Java, terminology clarification

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

In this case, A is the reference type while B is the instance type

Answer 4

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.

Answer 5

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.

Answer 6

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.