Java problem log

1. Is it impossible to use polymorphism after using generics? as follows

public class TwoTuple<A, B> {
    public final A first;
    public final B second;
    public TwoTuple(A a, B b) { first = a; second = b; }
    public String toString() {
        return "(" + first + ", " + second + ")";
    }
}

public class ThreeTuple<A, B, C> extends TwoTuple<A, B> {
    public final C third;
    public ThreeTuple(A a, B b, C c) {
        super(a, b);
        third = c;
    }
    public String toString() {
        return "(" + first + ", " + second + ", " + third + ")";
    }
}

public class TupleTest {
    static TwoTuple<String, Integer> f() {
        return new TwoTuple<String, Integer>("hi", 47);
    }

    static ThreeTuple<Amphibian, String, Integer>g() {
        return new ThreeTuple<Amphibian, String, Integer>(new Amphibian(), "hi", 47);
    }

    public static void main(String... args) {
        TwoTuple<String, Integer> ttsi = f();
        System.out.println(ttsi);
        // ttsi = g();
        System.out.println(g());
    }
}

The function g() returns the ThreeTuple type, which inherits from TwoTuple, but an error occurs when assigning it directly.