Radosław Łazarz Radosław Łazarz - 2 months ago 9
Java Question

Is it possible to define an optional flow or exception-like behaviour in Java 8 streams API?

Let us have a stream of objects, resulting from a sequence of operations (e.g. mapping, filtering, flatmapping, etc.). Now I want to do a certain operation on them, but only if a given predicate is true. Otherwise I want to immediately return something else.

A simple example. I have a stream of different food objects. If all of them are edible I want to perform a cook operation on them and return the list of cooked food. But if any of them turns out to not be edible I want to immediately return an empty list.

Few solutions come to my mind, but I am not satisfied with any of them.

I could first perform an

operation with
predicate on the stream, but it will result in terminating it and I would need to repeat preliminary operations once more.

I could persist the collection that is the result of preliminary operations before checking the edibility, but therefore I need to perform them for all elements. Which is suboptimal, because it may turn out, that the first of them is not edible and
would return much, much earlier.

Or I could design a hacky
routine, but it would also be unable to stop processing elements when predicate fails.

What I hope for is something like the code below. Is it possible with current API?

// some operations
.ifAny(food -> !food.isEdible(), new LinkedList<Food>())
// other operations if previous step not failed
.peek(food -> food.prepare())


Not exactly what you wanted, but using the ternary operator will make your two step solution look cleaner, it also should have optimal performance:

return source.stream()
    ? source.stream()
        .map(food -> {
           food.prepare(); // do stuff
           return food;
    : Collections.emptyList();