I'm trying to filter a List of objects that implements an interface. And I'm trying to create a generic method for all the classes.
Something like:
interface SomeInterface {
String getFlag();
}
class SomeObject implements SomeInterface {
public String getFlag() {
return "X";
}
}
List<SomeObject> someObjectList = new ArrayList<>();
// Compilation error here
List<SomeObject> filterList = filterGenericList(someObjectList, "X");
private List<?> filterGenericList(List<? extends SomeInterface> objects, String flag) {
return objects.stream()
.filter(it -> it.getFlag().equals(flag))
.collect(Collectors.toList());
}
How do I run away from the compilation Error?
Incompatible types.
Found: 'java.util.List<capture<?>>',
Required: 'java.util.List<SomeObject>'
When you return List<?>
that means the method returns a list of some unknown type. Java doesn't know that it's the same type of list as the input list. To signal that, create a generic type T
and have both the input and output lists be List<T>
.
private <T extends SomeInterface> List<T> filterGenericList(List<T> objects, String flag) {
return objects.stream()
.filter(it -> it.getFlag().equals(flag))
.collect(Collectors.toList());
}