Filter List<Map<String, String>> using streams

Question

I need to filter a list List<Map<String, String>> by value (if map have searched value, then map add to list) using streams. I'm trying to do this:

static Map<String, String> map1 = new HashMap<>();
static Map<String, String> map2 = new HashMap<>();
static Map<String, String> map3 = new HashMap<>();

static {
    map1.put("key", "value");
    map1.put("key2", "value2");

    map2.put("key3", "value3");
    map2.put("key2", "value2");

    map3.put("key3", "value3");
    map3.put("key4", "value4");
}

public static void main(String[] args) {
    List<Map<String, String>> list = new ArrayList<>();
    Map<String, String> resultMap = new HashMap<>();

    list.add(map1);
    list.add(map2);
    list.add(map3);

    List<Map<String, String>> result = list.stream()
            .flatMap(map -> map.entrySet().stream())
            .filter(value -> value.getValue().equals("value2"))
            .map(x -> resultMap.put(x.getKey(), x.getValue()))
            .collect(Collectors.toList());

}

For execution this code i have error: java: incompatible types: inference variable T has incompatible bounds equality constraints: java.util.Map lower bounds: java.lang.String

Does this approach work? If not, do you get an error? Or do you get an unexpected result? Which? — user11044402
– user11044402, Commented Sep 2, 2019 at 9:59

Eran · Accepted Answer · 2019-09-02 10:14:33Z

2

If you want all the Maps of the input List that contain the "value2" value to appear in the output List, you need:

List<Map<String, String>> result = 
    list.stream()
        .filter(map -> map.entrySet().stream().anyMatch (e->e.getValue().equals("value2")))
        .collect(Collectors.toList());

Or (as Eritrean commented):

List<Map<String, String>> result = 
    list.stream()
        .filter(map -> map.containsValue("value2"))
        .collect(Collectors.toList());

edited Sep 2, 2019 at 10:14

answered Sep 2, 2019 at 10:04

Eran

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2 Comments

Eritrean Over a year ago

Wouldn't it be sufficient to use map.containsValue? list.stream() .filter(map -> map.containsValue("value2")).collect(Collectors.toList());

Holger Over a year ago

If you want to use a Stream operation instead of the straight-forward map.containsValue("value2"), it would still be more natural to use map.values().stream().anyMatch("value2"::equals) instead of streaming over the entrySet() and having to call getValue() for each entry.

robingood · Accepted Answer · 2019-09-02 12:44:09Z

0

Another quick solution is removeIf function. Has the advantage of reusing the same list object.

 list.removeIf(map -> !map.containsValue("value2"));

answered Sep 2, 2019 at 12:44

robingood

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