Question

I’m looking over the code in generics. I’m tyring to understand the code but couldn’t make sense about the usage of parameterized usage of T. Why T has to be used at all why not make it to work with E and R. As far as I understand E refers element and R refers to Tree. Coudn’t make up the usage of generics in the code. Please someone expalin the code.

abstract class Tree<E> {
    public interface Visitor<E, R> {
        public R leaf(E elt);

        public R branch(R left, R right);
    }

    public abstract <R> R visit(Visitor<E, R> v);

    public static <T> Tree<T> leaf(final T e) {
        return new Tree<T>() {
            public <R> R visit(Visitor<T, R> v) {
                return v.leaf(e);
            }
        };
    }

    public static <T> Tree<T> branch(final Tree<T> l, final Tree<T> r) {
        return new Tree<T>() {
            public <R> R visit(Visitor<T, R> v) {
                return v.branch(l.visit(v), r.visit(v));
            }
        };
    }
}

Answer 1

The generic type <E> in this class is not visible for Class’s static method or fields. So you are able to replace the <T> with <E>, it’s only a name, but actually this <E> in the static method is different from the Tree<E>. You can try to change the signature as below to get the compile time error message.

public static Tree<E> leaf(final E e)

To remove the ambiguity, it’s much better to use a different generic type name in the static method.