Let’s Stream a Map in Java 8 with jOOλ

Lukas EderOctober 25th, 2014Last Updated: October 25th, 2014

0 29 2 minutes read

I wanted to find an easy way to stream a Map in Java 8. Guess what? There isn’t!

What I would’ve expected for convenience is the following method:

public interface Map<K, V> {
 
    default Stream<Entry<K, V>> stream() {
        return entrySet().stream();
    }    
}

But there’s no such method. There are probably a variety of reasons why such a method shouldn’t exist, e.g.:

There’s no “clear” preference for entrySet() being chosen over keySet() or values(), as a stream source
Map isn’t really a collection. It’s not even an Iterable
That wasn’t the design goal
The EG didn’t have enough time

Well, there is a very compelling reason for Map to have been retrofitted to provide both an entrySet().stream() and to finally implement Iterable<Entry<K, V>>. And that reason is the fact that we now have Map.forEach():

default void forEach(
        BiConsumer<? super K, ? super V> action) {
    Objects.requireNonNull(action);
    for (Map.Entry<K, V> entry : entrySet()) {
        K k;
        V v;
        try {
            k = entry.getKey();
            v = entry.getValue();
        } catch(IllegalStateException ise) {
            // this usually means the entry is no longer in the map.
            throw new ConcurrentModificationException(ise);
        }
        action.accept(k, v);
    }
}

forEach() in this case accepts a BiConsumer that really consumes entries in the map. If you search through JDK source code, there are really very few references to the BiConsumer type outside of Map.forEach() and perhaps a couple of CompletableFuture methods and a couple of streams collection methods.

So, one could almost assume that BiConsumer was strongly driven by the needs of this forEach() method, which would be a strong case for making Map.Entry a more important type throughout the collections API (we would have preferred the type Tuple2, of course).

Let’s continue this line of thought. There is also Iterable.forEach():

public interface Iterable<T> {
    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }
}

Both Map.forEach() and Iterable.forEach() intuitively iterate the “entries” of their respective collection model, although there is a subtle difference:

Iterable.forEach() expects a Consumer taking a single value
Map.forEach() expects a BiConsumer taking two values: the key and the value (NOT a Map.Entry!)

Think about it this way:

This makes the two methods incompatible in a “duck typing sense”, which makes the two types even more different

Bummer!

Improving Map with jOOλ

We find that quirky and counter-intuitive. forEach() is really not the only use-case of Map traversal and transformation. We’d love to have a Stream<Entry<K, V>>, or even better, a Stream<Tuple2<T1, T2>>. So we implemented that in jOOλ, a library which we’ve developed for our integration tests at jOOQ. With jOOλ, you can now wrap a Map in a Seq type (“Seq” for sequential stream, a stream with many more functional features):

Map<Integer, String> map = new LinkedHashMap<>();
map.put(1, "a");
map.put(2, "b");
map.put(3, "c");
 
assertEquals(
  Arrays.asList(
    tuple(1, "a"), 
    tuple(2, "b"), 
    tuple(3, "c")
  ),
 
  Seq.seq(map).toList()
);

What you can do with it? How about creating a new Map, swapping keys and values in one go:

System.out.println(
  Seq.seq(map)
     .map(Tuple2::swap)
     .toMap(Tuple2::v1, Tuple2::v2)
);
 
System.out.println(
  Seq.seq(map)
     .toMap(Tuple2::v2, Tuple2::v1)
);

Both of the above will yield:

1	`{a=1, b=2, c=3}`

Just for the record, here’s how to swap keys and values with standard JDK API:

System.out.println(
  map.entrySet()
     .stream()
     .collect(Collectors.toMap(
         Map.Entry::getValue, 
         Map.Entry::getKey
     ))
);