问号(?)代表通配符，代表泛型中的未知类型。有时您可能想限制传递给类型参数的种类。如，对数字进行操作的方法可能只希望接受Number或其子类。
要声明一个上限的通配符参数，需要使用 “?”，其次是extends关键字，然后是其上限。
以下示例说明了如何使用extends来指定上限通配符。
package com.learnfk;
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));
</span><span class="typ">List</span><span class="pun"><</span><span class="typ">Double</span><span class="pun">></span><span class="pln"> doubleList </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Arrays</span><span class="pun">.</span><span class="pln">asList</span><span class="pun">(</span><span class="lit">1.2</span><span class="pun">,</span><span class="pln"> </span><span class="lit">2.3</span><span class="pun">,</span><span class="pln"> </span><span class="lit">3.5</span><span class="pun">);</span><span class="pln">
</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"sum="</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> sum</span><span class="pun">(</span><span class="pln">doubleList</span><span class="pun">));</span><span class="pln">
}
}
这将产生以下输出 -
sum=6.0 sum=7.0
