java 8's Optional not be used in arguments Why use Optional.of over Optional.ofNullable? Java 8 - Difference between Optional.flatMap and Optional.map Check string for nil & empty How to execute logic on Optional if not present? Swift: Testing optionals for nil Proper usage of Optional.ifPresent()