泛型：列表<？延伸动物&gt;与List&lt; Animal&gt;相同吗？

问题描述

名单< ;? extends Animal> 表示我们可以在扩展Animal> ，但不是 List< Animal> 。

的子类型。

是列表< Dog> 不是列表< Animal> 的子类型？考虑下面的例子：

  void mySub（List< Animal> myList）{
 myList.add（new Cat ））; 
 
 
 
 
 
 
如果您被允许传递列表< Dog> ; 到这个函数，你会得到一个运行时错误。
 
 
 
 
 
 
编辑：Now ，如果我们使用 List ，则会发生以下情况： 
 
 
  void mySub（List< ;? extends Animal> myList）{
 myList.add（new Cat（））; //在这里编译错误
 Animal a = myList.get（0）; //正常工作
 
 $ / code $ / pre 
 $ b $您可以传递一个列出< Dog> 这个函数，但是编译器意识到添加一些东西到列表可能会让你陷入困境。如果您使用 super 而不是 extends （允许您传递 List< LifeForm> ），这是相反的方式。 
 
 
  void mySub（List< ;? super Animal> myList）{ 
 myList.add（new Cat（））; //正常工作
 Animal a = myList.get（0）; //在此处编译错误，因为列表条目可能是Plant 
} 
  
理论在这之后是。
 
I am just trying to understand the extends keyword in Java Generics.
List<? extends Animal> means we can stuff any object in the List which IS A Animal
then won't the following also mean the same thing:
List<Animal>
Can someone help me know the difference between the above two? To me extends just sound redundant here.
Thanks!
 解决方案 
List<Dog> is a subtype of List<? extends Animal>, but not a subtype of List<Animal>.
Why is List<Dog> not a subtype of List<Animal>? Consider the following example:
void mySub(List<Animal> myList) {
    myList.add(new Cat());
}
If you were allowed to pass a List<Dog> to this function, you would get a run-time error.
EDIT: Now, if we use List<? extends Animal> instead, the following will happen:
void mySub(List<? extends Animal> myList) {
    myList.add(new Cat());     // compile error here
    Animal a = myList.get(0);  // works fine
}
You could pass a List<Dog> to this function, but the compiler realizes that adding something to the list could get you into trouble. If you use super instead of extends (allowing you to pass a List<LifeForm>), it's the other way around.
void mySub(List<? super Animal> myList) {
    myList.add(new Cat());     // works fine
    Animal a = myList.get(0);  // compile error here, since the list entry could be a Plant
}
The theory behind this is Co- and Contravariance.
                        
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