Is there an equivalent of Javascript's Array.map in Java 8?

Question

Is there an equivalent of Javascript's Array.map in Java?

I have been playing with Java 8 :

List<Long> roleList = siteServiceList.stream()
        .map(s -> s.getRoleIdList()).collect(Collectors.toList());

but this doesn't work I don't know why the warning says Incompatible Type.

How can I do this in Java8?

Define "this doesn't work". Post all the relevant code, and the exact and complete error you get. Error messages are important. — JB Nizet
– JB Nizet, Commented Nov 26, 2017 at 10:45

Youcef LAIDANI · Accepted Answer · 2017-11-26 17:46:03Z

19

If roleIdList is a List<Long> and you want to get a List<Long> you have to use flatMap instead :

List<Long> roleList = siteServiceList.stream()
                .flatMap(s -> s.getRoleIdList().stream())
                .collect(Collectors.toList());

If you insist using map the return type should be List<List<Long>> :

List<List<Long>> roleList = siteServiceList.stream()
    .map(MyObject::getRoleIdList)
    .collect(Collectors.toList());

edited Nov 26, 2017 at 17:46

answered Nov 26, 2017 at 10:50

Youcef LAIDANI

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