How to use stream API with HashMap

Question

I have the below code and I am using lamda expression to set the key and value pair of the hashMap to the list of Stats. I am new to stream API and I want to do it in stream API, can anyone help with this. Thanks.

    final List<Stats> values = new ArrayList<>();
        if (countMap != null) {
                    countMap.forEach((k, v) -> {
                        final Stats value = new Stats();
                        value.setType(k);
                        value.setCount(v);
                        final double percent = getPercentage(v, total.get());
                        value.setPercent(percent);
                        values.add(value);
                    });
                }

private double getPercentage(final double count, final double total) {
        final double percent = (count / (total * 1.0)) * 100;
        final BigDecimal bd = new BigDecimal(percent).setScale(2, RoundingMode.HALF_UP);
        return bd.doubleValue();
    }

this would be _a lot_ simpler if you could provide a constructor for `Stats` that would take a `k` and `v` (whatever those types are...) — Eugene, Oct 17 '19 at 19:38
Possible duplicate of [How to convert a Map to Arraylist of Objects?](https://stackoverflow.com/questions/42694964/how-to-convert-a-map-to-arraylist-of-objects) — Michał Krzywański, Oct 17 '19 at 19:52
Another [one](https://stackoverflow.com/questions/49030336/converting-mapstring-string-to-listobject-in-java8) and also [this](https://stackoverflow.com/questions/21233183/java-8-stream-mapk-v-to-listt) — Michał Krzywański, Oct 17 '19 at 19:54

Naman · Answer 1 · 2019-10-17T19:53:28.327

You can simplify the code with an all argument constructor defined, to something like:

final List<Stats> values = countMap.entrySet().stream()
        .map(e -> new Stats(e.getKey(), e.getValue(), getPercentage(e.getValue(), total.get())))
        .collect(Collectors.toList());

where the constructor resolves to

new Stats(<type>, <count>, <percent>)

Note: Avoid assigning null value to a Map, instead initialize it as empty. The code with that additional check would look like:

final List<Stats> values = countMap != null ? countMap.entrySet().stream()
        .map(e -> new Stats(e.getKey(), e.getValue(), getPercentage(e.getValue(), total.get())))
        .collect(Collectors.toList()) : Collections.emptyList();

score 0 · Answer 2 · answered Oct 17 '19 at 19:39

0

you can grab entrySet and iterate over it using streams

answered Oct 17 '19 at 19:39

Ilya Aliaksandrovich

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How to use stream API with HashMap

2 Answers2