无法从ArrayList< Parcelable>转换到ArrayList< ClSprite> [英] Cannot cast from ArrayList<Parcelable> to ArrayList<ClSprite>

问题描述

在我编写的一段代码中，我有这一行：

In a piece of code I've written, I have this line:

         AllSprites = (ArrayList<ClSprite>) savedInstanceState.getParcelableArrayList("AllSprites");

我收到来自 ArrayList< Parcelable>的无效演员的错误; 到 ArrayList< ClSprite> 。为什么这不合法呢？

I'm getting an error about an invalid cast from an ArrayList<Parcelable> to ArrayList<ClSprite>. Why isn't this legal?

推荐答案

施放 ArrayList< Derived> <从根本上说是不安全的/ code>到 ArrayList< Base> ，反之亦然。这样做会在类型系统中打开一个漏洞，如果您尝试这样做，Java将在运行时抛出 ClassCastException 。

It is fundamentally unsafe to cast an ArrayList<Derived> to an ArrayList<Base> or vice-versa. Doing so opens up a hole in the type system, and Java will throw a ClassCastException at runtime if you try this.

原因是我可以这样做：

ArrayList<Derived> derived = new ArrayList<Derived>();
ArrayList<Base> base = (ArrayList<Derived>) derived; // Not legal!
base.add(new Base()); // Just put a Base into the list, but it only holds Derived!
derived.get(0).doSomethingOnlyInDerived(); // Error!  It's not really a Derived!

顺便说一下，这就是Java在数组之间的隐式转换被破坏的原因以及为什么 ArrayStoreException信息。在所有情况下，此演员阵容都不安全。

This is the reason, by the way, that Java's implicit conversions between arrays are broken and why there's ArrayStoreException. This cast isn't safe under all cases.

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