Java8 stream with nested objects

Question

So I'm new to Java8. I've read about streams, but most examples are very basic. I was wondering how it'd be done with nested objects. Here is an example from my code:

for (Gas gas : gases) {
  resourceNodes.add(gas.getRefinery().unit());
}

It seems like I should be able to one-line this with a stream, but I can't quite figure it out. Could someone provide an answer with a stream. Is there a way to use :: syntax with nested methods too?

Edit: To clarify the example, getRefinery() returns an object of type: UnitInPool, whose method unit() returns an object of type: Unit. resourceNodes is an ArrayList of Unit.

Answer 1

The :: syntax that you refer to is what's known as a method reference.

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Assuming resourceNodes is unassigned (or empty, in which case you can remove any previous assignment) prior to the for-loop, then you'd want to first map each Gas to whatever type unit() returns, and then collect the Stream to a List:

resourceNodes = gases.stream()
                     .map(Gas::getRefinery)
                     .map(GasRefinery::unit)
                     .collect(Collectors.toList());

Otherwise, if your goal is to simply add to resourceNodes, then it would be very similar:

resourceNodes.addAll(gases.stream()
                          .map(Gas::getRefinery)
                          .map(GasRefinery::unit)
                          .collect(Collectors.toList()));