Example of a problem where I'm trying to have a class implement an interface but return a couple of extra methods, but I cannot use that class in places where an interface method expects the return type to be the base interface. Edit: Now with a solution.

Test.java

import java.util.*;
public class Test {
    public static interface Position {}
    
    public static interface Axis {
        public List<? extends Position> getPositions();
    }
    
    public static class GoodAxis implements Axis {
        private List<Position> positions = new ArrayList<>();

        @Override
        public List<Position> getPositions() {
            return positions;
        }
    }
    
    public static class MyPosition implements Position {
        public String exclaim() { return "YAY!"; }
    }
    
    public static class MyBadPositionList extends AbstractList<MyPosition> {
        @Override
        public MyPosition get(int index) {
            return null;
        }

        @Override
        public int size() {
            return 0;
        }

        public String getExclamationForPosition(int index) {
            return get(index).exclaim();
        }
    }
    
    /* XXX: If uncommented, javac reports incompatible types.
    public static class BadAxis implements Axis {
        private MyBadPositionList myPositions = new MyBadPositionList();
        
        @Override
        public List<Position> getPositions() {
            return myPositions;
        }
    }
    */

    /* XXX: If uncommented, javac reports an unchecked conversion
    public static class MyOkayPositionList<T> extends AbstractList<MyPosition> {
        @Override
        public MyPosition get(int index) {
            return null;
        }

        @Override
        public int size() {
            return 0;
        }

        public String getExclamationForPosition(int index) {
            return get(index).exclaim();
        }
    }


    public static class OkayAxis implements Axis {
        private MyOkayPositionList<MyPosition> myPositions = new MyOkayPositionList<MyPosition>();

        @Override
        public List<MyPosition> getPositions() {
            // XXX: javac reports unchecked conversion
            return myPositions;
        }
    }
    */


    public static class MyGoodPositionList extends AbstractList<Position> {
        private List<MyPosition> list = new ArrayList<>();

        @Override
        public Position get(int index) {
            return list.get(index);
        }

        @Override
        public int size() {
            return list.size();
        }

        public String getExclamationForPosition(int index) {
            return list.get(index).exclaim();
        }

        public List<Position> getDowncastPositions() {
            return new AbstractList<Position>() {
                @Override
                public Position get(int index) {
                    return list.get(index);
                }

                @Override
                public int size() {
                    return size();
                }
            };
        }
    }


    public static class MyGoodAxis implements Axis {
        private MyGoodPositionList myPositions = new MyGoodPositionList();

        @Override
        public List<Position> getPositions() {
            return myPositions.getDowncastPositions();
        }

        public String getExclamationForPosition(int index) {
            return myPositions.getExclamationForPosition(index);
        }
    }
}