Converting list of objects with map to array of primitives

Question

I keep finding pieces of something I need to do, but I'm having trouble putting it all together. To start, here is my object, put simply:

Object1
    Object2
        Map<String, Double>

What I need to do is, starting with a list of Object1, get a double[] for the values of the map given a specific key (all Objects in the list have the same N keys in the map).

Here was my starting attempt:

myList.stream().map(Object1::getObject2).map(Object2::getMyMap).map(m -> m.get(key).collect(Collectors.toCollection(ArrayList::new))

I'm not sure how to get to the primitive array from here. If this is good so far, where do I go from here? If there a better way to do this whole thing, I'm open to suggestions. Any help is appreciated, thank you.

Misha · Accepted Answer · 2017-06-05 21:59:47Z

7

Use .mapToDouble to make a DoubleStream:

myList.stream()
    .map(Object1::getObject2)
    .map(Object2::getMyMap)
    .mapToDouble(m -> m.get(key))  // or throw if key is not in map
    .toArray();

answered Jun 5, 2017 at 21:59

Misha

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4 Comments

lexicore Over a year ago

Careful with null values.

Andreas Over a year ago

To not throw, do this:

myList.stream().map(o -> o.getObject2().getMyMap().get(key)).filter(d -> d != null).mapToDouble(Double::doubleValue).toArray()

Misha Over a year ago

Based on OP's description, throwing is probably more appropriate. If one of the maps is missing the key, the data is invalid and the code should fail fast rather than return a shorter array.

fps Over a year ago

Or maybe mapToDouble(m -> m.getOrDefault(key, 0.0)). Who knows? We are speculating here. Let's let the OP decide what's better when there's no entry for the given key.

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