为什么我们不能做List< Parent> mylist = ArrayList< child>（）; [英] Why we can't do List<Parent> mylist = ArrayList<child>();

问题描述

为什么我们不能做

 列表< Parent> mylist = ArrayList< child>（）; 
  

解决方案

然后，这个程序必须很好：

  ArrayList< Banana> bananas = new ArrayList< Banana>（）; 
 List< Fruit>水果=香蕉; 
 fruit.add（new Apple（））; 
 
香蕉蕉= bananas.get（0）; 
  

这显然不是类型安全 - 你结束了一个苹果在香蕉的集合。 / p>

您可以做的是：

  List< ;? extends Fruit> fruit = new ArrayList< Banana>（）; 
  

这是安全的，因为编译器不会让你尝试 add 到水果列表。它知道它是一个某种水果的列表，所以你可以写：

 水果firstFruit = fruit.get（0）; 
  

但它不知道什么确切的水果它是一个列表，不要做错事。

请参阅 Java泛型常见问题另一种解释。

Why we can't do

List<Parent> mylist = ArrayList<child>();

解决方案

Suppose we could. Then this program would have to be fine:

ArrayList<Banana> bananas = new ArrayList<Banana>();
List<Fruit> fruit = bananas;
fruit.add(new Apple());

Banana banana = bananas.get(0);

That's clearly not type safe - you've ended up with an apple in the collection of bananas.

What you can do is:

List<? extends Fruit> fruit = new ArrayList<Banana>();

this is safe, because the compiler won't then let you try to add to the list of fruit. It knows that it's a list of some kind of fruit, so you could write:

Fruit firstFruit = fruit.get(0);

but it doesn't know what exact kind of fruit it's a list of, and make sure you can't do the wrong thing.

See the Java generics FAQ another explanation.

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