问号(?)代表通配符，代表泛型中的未知类型，有时您可能想限制允许传递给参数的下限类型。
要声明一个下限的通配符参数，使用 “?”，然后使用super关键字，再列出其下限。
以下示例说明了如何使用super指定下界通配符。
package com.learnfk;
import java.util.ArrayList;
import java.util.List;
public class GenericsTester {
public static void addCat(List<? super Cat> catList) {
catList.add(new RedCat());
System.out.println("Cat Added");
}
public static void main(String[] args) {
List<Animal> animalList= new ArrayList<Animal>();
List<Cat> catList= new ArrayList<Cat>();
List<RedCat> redCatList= new ArrayList<RedCat>();
List<Dog> dogList= new ArrayList<Dog>();
</span><span class="com">//add list of super class Animal of Cat class</span><span class="pln">
addCat</span><span class="pun">(</span><span class="pln">animalList</span><span class="pun">);</span><span class="pln">
</span><span class="com">//add list of Cat class</span><span class="pln">
addCat</span><span class="pun">(</span><span class="pln">catList</span><span class="pun">);</span><span class="pln">
</span><span class="com">//compile time error</span><span class="pln">
</span><span class="com">//can not add list of subclass RedCat of Cat class</span><span class="pln">
</span><span class="com">//addCat(redCatList);</span><span class="pln">
</span><span class="com">//compile time error</span><span class="pln">
</span><span class="com">//can not add list of subclass Dog of Superclass Animal of Cat class</span><span class="pln">
</span><span class="com">//addCat.addMethod(dogList); </span><span class="pln">
}
}
class Animal {}
class Cat extends Animal {}
class RedCat extends Cat {}
class Dog extends Animal {}
这将产生以下输出-
Cat Added Cat Added
