Find a map in list of map java stream

Question

I am iterating through a List of Hashmap to find the required HashMap object using the following code.

public static Map<String, String> extractMap(List<Map<String, String>> mapList, String currentIp) {
    for (Map<String, String> asd : mapList) {
        if (asd.get("ip").equals(currentIp)) {
            return asd;
        }
    }
    return null;
}

I was thinking about using Java 8 stream. This is the code I used to display the required object.

public static void displayRequiredMapFromList(List<Map<String, String>> mapList, String currentIp) {
    mapList.stream().filter(e -> e.get("ip").equals(currentIp)).forEach(System.out::println);
}

I couldn't get the required Map from the stream using following code

public static Map<String, String> extractMapByStream(List<Map<String, String>> mapList, String currentIp) {
    return mapList.stream().filter(e -> e.get("ip").equals(currentIp))
            .collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
}

This causes syntax error Type mismatch: cannot convert from Map to Map. What do I have to put here to get Map?

mapList.stream().filter(e -> e.get("ip").equals(currentIp)).findFirst() ... Note that this returns an Optional<Map ...> — griFlo
– griFlo, Commented Jan 12, 2018 at 10:54
Consider defining classes rather than nested collections. It gets hard to understand what a List<Map<Set<String>, Map<Integer, List<HttpClient>>>> is supposed to represent. — Michael
– Michael, Commented Jan 12, 2018 at 10:55

Christoffer Hammarström · Accepted Answer · 2018-01-12 10:51:58Z

7

You don't want to .collect anything. You want to find the first map that matches the predicate.

So you should use .findFirst() instead of .collect().

toMap() is for building a Map from the elements in the stream.

But you don't want to do that, each element is already a Map.

answered Jan 12, 2018 at 10:51

Christoffer Hammarström

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4 Comments

Lino Over a year ago

if order is not needed, one might also use findAny()

Muneeb Mirza Over a year ago

I have accepted @Andrew's answer only because it completely translated my for loop method to stream with else null that is. Thank you.

Christoffer Hammarström Over a year ago

@MuneebMirza That's fine. :) I recommend that you return the Optional<Map<String,String>> instead of .orElse(null). What are you doing with the null that is returned anyway?

Muneeb Mirza Over a year ago

@ChristofferHammarström so you are saying that I should instead return at findFirst()? Yes i think this makes more sense. Thanks :)

Andrew · Accepted Answer · 2018-01-12 11:00:22Z

1

This will will work, the other examples without orElse() don't compile (at least they don't in my IDE).

mapList.stream()
    .filter(asd -> asd.get("ip").equals(currentIp))
    .findFirst()
    .orElse(null);

The only thing I would add as a suggestion is to return Collections.emptyMap(), this will save a null check in the calling code.

To get the code to compile without orElse you need to change the method signature to:

public static Optional<Map<String, String>> extractMap(List<Map<String, String>> mapList, String currentIp)

answered Jan 12, 2018 at 11:00

Andrew

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Comments

Rishikesh Dhokare · Accepted Answer · 2018-01-12 14:54:28Z

1

User this

    public static Map<String, String> extractMapByStream(List<Map<String, String>> mapList, String currentIp) {
        return mapList.stream().filter(e -> e.get("ip").equals(currentIp))
            .findFirst().get();
}

edited Jan 12, 2018 at 14:54

answered Jan 12, 2018 at 10:53

Rishikesh Dhokare

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3 Comments

Lino Over a year ago

compiler error: Type Optional<Map<String, String>> is not assignable to type Map<String, String>

Rishikesh Dhokare Over a year ago

thats because findFirst() returns Optional and we have to call get() on it. I have updated the answer.

Christoffer Hammarström Over a year ago

@RishikeshDhokare No, you don't blindly call .get() on an Optional if you're not sure that it's non-empty. That defeats the whole purpose of Optional.

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