Casting-(Base) o- 它到底有什么作用？
class Rectangle {}
class ColoredRectangle extends Rectangle {}
class Base {
void foo(Rectangle x) {
System.out.println("Base: foo(Rectangle)");
}
}
class Sub extends Base {
@Override
void foo(Rectangle x) {
System.out.println("Sub: foo(ColoredRectangle)");
}
}
public class WhatHappensHere {
public static void main(String[] args) {
Sub o = new Sub();
Rectangle rc = new ColoredRectangle();
((Base) o).foo(rc);
}
}
我很难意识到这个 (Base) o 是否改变了 o 的静态类型。我记得我在这里被告知它不会改变它，那么它有什么作用呢？为什么我需要这样做？在什么条件下以及出于什么目的？
谢谢
class Rectangle {}
class ColoredRectangle extends Rectangle {}
class Base {
void foo(Rectangle x) {
System.out.println("Base: foo(Rectangle)");
}
}
class Sub extends Base {
@Override
void foo(Rectangle x) {
System.out.println("Sub: foo(ColoredRectangle)");
}
}
public class WhatHappensHere {
public static void main(String[] args) {
Sub o = new Sub();
Rectangle rc = new ColoredRectangle();
((Base) o).foo(rc);
}
}
I'm having a hard time realize if this (Base) o changes the static type of o. I remember that I was told here that it doesn't change it, so what does it do? why do I need to do that? in what conditions and for what purpose?
Thanks
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评论（2）
它不会改变
o的静态类型，因为表达式o仍然是Sub类型 - 但表达式(Base) o的类型为Base。我认为您的意思可能是它不会更改o引用的对象的类型。在这种情况下，转换为Base根本没有任何意义。它不会改变foo()调用的执行。然而，在各种不同的情况下，强制转换确实有意义。It's not changing the static type of
o, in that the expressionois still of typeSub- but the expression(Base) ois of typeBase. What I think you may mean is that it doesn't change the type of the object thatorefers to. In this case, there is no purpose in casting toBaseat all. It doesn't change the execution of thefoo()call. There are various and different cases where casting does make sense, however.它什么也不做。在某些其他语言中，这可能会调用
Base中定义的foo，但在这种情况下，它仍然调用Sub.fooIt does nothing. In some other language this might call the
foodefined inBasebut in this case it still callsSub.foo