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Method reference does not work with the Decorator pattern

Hello all!
I faced with a problem, when I use method reference as a function in Collectors.groupingBy I get an error: "no suitable method found for collect". But! if I replaced method reference with a lambda expression everything works fine.
Here is the code sample:
interface Iface{ public int getPropertyOfClassA(); public void setPropertyOfClassA(int propertyOfClassA); } class A implements Iface{ private int propertyOfClassA; @Override public int getPropertyOfClassA() { return propertyOfClassA; } @Override public void setPropertyOfClassA(int propertyOfClassA) { this.propertyOfClassA = propertyOfClassA; } } class B{ private A objectA; public int getPropertyOfClassA() {return objectA.getPropertyOfClassA();} } class C extends B { int propertyOfClassC; public int getPropertyOfClassC() { return propertyOfClassC; } public void setPropertyOfClassC(int propertyOfClassC) { this.propertyOfClassC = propertyOfClassC; } } public class example { public static void main(String[] ars){ List<B> listOfB = new ArrayList<>(); Map<Integer, List<B>> groupedByPropertyOfClassA = listOfB .stream() .collect(Collectors.groupingBy(C::getPropertyOfClassA)); } }
Change "C::getPropertyOfClassA" with "objC -> objC.getPropertyOfClassA()" and it works!
Any ideas?
Best Answer
-
You have a Stream<B>. Not every B is a C, so you can't apply a method of C to each element (even though that method is defined in
.
It also works if you change it to
collect(Collectors.groupingBy(B::getPropertyOfClassA));
Answers
-
You have a Stream<B>. Not every B is a C, so you can't apply a method of C to each element (even though that method is defined in
.
It also works if you change it to
collect(Collectors.groupingBy(B::getPropertyOfClassA));
-
It really helps! Stupid me )
If you change C::getPropertyOfClassA to B::getPropertyOfClassA everything works fine.
Thanx )