convert list of map to map using flatMap

问题

How I can merge List<Map<String,String>> to Map<String,String> using flatMap?

Here's what I've tried:

final Map<String, String> result = response
    .stream()
    .collect(Collectors.toMap(
        s -> (String) s.get("key"),
        s -> (String) s.get("value")));
result
    .entrySet()
    .forEach(e -> System.out.println(e.getKey() + " -> " + e.getValue()));

This does not work.

回答1:

Assuming that there are no conflicting keys in the maps contained in your list, try following:

Map<String, String> maps = list.stream()
    .flatMap(map -> map.entrySet().stream())
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

回答2:

If you're ok with overriding the keys, you can just merge the Maps into a single map with collect, even without flatMaps:

public static void main(String[] args) throws Exception {
    final List<Map<String, String>> cavia = new ArrayList<Map<String, String>>() {{
        add(new HashMap<String, String>() {{
            put("key1", "value1");
            put("key2", "value2");
            put("key3", "value3");
            put("key4", "value4");
        }});
        add(new HashMap<String, String>() {{
            put("key5", "value5");
            put("key6", "value6");
            put("key7", "value7");
            put("key8", "value8");
        }});
        add(new HashMap<String, String>() {{
            put("key1", "value1!");
            put("key5", "value5!");
        }});
    }};

    cavia
            .stream()
            .collect(HashMap::new, HashMap::putAll, HashMap::putAll)
            .entrySet()
            .forEach(System.out::println);
}

Will output:

key1=value1!
key2=value2
key5=value5!
key6=value6
key3=value3
key4=value4
key7=value7
key8=value8

回答3:

A very simple way would be to just use putAll:

Map<String, String> result = new HashMap<>();
response.forEach(result::putAll);

If you particularly want to do this in a single stream operation then use a reduction:

response.stream().reduce(HashMap<String, String>::new, Map::putAll);

Or if you really want to use flatMap:

response.stream().map(Map::entrySet).flatMap(Set::stream)
    .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, Map::putAll));

Note the merging function in the final alternative.

来源：https://stackoverflow.com/questions/46964641/convert-list-of-map-to-map-using-flatmap

标签

java

java-8

flatmap