compileOutput.txt

$ javac Generics.java -Xlint:unchecked
Generics.java:17: error: method dblList in class Generics cannot be applied to given types;
      dblList(lst);
      ^
  required: List<EType>
  found: List<CAP#1>
  reason: inferred type does not conform to declared bound(s)
    inferred: CAP#1
    bound(s): Enum<CAP#1>
  where EType is a type-variable:
    EType extends Enum<EType> declared in method <EType>dblList(List<EType>)
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Enum<?> from capture of ? extends Enum<?>
1 error

Generics.java

import java.util.*;

class Generics {
    public enum A { A1, A2 }
    public enum B { B1, B2 }

    public static List<? extends Enum<?>> listFactory(String[] args) { 
        if (args.length == 0) {
            return new ArrayList<>(Arrays.asList(A.A1, A.A2));
        } else {
            return new ArrayList<>(Arrays.asList(B.B1, B.B2));
        }
    }

    public static void main(String[] args) {
        List<? extends Enum<?>> lst = listFactory(args);
        dblList(lst);
        System.out.println(lst);
    }

    public static <EType extends Enum<EType>> void dblList(List<EType> lst) {
        int size = lst.size();
        for (int i = 0; i < size; i++) {
            lst.add(lst.get(i));
        }
    }
}