What does the following Java code output?

public class GenericExample<T extends Number> {
    private T num;
    public GenericExample(T num) {
        this.num = num;
    }
    public double square() {
        return num.doubleValue() * num.doubleValue();
    }
    public static void main(String[] args) {
        GenericExample<Double> example = new GenericExample<>(2.0);
        System.out.println(example.square());
    }
}