Question

I’m going through example.

public interface Visitor<T> {
    public void visit(T e);
}

class Sum<T extends Integer> implements Visitor<T> {

    private int sum = 0;



    @Override
        public void visit(T element) {
            sum += element;
        }

        public T value() {
            return sum;
        }

}

The return statement T value() return sum is giving error stating that Type mismatch: cannot convert from int to T. Why it is so and how to fix the problem.

Answer 1

In your case there are two problems:

1. T extends Integer is not exactly valid, because Integer cannot be extended, i.e. it’s a final class. Therefore you can safely change your class declaration to

   class Sum implements Visitor<Integer> { ...
   

2. Secondly, T value() method is not part of the interface and therefore does not need (should not) be of type T. You can safely replace it with public int value() or public Integer value().

Therefore the resulting code will look like this:

class Sum implements Visitor<Integer> {
    private int sum = 0;

    @Override
    public void visit(Integer element) {
        sum += element.intValue();
    }

    public int value() {
        return sum;
    }
}