Java泛型上限通配符简介说明

上限通配符的简介

上限通配符的简介:
     上限通配符只需在？后面 紧跟extends关键字
	 然后加上"上限类型"

package com.java265;
import java.util.Arrays;
import java.util.list;

public class GenericsTester {
   public static double sum(List<? extends Number> numberlist) {
      double sum = 0.0;
      for (Number n : numberlist) sum += n.doubleValue();
      return sum;
   }

   public static void main(String args[]) {
      List<Integer> integerList = Arrays.asList(1, 2, 3);
      System.out.println("sum = " + sum(integerList));

      List<Double> doubleList = Arrays.asList(1.2, 2.3, 3.5);
      System.out.println("sum = " + sum(doubleList));
   }
}

-----运行以上代码，将输出以下信息-----
sum = 6.0
sum = 7.0