In Java, an Object can have a runtime type (which is what it was created as) and a casted type (the type you have casted it to be).
我想知道这些类型的专有名称是什么。例如
class A {
}
class B extends A {
}
A a = new B();
a was created as a B however it was declared as an A. What is the proper way of referring to the type of a using each perspective?
In this case,
Ais the reference type whileBis the instance typeThe Java Language Specification speaks about a variable's declared type, the javadoc of
getClass()about an object's runtime class.Note that there is no such thing as a runtime type in Java;
List<String>andList<Integer>are different types, but their instances share the same runtime class.我认为，区分对象（存在于执行时，只是具有执行时间类型）和具有编译时类型的表达式（例如变量）之间是很重要的。
因此，在这种情况下：
ais a variable, of typeA. Its value at execution time is a reference to an object of typeB.The Java language specification uses "run-time class" (e.g. for the purpose of overriding, as in section 15.12.4.4) for the type of an object. Elsewhere I think it just uses "type" for the type of an expression, meaning the compile-time type.
您正在寻找的术语是表观类型和实际类型。
表观类型为A，因为编译器仅知道该对象的类型为A。因此，此时您不能引用B的任何特定方法。
实际类型为B。允许访问该对象（即更改其外观类型）以访问B特定方法。
The type of the variable
aisA. There's no changing that, since it's a reference. It happens to refer to an object of typeB. While you're referring to thatBobject through anAreference you can only treat it as though it were of typeA.您以后可以将其转换为更具体的类型
and use the
Bmethods on that object.我要说的是，您要区分变量/引用的类型和对象的类型。在这种情况下
the variable/reference would be of type
Abut the object of typeB.To determine
ais object of which class you can use:Section 15.5. Expressions and Run-Time Checks differentiates between
例如，
因此，将上述语言应用于
我们可能会说类似
我个人以以下方式解释这两个概念（但是请注意，我不确定其正确性）：