Merging two Map<String, Integer> with Java 8 Stream API
I have two (or more) Map<String, Integer>
objects. I'd like to merge them with Java 8 Stream API in a way that values for common keys should be the maximum of the values.
@Test
public void test14() throws Exception {
Map<String, Integer> m1 = ImmutableMap.of("a", 2, "b", 3);
Map<String, Integer> m2 = ImmutableMap.of("a", 3, "c", 4);
List<Map<String, Integer>> list = newArrayList(m1, m2);
Map<String, Integer> mx = list.stream()... // TODO
Map<String, Integer> expected = ImmutableMap.of("a", 3, "b", 3, "c", 4);
assertEquals(expected, mx);
}
How can I make this test method green?
I've played with collect
and Collectors
for a while without any success.
(ImmutableMap
and newArrayList
are from Google Guava.)
Solution 1:
@Test
public void test14() throws Exception {
Map<String, Integer> m1 = ImmutableMap.of("a", 2, "b", 3);
Map<String, Integer> m2 = ImmutableMap.of("a", 3, "c", 4);
Map<String, Integer> mx = Stream.of(m1, m2)
.map(Map::entrySet) // converts each map into an entry set
.flatMap(Collection::stream) // converts each set into an entry stream, then
// "concatenates" it in place of the original set
.collect(
Collectors.toMap( // collects into a map
Map.Entry::getKey, // where each entry is based
Map.Entry::getValue, // on the entries in the stream
Integer::max // such that if a value already exist for
// a given key, the max of the old
// and new value is taken
)
)
;
/* Use the following if you want to create the map with parallel streams
Map<String, Integer> mx = Stream.of(m1, m2)
.parallel()
.map(Map::entrySet) // converts each map into an entry set
.flatMap(Collection::stream) // converts each set into an entry stream, then
// "concatenates" it in place of the original set
.collect(
Collectors.toConcurrentMap( // collects into a map
Map.Entry::getKey, // where each entry is based
Map.Entry::getValue, // on the entries in the stream
Integer::max // such that if a value already exist for
// a given key, the max of the old
// and new value is taken
)
)
;
*/
Map<String, Integer> expected = ImmutableMap.of("a", 3, "b", 3, "c", 4);
assertEquals(expected, mx);
}
Solution 2:
Map<String, Integer> mx = new HashMap<>(m1);
m2.forEach((k, v) -> mx.merge(k, v, Integer::max));
Solution 3:
mx = list.stream().collect(HashMap::new,
(a, b) -> b.forEach((k, v) -> a.merge(k, v, Integer::max)),
Map::putAll);
This covers the general case for any size list and should work with any types, just swap out the Integer::max
and/or HashMap::new
as desired.
If you don't care which value comes out in a merge, there's a much cleaner solution:
mx = list.stream().collect(HashMap::new, Map::putAll, Map::putAll);
And as generic methods:
public static <K, V> Map<K, V> mergeMaps(Stream<? extends Map<K, V>> stream) {
return stream.collect(HashMap::new, Map::putAll, Map::putAll);
}
public static <K, V, M extends Map<K, V>> M mergeMaps(Stream<? extends Map<K, V>> stream,
BinaryOperator<V> mergeFunction, Supplier<M> mapSupplier) {
return stream.collect(mapSupplier,
(a, b) -> b.forEach((k, v) -> a.merge(k, v, mergeFunction)),
Map::putAll);
}
Solution 4:
I've created a visual representation of what @srborlongan did, for anyone who might be interested.
Solution 5:
I added my contribution to the proton pack library which contains utility methods for the Stream API. Here's how you could achieve what you want:
Map<String, Integer> mx = MapStream.ofMaps(m1, m2).mergeKeys(Integer::max).collect();
Basically mergeKeys
will collect the key-value pairs in a new map (providing a merge function is optional, you'll end up with a Map<String, List<Integer>>
otherwise) and recall stream()
on the entrySet()
to get a new MapStream
. Then use collect()
to get the resulting map.