How to convert list without generics to list with generics using streams in java?

Question

I would like convert a List without generics to List<MyConcreteType>.

I also need to filter out only my concrete types.

My current stream logic is like:

List list = new ArrayList();
Object collect1 = list.stream().filter((o -> o instanceof MyConcreteType)).collect(Collectors.toList());

But as a result I'm getting an Object instead of a List. Is there a way to convert this Stream to a List<MyConcreteType>?

Just cast it List<MyConcreteType> typedList = (List<MyConcreteType>) list. — Boris the Spider
– Boris the Spider, Commented Dec 19, 2017 at 13:34
@BoristheSpider but I need to filter it first - cause it can contain other items as well — pixel
– pixel, Commented Dec 19, 2017 at 13:35
list.stream().filter(MyConcreteType.class::isInstance).map(MyConcreteType.class::cast).collect(Collectors.toList()) then — Boris the Spider
– Boris the Spider, Commented Dec 19, 2017 at 13:36

Eran · Accepted Answer · 2017-12-19 13:45:53Z

10

Use parameterized types instead of raw types, and use map to cast the objects that pass the filter to MyConcreteType:

List<?> list = new ArrayList();
List<MyConcreteType> collect1 = 
    list.stream()
        .filter((o -> o instanceof MyConcreteType))
        .map(s-> (MyConcreteType) s)
        .collect(Collectors.toList());

or (similar to what Boris suggested in comment):

 List<?> list = new ArrayList();
 List<MyConcreteType> collect1 = 
     list.stream()
         .filter(MyConcreteType.class::isInstance)
         .map(MyConcreteType.class::cast)
         .collect(Collectors.toList());

edited Dec 19, 2017 at 13:45

answered Dec 19, 2017 at 13:35

Eran

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