How do I keep the iteration order of a List when using Collections.toMap() on a stream?

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I am creating a Map from a List as follows:

List<String> strings = Arrays.asList("a", "bb", "ccc");

Map<String, Integer> map = strings.stream()
    .collect(Collectors.toMap(Function.identity(), String::length));

I want to keep the same iteration order as was in the List. How can I create a LinkedHashMap using the Collectors.toMap() methods?

Answer

The 2-parameter version of Collectors.toMap() uses a HashMap:

public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(
    Function<? super T, ? extends K> keyMapper, 
    Function<? super T, ? extends U> valueMapper) 
{
    return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
}

To use the 4-parameter version, you can replace:

Collectors.toMap(Function.identity(), String::length)

with:

Collectors.toMap(
    Function.identity(), 
    String::length, 
    (u, v) -> {
        throw new IllegalStateException(String.format("Duplicate key %s", u));
    }, 
    LinkedHashMap::new
)

Or to make it a bit cleaner, write a new toLinkedMap() method and use that:

public class MoreCollectors
{
    public static <T, K, U> Collector<T, ?, Map<K,U>> toLinkedMap(
        Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends U> valueMapper)
    {
        return Collectors.toMap(
            keyMapper,
            valueMapper, 
            (u, v) -> {
                throw new IllegalStateException(String.format("Duplicate key %s", u));
            },
            LinkedHashMap::new
        );
    }
}


Source: stackoverflow