How to convert List<Object> into List<Map<String,String>> in java8

Question

I know how to convert List<Object> into Map<String,String> as you can see in this code:

Map<Long, String> divMap = divs.stream()
  .collect(Collectors.toMap(Div::getId, Div::getName));

The output of this is:

"1": "A", "2": "B"

But I want to return List<Map<String,String>> with the output given below:

["id":"1",Name="A"] ["id":"2",Name="B"]

Try using Map<String, String> result1 = list.stream().collect( Collectors.toMap(string1, string2));, Collectors.toMap is an API fpr doing that — Fady Saad
– Fady Saad, Commented Aug 5, 2017 at 20:53
Please read the question carefully this is what i was doing but i want to return List <Map<String,String> not Map<String,String> — Kunal Batra
– Kunal Batra, Commented Aug 5, 2017 at 21:03

Grzegorz Piwowarek · Accepted Answer · 2017-08-06 07:33:05Z

5

The easiest way would be to help yourself by writing a separate toMap helper method that converts a Div into a Map<String, String>.

private Map<String, String> toMap(Div div) {
    Map<String, String> result = new HashMap<>();
    result.put("id", div.getId().toString());
    result.put("name", div.getName());
    return result;
}

...and everything becomes much easier then:

List<Map<String, String>> divMap = divs.stream()
  .map(this::toMap)
  .collect(Collectors.toList());

Also, it looks like you are trying to reinvent a wheel. Consider using some serialization/deserialization libraries.

edited Aug 6, 2017 at 7:33

answered Aug 5, 2017 at 21:10

Grzegorz Piwowarek

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Szymon Stepniak · Accepted Answer · 2017-08-05 20:59:23Z

2

You can use Stream's reduce method, like:

final List<Div> divs = Arrays.asList(
       new Div("1", "A"),
       new Div("2", "B")
);

final List<Map<String, String>> result = divs.stream()
       .reduce(new ArrayList<>(), (list, div) -> {
            list.add(new HashMap<String, String>() {{
               put("id", div.getId());
               put("name", div.getName());
            }});
            return list;
        }, (a, b) -> a);

Reduce method expects 3 parameters:

initial value (empty list in this case)
BiFunction that provides 2 parameters: accumulator (type List<Map<String,String>> in your case) and current value taken from stream iteration (Div object) and returns a value of accumulator type
comparator function (in this case lambda (a,b) -> a is enough.

This approach pretty similar to folding known from functional programming.

edited Aug 5, 2017 at 20:59

answered Aug 5, 2017 at 20:54

Szymon Stepniak

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2 Comments

Grzegorz Piwowarek Over a year ago

The reduce is an overkill, check my answer :)

Szymon Stepniak Over a year ago

@GrzegorzPiwowarek Indeed, thumbs up for more clever approach :D :+1: I don't know why I thought of reducing in the very first place :)

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