重载遵守变异（第3.4节）的规则。也就是说，它们的参数类型允许逆变（不那么特定的类型）而它们的返回类型允许共变（更特别的类型）。 > Overriding adheres to the rules of variance (3.4). That is, their argument types allow contravariance (less specific types) while their return type allows covariance (more specific types): ~~~ class Base { public function new() { } } class Child extends Base { private function method(obj:Child):Child { return obj; } } class ChildChild extends Child { public override function method(obj:Base):ChildChild { return null; } } class Main { static public function main() { } } ~~~ 直观的说，这是因为参数被“写入”到函数中，而返回值是从函数中“读取”。 > Intuitively, this follows from the fact that arguments are “written to” the function and the return value is “read from” it. 示例也展示了如何改变可见性（第4.4.1节）：一个重载的字段可能是 public ，如果被重载的字段是private，但是相反则不行。 > The example also demonstrates how visibility (4.4.1) may be changed: An overriding field may be public if the overridden field is private, but not the other way around. 不可能重载声明为内联（第4.4.2节）的字段。这是由于冲突的概念：当内联在编译时通过替换一个函数体的调用，重载字段必须被在运行时决定。 > It is not possible to override fields which are declared as inline (4.4.2). This is due to the conflicting concepts: While inlining is done atcompile-time by replacinga call with thefunction body, overriding fields necessarily have to be resolved at runtime.