扩展和超级的通用概念 [英] Generic concept of extends and super

问题描述

通用扩展背后的概念是什么，为什么不允许修改 列表;为什么在我将一个字符串添加到list时，它会引发编译时错误，因为String扩展了Object并且应该是合法的.

What is the concept behind the Generic extend that why is it not allowed to modify the list; why does it throw a compile time error when I add a string to list , since String extends Object and should be legal.

如果这会导致编译错误，那么创建的列表的用途是什么.

If this gives compilation error , then what is the use of that list that is created then.

         List<? extends Object>  ls=new ArrayList<String>();
         ls.add("asd");  // compilation error

并且在超级的情况下进行编译.

And it compiles in the case of super.

         List<? super Integer> ls1=new ArrayList<Object>();
         ls1.add(1);

我已经阅读了Kathy Sierra和Javadoc，但无法理解这意味着什么.请通过示例详细说明我.

I have read Kathy Sierra and Javadoc, but am not able to understand what this means. Please give me a detailed explanation with examples to understand this.

推荐答案

您不能将字符串添加到List<? extends Object>，因为?可以是任何东西.

You can't add Strings to a List<? extends Object> because ? could be anything.

如果要将放入列表，则其类型参数应该是要放入的类型的超类.(包括类型本身.)

If you want to put things into a list, its type parameter should be a superclass of the type you want to put in. (This includes the type itself.)

如果要从列表中获取 ，则其type参数应该是要取出的类型的子类. (这包括类型本身.)

If you want to get things from a list, its type parameter should be a subclass of the type you want to take out. (This includes the type itself.)

这可以用PECS的缩写来记住-生产者扩展，消费者超级.

This can be remembered with the acronym PECS - producer-extends, consumer-super.

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