Lambda expression used to replace the nested for-loop, for some reason, does not work

Question

I have the following situation, where the lambda expression i used to replace a working for loop does not work. Have no clue as to why this fails

public class Abc implements IAbc {

    // some fields
    ...
    // field i'm interested in
    @Inject @Any
    private Instance<HandlerInterface> handlers;

    // more members
    ...
    // method i'm interested in
    @Override
    public boolean hasHandler(List<Order> orders) {
        for (Order anOrder : orders) {
            for (HandlerInterface aHandler : handlers) {
                // following canHandler() is implemented by each 
                // handler that implements the HandlerInterface
                if(aHandler.canHandle(anOrder)) {
                    return true;
                }
            }
        return false;
    }
    // rest of the class content
    .....
}

So I was actually trying to replace the above code, within the method, with Lambdas (to which i'm new). The following was my replacement code

public boolean hasHandler(List<Order> orders) {
    return orders.stream().anyMatch(order ->
        Stream.of(handlers).map(Provider::get).anyMatch(handler ->
            handler.canHandle(order)));
}

The above lambda expression fails at handler.canHandle with AmbiguousResolutionException. I can't find out why it works with the for loop and not with stream. Im sure im doing something wrong here - but have no clue as to what. Any help on this is greatly appreciated.

Why are you inserting a Provider::get step that doesn’t exist in the loop code? — Holger
– Holger, Commented May 4, 2018 at 8:07

Eran · Accepted Answer · 2018-05-04 07:57:18Z

2

I'm assuming Instance<HandlerInterface> implements Iterable<HandlerInterface> (or you wouldn't be able to use it in the enhanced for loop).

This means you can create a Stream of the elements of this Iterable by calling StreamSupport.stream(handlers.spliterator(), false);

So now, your method can be converted to use Streams as follows:

public boolean hasHandler(List<Order> orders) {
    return orders.stream()
                 .anyMatch(o -> StreamSupport.stream(handlers.spliterator(), false)
                                             .anyMatch(h -> h.canHandle(o)));
}

Note: I removed the .map(Provider::get) step, since it has no corresponding step in your original nested loop code.

answered May 4, 2018 at 7:57

Eran

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1 Comment

Jogin Joy Over a year ago

I tried this code out and it worked like magic. Thanks a lot.

Dhawal Kapil · Accepted Answer · 2018-05-04 08:10:29Z

0

In such cases create predicates starting from the innermost condition

First predicate is

if(aHandler.canHandle(anOrder)) {
  return true;
}

You can write this in lambda as

private static Predicate<? super HandlerInterface> p1(Order o) {
    return h -> h.canHandle(o);
}

Second(though is not very intuitive) but it is you want to check that for your order any handler that matches predicate p1, hence write that as

private static Predicate<? super Order> p2(List<HandlerInterface> handlers){
    return o -> handlers.stream().anyMatch(p1(o));
}

Hence complete solution becomes

orders.stream().anyMatch(p2(handlers));

When written inline and removing braces because of single-line predicates you can re-write this as

orders.stream().anyMatch(o -> handlers.stream().anyMatch(h -> h.canHandle(o)));

edited May 4, 2018 at 8:10

answered May 4, 2018 at 8:00

Dhawal Kapil

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3 Comments

Jogin Joy Over a year ago

thanks for the response. The handlers.stream() would not work i think as it's not a Collection

Dhawal Kapil Over a year ago

it should be iterable for sure else we can't use it in advanced for loop

Jogin Joy Over a year ago

handlers is of type Instance here. It's not a collection. The iterable implementations are injected by CDI at runtime. Im new to using all these so im not sure, but this is my guess

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