How to create a multiple new Hashmaps using java 8 streams?

Question

I want to achieve the below snippet using java 8 streams.Any answers would be appreciable.I need to set different values for the same key 'amount'.so that i am creating new hashmap inside a loop.It will be like [{amount=100},{amount=200}].

List<String> data = Arrays.asList('', '', '');
List<Map<String,Object> finalList = new ArrayList();
for(String dataIterate : data) {
    Map<String,Object> map = new HashMap();
    map.put("amount",dataIterate);
    finalList.add(map);
}

And what are you going to do with all those `HashMap`s? Simply let the garbage collector release them? — Eran, Jul 12 '18 at 10:18
Firstly this this code doesn't compile. Secondly, you try to add one key with different value? Finally, what you are going to do with map? Not clearly — Maxim Kasyanov, Jul 12 '18 at 10:19
and why not a `Pair` or `SimpleEntry` for a key value instead of a `Map` with a single entry — Eugene, Jul 12 '18 at 10:19
I want to set different values for the same key 'amount'.so that i am creating new hashmap inside a loop.It will be like [{amount=100},{amount=200}].I for to add list in that loop — RathanaKumar, Jul 12 '18 at 10:50

Ravindra Ranwala · Answer 1 · 2018-07-12T10:26:23.000

2

Is that what you are asking for,

List<Map<String, String>> result = data.stream()
        .map(s -> Stream.of(s).collect(Collectors.toMap(s2 -> "amount", Function.identity())))
        .collect(Collectors.toList());

Get each element in the source, create a Map with the given constant key and that elements value. Finally collect them into a List

edited Jul 12 '18 at 10:26

answered Jul 12 '18 at 10:21

Ravindra Ranwala

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Ravi,am getting compilation error like cannot infer type arguments – RathanaKumar Jul 12 '18 at 10:53
In which line? It's working fine for me. – Ravindra Ranwala Jul 12 '18 at 10:56
Use this as data source, `List data = Arrays.asList("a", "b", "c", "d")` – Ravindra Ranwala Jul 12 '18 at 10:58

score 2 · Answer 2 · answered Jul 12 '18 at 10:26

2

 data.stream()
     .map(dataIterate -> {
           Map<String,Object> map = new HashMap();
           map.put("amount", dataIterate);
           return map;
     })
     .collect(Collectors.toList()) // or any other terminal operation you might need

But it's really unclear why not a simpler SimpleEntry or Pair, or anything else that can hold a key/value instead of a Map with a single Entry

answered Jul 12 '18 at 10:26

Eugene

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Thanks a lot Eugene.This is what ,am expected!!. – RathanaKumar Jul 12 '18 at 10:55
2

`.map(dataIterate -> Collections.singletonMap("amount", dataIterate))` or, if it has to be a `HashMap`, `.map(dataIterate -> new HashMap<>(Collections.singletonMap("amount", dataIterate)))` – Holger Jul 12 '18 at 11:04
Thank you @Holger.I will try – RathanaKumar Jul 12 '18 at 11:06

score 0 · Answer 3 · answered Jul 12 '18 at 11:14

0

You can do as follow, using initialization block for the Map :

List<Map<String, String>> map = 
           data.stream().map(data -> new HashMap<String, String>(){{put("amount", data);}})
                        .collect(Collectors.toList());

Workable Demo

But to store multiples values for the same keys, you'd better use a Map<String, List<String>>

answered Jul 12 '18 at 11:14

azro

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This is prone to memory leaks, as this subclass of `HashMap` will keep a reference to `data` even if you remove the entry, as well as a reference to the outer instance if you use this in a non-static context. – Holger Jul 12 '18 at 11:21
@Holger nore sure to understand, how it is so different of what you proposed as comment below ? – azro Jul 12 '18 at 11:22
The curly braces create an anonymous inner class subclassing `HashMap` and capturing the variables used inside the body (i.e. `data`). Further, unlike lambda expressions, anonymous inner classes always capture a reference to the outer `this`, even if it’s not used. This is entirely different to performing a method invocation to `Collections.singletonMap("amount", dataIterate)` which will just return an instance of a map containing the specified mapping. See also [What is Double Brace initialization in Java?](https://stackoverflow.com/a/27521360/2711488)… – Holger Jul 12 '18 at 11:27
Thank u @azro.This is the thing i expected. – RathanaKumar Jul 12 '18 at 13:35

How to create a multiple new Hashmaps using java 8 streams?

3 Answers3