具有绑定类型参数的上限通配符有什么区别？

Question

Both are using as restriction mechanisms and java document say that upper bound wild card will allow all the sub classes as variables while bound type parameters only accept declared class reference variables only.

My question is. for example if we take Number super class and declared it as a bound type parameters usually it will accept all the sub class variables too in practical scenario regardless of the subclass type. So why the wild cards?

Answer 1

As per documentation The term List<Number> is more restrictive than List<? extends Number> because the former matches a list of type Number only, whereas the latter matches a list of type Number or any of its subclasses. The term List<Number> is more restrictive than List<? extends Number> because the former matches a list of type Number only, whereas the latter matches a list of type Number or any of its subclasses.

upperBounded

Consider this example

import java.util.Arrays;
import java.util.List;


public class Sample {

    public static double sumOfList(List<? extends Number> list) {
        double s = 0.0;
        for (Number n : list)
            s += n.doubleValue();
        return s;   
    }
    public static void main(String[] args) {
        List<Double> ld = Arrays.asList(1.2, 2.3, 3.5);
        System.out.println("sum = " + sumOfList(ld));
    }

}

Above works fine as expected but if you change this line to public static double sumOfList(List<Number> list) {

then this line will complain

System.out.println("sum = " + sumOfList(ld)); that change it to Number ie List<Number> ld or change

or change sub to public static double sumOfList(List<Double> list) {