Core Java

Streaming over Maps with Java 8

mapping-streams-smallIn this article I will show you how Speedment Open Source stream efficiently over standard Java maps, expanding the Stream interface into something called a MapStream! This addition will make it easier to keep your streams concrete and readable even in complex scenarios. Hopefully this will allow you to keep streaming without prematurely collecting the result.

One of the largest features in Java 8 was the ability to stream over collections of objects. By adding the .stream()-method into the Collection interface, every collection in the java language was suddenly expanded with this new ability. Other data structures like the Map-interface, does not implement the method as they are not strictly speaking collections.

The MapStream will take two type parameters, a key and a value. It will also extends the standard Stream interface by specifying Map.Entry<K, V> as type parameter. This will allow us to construct a MapStream directly from any Java map.

public interface MapStream<K, V> extends Stream<Map.Entry<K, V>> {

The concept of polymorphism tells us that a child component may change the return type of an overidden method as long as the new return type is a more concrete implementation of the old return type. We will use this when defining the MapStream interface so that for each chaining operation, a MapStream is returned instead of a Stream.

public interface MapStream<K, V> extends Stream<Map.Entry<K, V>> {

    MapStream<K, V> filter(Predicate<? super Map.Entry<K, V>> predicate);

    MapStream<K, V> distinct();

    MapStream<K, V> sorted(Comparator<? super Map.Entry<K, V>> comparator);


Some operations will still need to return an ordinary Stream. If the operation change the type of the streamed element, we can’t ensure that the new type will be a Map.Entry. We can, however, add additional methods for mapping between types with Key-Value pairs.

    <R> Stream<R> map(Function<? super Map.Entry<K, V>, ? extends R> mapper);
    <R> Stream<R> map(BiFunction<? super K, ? super V, ? extends R> mapper);

In addition to the Function that let the user map from an Entry to something else, he or she can also map from a Key-Value-pair to something else. This is convenient, sure, but we can also add more specific mapping operations now that we are working with pairs of values.

<R> MapStream<R, V> mapKey(BiFunction<? super K, ? super V, ? extends R> mapper);

    <R> MapStream<K, R> mapValue(BiFunction<? super K, ? super V, ? extends R> mapper);

The difference doesn’t look like much, but the difference is apparent when using the API:

// With MapsStream
final Map<String, List<Long>> map = ...;
    .mapKey((k, v) -> k + " (" + v.size() + ")")
    .flatMapValue((k, v) ->
    .map((k, v) -> k + " >> " + v)

// Without MapStream
final Map<String, List<Long>> map = ...;
    .map(e -> new AbstractMap.SimpleEntry<>(
         e.getKey() + " (" + e.getValue().size() + ")"),
    .flatMap(e -> e.getValue().stream()
        .map(v -> new AbstractMap.SimpleEntry<>(e.getKey(), v))
    .map(e -> e.getKey() + " >> " + e.getValue())
Reference: Streaming over Maps with Java 8 from our JCG partner Emil Forslund at the Age of Java blog.
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