Java terminology for differentiating runtime type from compile-time type

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

Answer 1

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

Answer 2

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());

Answer 3

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 4

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 5

Section 15.5. Expressions and Run-Time Checks differentiates between

• the type of an expression
• the class of the referenced object

For example,

If the type of an expression is a reference type, then the class of the referenced object, or even whether the value is a reference to an object rather than null, is not necessarily known at compile time. There are a few places in the Java programming language where the actual class of a referenced object affects program execution in a manner that cannot be deduced from the type of the expression...

[snip]

An expression whose type is a reference type may be tested using instanceof to find out whether the class of the object referenced by the run-time value of the expression

Hence, applying the above language to

A a = new B();

we might say something like

The static type of the expression a is A, despite the fact the value stored in a is a reference to the object of runtime class B.

Personally, I interpret the two concepts in the following manner (but beware I am unsure of its correctness):

• static type of the expression is purely syntactic concept existing in source code at compile time for processing by Java compiler
• runtime class of the object is actual machine construct existing in machine memory at runtime for processing by Java Virtual Machine

Answer 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.