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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?

Answer 1

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 .

Answer 2

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();
}

Answer 3

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)