为什么这种通用方法不安全？ [英] Why is this generic method unsafe?

问题描述

以下方法会产生警告，但对我来说看起来很安全。我确信问题出在我身上：

  public< S扩展了CharSequence> S foo（S s）{
 return（S）new StringBuilder（s）; 
} 
  

它看起来像总是返回参数 s 。任何人都可以显示一个会导致此方法抛出异常的例子吗？

编辑：我对于是否特别感兴趣泛型在这里是必要的。相反，我正在寻找这种方法如何不安全的演示。

解决方案

foo（test）; p>

足以让java尝试在String中投射一个StringBuilder。

你的代码保证是错误的，你能解释一下你想要达到的目标吗？

The following method generates a warning, but it looks safe to me. I'm sure the problem is with me:

public <S extends CharSequence> S foo(S s) {
    return (S) new StringBuilder(s);
}

It looks like this will always return the argument s. Can anyone show an example that would cause this method to throw an exception?

Edit: I'm not particularly interested in the question of whether generics are necessary here. Rather, I'm looking for a demonstration of how this method is unsafe.

解决方案

foo("test");

is enough to make java try to cast a StringBuilder in a String.

Your code is guaranteed o be wrong, can you explain what you're trying to achieve plase ?

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