Eine (typ)generische Stack-Implementierung

StackGen.java

// Zur Erläuterung mein Video auf YouTube: https://youtu.be/rSuE2vIUzzk

interface Stackable<T> {
    boolean isEmpty();
    default T top() { throw new UnsupportedOperationException(); }
    default Stackable<T> pop() { throw new UnsupportedOperationException(); }
    Stackable<T> push(T element);
}

class StackWithElements<T> implements Stackable<T> {
    private T element;
    private Stackable<T> previous;
    @SafeVarargs static <T> Stackable<T> of(T... elements) {
        Stackable<T> s = new EmptyStack<T>();
        for (T element : elements) {
            s = s.push(element);
        }
        return s;
    }
    StackWithElements(Stackable<T> previous, T element) {
        assert Objects.nonNull(previous);
        this.previous = previous;
        this.element = element;
    }
    public boolean isEmpty() { return false; }
    public Stackable<T> push(T element) {
        return new StackWithElements<T>(this, element);
    }
    public T top() { return element; }
    public Stackable<T> pop() { return previous; }
    @Override
    public String toString() {
        return pop() + " <- " + top(); 
    }
}

class EmptyStack<T> implements Stackable<T> {
    public boolean isEmpty() { return true; }
    public Stackable<T> push(T element) {
        return new StackWithElements<T>(this, element);
    }
    @Override
    public String toString() {
        return "[ ]";
    }
}