问题
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).
I'm wondering what are the proper name for these types. For instance
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?
回答1:
I think it's important to distinguish between the object (which exists at execution time, and just has its execution time type) and an expression (such as a variable) which has a compile-time type.
So in this case:
A a = new B();
a is a variable, of type A. Its value at execution time is a reference to an object of type B.
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.
回答2:
The 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> and List<Integer> are different types, but their instances share the same runtime class.
回答3:
In this case, A is the reference type while B is the instance type
回答4:
I would say that you differentiate between the type of the variable/reference and the type of the object. In the case
A a = new B();
the variable/reference would be of type A but the object of type B.
回答5:
The type of the variable a is A. There's no changing that, since it's a reference. It happens to refer to an object of type B. While you're referring to that B object through an A reference you can only treat it as though it were of type A.
You can later cast it to its more specific type
B b = (B)a;
and use the B methods on that object.
回答6:
The terminology you are looking for is the Apparent Type and the Actual Type.
A a = new B();
The Apparent Type is A because the compiler only knows that the object is of type A. As such at this time you cannot reference any of the B specific methods.
The Actual Type is B. You are allowed to cast the object (that is change its apparent type) in order to access the B specific methods.
回答7:
To determine a is object of which class you can use:
/*The java.lang.Object.getClass() method returns the runtime class of an object*/
System.out.println("a is object of: "+a.getClass());
来源：https://stackoverflow.com/questions/1514214/java-terminology-clarification