How to use collect call in Java 8?

Question

Lets say we have this boring piece of code that we all had to use:

ArrayList<Long> ids = new ArrayList<Long>();
for (MyObj obj : myList){
    ids.add(obj.getId());
}

After switching to Java 8, my IDE is telling me that I can replace this code with collect call, and it auto-generates:

ArrayList<Long> ids = myList.stream().map(MyObj::getId).collect(Collectors.toList());

However its giving me this error:

collect(java.util.stream.Collector) in Steam cannot be applied to: (java.util.stream.Collector, capture, java.util.List>)

I tried casting the parameter but its giving me undefined A and R, and the IDE isn't giving any more suggestions.

I'm curious as how can you use collect call in this scenario, and I couldn't find any information that could guide me properly. Can anyone shed a light?

Answer 1

The issue is that Collectors.toList, not surprisingly, returns a List<T>. Not an ArrayList.

List<Long> ids = remove.stream()
        .map(MyObj::getId)
        .collect(Collectors.toList());

Program to the interface.

From the documentation:

Returns a Collector that accumulates the input elements into a new List. There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier).

Emphasis mine - you cannot even assume that the List returned is mutable, let alone that it is of a specific class. If you want an ArrayList:

ArrayList<Long> ids = remove.stream()
        .map(MyObj::getId)
        .collect(Collectors.toCollection(ArrayList::new));

Note also, that it is customary to use import static with the Java 8 Stream API so adding:

import static java.util.stream.Collectors.toCollection;

(I hate starred import static, it does nothing but pollute the namespace and add confusion. But selective import static, especially with the Java 8 utility classes, can greatly reduce redundant code)

Would result in:

ArrayList<Long> ids = remove.stream()
        .map(MyObj::getId)
        .collect(toCollection(ArrayList::new));

Answer 2

I use a lot of collector blocks where I create an empty Array and fill it using a loop so I decided I need a utility class of my own not to write the same lines again ad again, here it is:

public class Collections {

    public static <T, O> List<T> collect(Set<O> items, Function<? super O, ? extends T> mapper) {

    return items.stream().map(mapper).collect(Collectors.toCollection(ArrayList::new));
}

}

and use it like this

List<Product> prods = Collections.collect(basket.getOrderItems(), OrderItem::getProduct);

or like this

List<Long> prods = Collections.collect(basket.getOrderItems(), (item)->item.getProduct().getId());

Though it might look like much more easier to read, it seems streams are a little slower in these kind of scenarios, look here