为什么Java让您强制转换为集合？ [英] Why does Java let you cast to a collection?

问题描述

我有一个简单的 foo 类，并且能够转换为集合接口（ Map 或列表）而没有任何编译器错误。
请注意， Foo 类不会实现任何接口或扩展任何其他类。

I have a simple foo class, and I am able to cast to a collection interface (either Map or List) without any compiler error. Note that Foo class does not implement any interface or extends any other class.

public class Foo {

    public List<String> getCollectionCast() {
        return (List<String>) this;    // No compiler error
    }

    public Map<String, String> getCollection2Cast() {
        return (Map<String, String>) this;    // No compiler error
    }

    public Other getCast() {
        return (Other)this;     // Incompatible types. Cannot cast Foo to Other
    }

    public  static class Other {
        // Just for casting demo
    }

}

为什么当我尝试投射<时，Java编译器为什么不返回 incompatible types error ？ code> Foo 类到集合吗？

Why does the Java compiler not return incompatible types error when I try to cast the Foo class to a collection?

Foo 未实现集合。我会期望出现不兼容的类型错误，因为给定当前的 Foo 类签名，所以它不能是 Collection 。

Foo does not implement Collection. I would expect an incompatible types error, because given the current Foo class signature, this cannot be a Collection.

推荐答案

不是因为它们是集合类，而是因为它们是接口。 Foo 不会实现它们，但可以实现它的子类。因此，这不是编译时错误，因为这些方法可能对子类有效。在运行时，如果这个不是实现那些接口的类，则自然是运行时错误。

It's not because they're collection classes, it's because they're interfaces. Foo doesn't implement them, but subclasses of it could. So it's not a compile-time error, since those methods may be valid for subclasses. At runtime, if this isn't of a class that implements those interfaces, naturally it's a runtime error.

如果将 List< String> 更改为 ArrayList< String> ，您将得到同样，这也是一个编译时错误，因为 Foo 子类可以实现 List ，但不能扩展 ArrayList （因为 Foo 没有）。同样，如果您使 Foo final ，则编译器将为您的接口类型转换提供错误，因为它知道它们可以永远都不是真的（因为 Foo 不能有子类，并且不能实现那些接口）。

If you change List<String> to ArrayList<String>, you'll get a compiler-time error for that, too, since a Foo subclass could implement List, but can't extend ArrayList (since Foo doesn't). Similarly, if you make Foo final, the compiler will give you an error for your interface casts because it knows they can never be true (since Foo can't have subclasses, and doesn't implement those interfaces).

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