Functional style of Java 8's Optional.ifPresen

Question

In Java 8, I want to do something to an Optional object if it is present, and do another thing if it is not present.

if (opt.isPresent()) {
  System.out.println("found");
} else {
  System.out.println("Not found");
}

This is not a 'functional style', though.

Optional has an ifPresent() method, but I am unable to chain an orElse() method.

Thus, I cannot write:

opt.ifPresent( x -> System.out.println("found " + x))
   .orElse( System.out.println("NOT FOUND"));

In reply to @assylias, I don't think Optional.map() works for the following case:

opt.map( o -> {
  System.out.println("while opt is present...");
  o.setProperty(xxx);
  dao.update(o);
  return null;
}).orElseGet( () -> {
  System.out.println("create new obj");
  dao.save(new obj);
  return null;
});

In this case, when opt is present, I update its property and save to the database. When it is not available, I create a new obj and save to the database.

Note in the two lambdas I have to return null.

But when opt is present, both lambdas will be executed. obj will be updated, and a new object will be saved to the database . This is because of the return null in the first lambda. And orElseGet() will continue to execute.

Answer 1

If you are using Java 9, you can use ifPresentOrElse() method:

opt.ifPresentOrElse(
   value -> System.out.println("Found: " + value),
   () -> System.out.println("Not found")
);

Answer 2

In case you want store the value:

Pair.of<List<>, List<>> output = opt.map(details -> Pair.of(details.a, details.b))).orElseGet(() -> Pair.of(Collections.emptyList(), Collections.emptyList()));

Answer 3

Another solution would be to use higher-order functions as follows

opt.<Runnable>map(value -> () -> System.out.println("Found " + value))
   .orElse(() -> System.out.println("Not Found"))
   .run();

Answer 4

For me the answer of @Dane White is OK, first I did not like using Runnable but I could not find any alternatives, here another implementation I preferred more

public class OptionalConsumer<T> {
    private Optional<T> optional;

    private OptionalConsumer(Optional<T> optional) {
        this.optional = optional;
    }

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

    public OptionalConsumer<T> ifPresent(Consumer<T> c) {
        optional.ifPresent(c);
        return this;
    }

    public OptionalConsumer<T> ifNotPresent(Runnable r) {
        if (!optional.isPresent())
            r.run();
        return this;
    }
}

Then :

Optional<Any> o = Optional.of(...);
OptionalConsumer.of(o).ifPresent(s ->System.out.println("isPresent "+s))
            .ifNotPresent(() -> System.out.println("! isPresent"));

Update 1:

the above solution for traditional way of development when you have the value and want to process it but what if I want to define the functionality and the execution will be then, check below enhancement;

public class OptionalConsumer<T> implements Consumer<Optional<T>> {
private final Consumer<T> c;
private final Runnable r;

public OptionalConsumer(Consumer<T> c, Runnable r) {
    super();
    this.c = c;
    this.r = r;
}

public static <T> OptionalConsumer<T> of(Consumer<T> c, Runnable r) {
    return new OptionalConsumer(c, r);
}

@Override
public void accept(Optional<T> t) {
    if (t.isPresent())
        c.accept(t.get());
    else
        r.run();
}
}

Then could be used as:

    Consumer<Optional<Integer>> c=OptionalConsumer.of(System.out::println, ()->{System.out.println("Not fit");});
    IntStream.range(0, 100).boxed().map(i->Optional.of(i).filter(j->j%2==0)).forEach(c);

In this new code you have 3 things:

1. can define functionality before existing of object easy.
2. not creating object refrence for each Optional, only one,you have so less memory then less GC.
3. it is implementing consumer for better usage with other components.

by the way now its name is more descriptive it is actually Consumer>

Answer 5

See excellent Optional in Java 8 cheat sheet.

It provides all answers for most use cases.

Short summary below

ifPresent() - do something when Optional is set

opt.ifPresent(x -> print(x)); 
opt.ifPresent(this::print);

filter() - reject (filter out) certain Optional values.

opt.filter(x -> x.contains("ab")).ifPresent(this::print);

map() - transform value if present

opt.map(String::trim).filter(t -> t.length() > 1).ifPresent(this::print);

orElse()/orElseGet() - turning empty Optional to default T

int len = opt.map(String::length).orElse(-1);
int len = opt.
    map(String::length).
    orElseGet(() -> slowDefault());     //orElseGet(this::slowDefault)

orElseThrow() - lazily throw exceptions on empty Optional

opt.
filter(s -> !s.isEmpty()).
map(s -> s.charAt(0)).
orElseThrow(IllegalArgumentException::new);

Answer 6

There isn't a great way to do it out of the box. If you want to be using your cleaner syntax on a regular basis, then you can create a utility class to help out:

public class OptionalEx {
    private boolean isPresent;

    private OptionalEx(boolean isPresent) {
        this.isPresent = isPresent;
    }

    public void orElse(Runnable runner) {
        if (!isPresent) {
            runner.run();
        }
    }

    public static <T> OptionalEx ifPresent(Optional<T> opt, Consumer<? super T> consumer) {
        if (opt.isPresent()) {
            consumer.accept(opt.get());
            return new OptionalEx(true);
        }
        return new OptionalEx(false);
    }
}

Then you can use a static import elsewhere to get syntax that is close to what you're after:

import static com.example.OptionalEx.ifPresent;

ifPresent(opt, x -> System.out.println("found " + x))
    .orElse(() -> System.out.println("NOT FOUND"));