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

Question

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 1

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         );     } } 

Answer 2

Make your own Supplier, Accumulator and Combiner:

List<String> myList = Arrays.asList("a", "bb", "ccc");  // or since java 9 List.of("a", "bb", "ccc");      LinkedHashMap<String, Integer> mapInOrder = myList                         .stream()                         .collect(                           LinkedHashMap::new,                           // Supplier                           (map, item) -> map.put(item, item.length()),  // Accumulator                           Map::putAll);                                 // Combiner  System.out.println(mapInOrder);  // prints {a=1, bb=2, ccc=3}