Java中的泛型 [英] Generics in Java

问题描述

我今天遇到了一些奇怪的事情。看看这个代码片段：

  List< Rectangle> test1 = new LinkedList< Rectangle>（）; 
列表<形状> test2 = test1; //编译器错误; 
  

这当然是假设类Rectangle是Shape的子类。
有人可以向我解释为什么这是一个错误吗？

解决方案

如果此代码有效，您可以继续插入进入 test2 a Circle - 因此彻底打破了保证 test1 使得只有 Rectangle s会被插入。

一般原则（与语言无关 - - 一个逻辑问题 - 即使违反直觉）：一袋香蕉不是一袋水果......在一个可变对象的世界里（函数式编程世界中，每个对象一旦创建就不可变，就是很多更简单！）。那是因为你可以给一袋水果加一个苹果（因为苹果是一片水果），但是你不能把一个苹果加到一袋香蕉上（因为苹果不是香蕉）。

顺便说一下，这与矩形不是矩形的原因非常相似（再次，在可变对象的世界中）：因为给定一个（可变的）矩形，可以改变双方独立，但是，给一个正方形，你不能。 （在几何中，一个正方形确实是一个矩形 - 但这是因为，在函数式编程的几何中，没有变异一个对象的概念！ - ）。

I came across something peculiar today. Take a look at this code snippet:

List <Rectangle> test1 = new LinkedList<Rectangle>();
List <Shape> test2 = test1; //Compiler Error;

This is of course is assuming that class Rectangle is a subclass of Shape. Can someone explain to me why this is an error?

解决方案

If this code worked, you could continue by inserting into test2 a Circle -- thus utterly breaking the guarantee that test1 makes, that only Rectangles will ever be inserted in it.

The general principle (language independent -- a matter of logic -- even though counter-intuitive): a bag of bananas is NOT a bag of fruit... in a world of mutable objects (the functional programming world, where every object is immutable once created, is MUCH simpler!). That's because you can add an apple to a bag of fruit (since an apple is a piece of fruit), but you can't add an apple to a bag of bananas (since an apple is not a banana).

BTW, this is very similar to the reason a square is not a rectangle (again, in a world of mutable objects): because given a (mutable) rectangle you can mutate the two sides independently, but, given a square, you can't. (In geometry, a square IS indeed a rectangle - but that's because, in geometry like in functional programming, there is no concept of "mutating" an object!-).

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