Java 8 retrieve Map's values into an array with stream and filter

Question

Would someone help me with getting the array of the map values with stream and filtering?

public class TheMap extends HashMap<String, String> {
    public TheMap(String name, String title) {
        super.put("name", name);
        super.put("title", title);
    }

    public static void main(final String[] args) {
        Map<Long, Map<String, String>>map = new HashMap<>();            

        map.put(0L, null);
        map.put(1L, new TheMap("jane", "engineer"));
        map.put(2L, new TheMap("john", "engineer"));
        map.put(3L, new TheMap(null, "manager"));
        map.put(4L, new TheMap("who", null));
        map.put(5L, new TheMap(null, null));
    }
}

The result that I am looking for is an ArrayList<TheMap> with only these two entries:

TheMap("jane", "engineer")
TheMap("john", "engineer")

Basically, retrieve TheMap with none-null name and title.

This is weird, you're putting an overridden Map into another map? Are you sure you want to do this? Don't you want a bean with just name and title properties? — Daniel Scott
– Daniel Scott, Commented Apr 4, 2017 at 20:23

Sébastien Temprado · Accepted Answer · 2017-04-04 20:35:24Z

4

List<Map<String, String>> list = 
        map.values().stream().filter(v -> 
                              v != null && 
                              !v.entrySet().isEmpty() &&
                              !v.containsValue(null)).
        collect(Collectors.toList());

answered Apr 4, 2017 at 20:35

Sébastien Temprado

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4 Comments

Holger Over a year ago

what’s the point of v.entrySet().isEmpty()? That doesn’t produce different results than v.isEmpty(). Besides that, empty maps are not within the OP’s problem description anyway.

TX T Over a year ago

both produce the same result. my original problem statement did not include the empty Map, but it is good to have. this way, when I add a constructor TheMap() {}, the logic would still work.

TX T Over a year ago

Can I post a follow-up question here. I have code to search the List<TheMap> based on Map<String, String>. Just like to know if my code is good. Map<String, String> srch = new HashMap<>(); srch.put("name", "jane"); List<Map<String, String>>fnd = list.stream().filter(e -> { boolean f = true; for (Entry<String, String>s: srch.entrySet()) f &= (e.get(s.getKey()) == s.getValue()); return f;}).collect(Collectors.toList());

Sébastien Temprado Over a year ago

Please, post a new question or update this one. As a comment, it's difficult to read your new question

VHS · Accepted Answer · 2017-04-06 19:54:34Z

3

If you need an arrayList of TheMap, try the following way:

ArrayList<TheMap> as = map.values()
   .stream()
   .filter(v -> v != null && v.get("name") != null && v.get("title") != null)
   .map(m -> (TheMap)m)
   .collect(Collectors.toCollection(ArrayList::new)));

edited Apr 6, 2017 at 19:54

answered Apr 4, 2017 at 20:44

VHS

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5 Comments

Eugene Over a year ago

there are not guarantees that the returned List is actually an ArrayList, so your cast might break in a future release. Collectors.toCollection is what you want in this case

VHS Over a year ago

@Holger. Sorry, I should have paid more attention on the filter to see that it was only filtering out the null values but not the key / values inside the values. See my edit.

VHS Over a year ago

@Eugene, as per the source code of Collectors.toList, it does in fact instantiate and return an ArrayList. Am I missing anything?

Eugene Over a year ago

@VHS you are not missing anything, but that is exactly what makes this an implementation detail that might change. Look at the javadoc of toList and the guarantees that it does not make.

VHS Over a year ago

@Eugene, thanks for a nice suggestion. I edited my answer to use toCollection instead.

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