VarianceQuestion.java

public abstract class Parent<T> {
    
    public abstract Parent<T> goodMethod(Function<Integer, Parent<T>> f);

    public abstract <TT extends T> Parent<TT> badMethod(Function<Integer, Parent<TT>> f);

    private static final class Child<T> extends Parent<T> {

        T value;

        @Override
        public Parent<T> goodMethod(final Function<Integer, Parent<T>> f) {
            return this; //compile
        }

        @Override
        public <TT extends T> Parent<TT> badMethod(final Function<Integer, Parent<TT>> f) {
            return this; //does not compile
        }
    }

}