Apparent vs. Actual Type

Consider the following statement:

Student jane = new GradStudent("Jane", "jan@e.mail");

Notice jane is declared as Student but instantiated using GradStudent. We can break the statement into two statements for clarity:

Student jane;  // Declare jane's type is "Student"
// Then, instantiate it as GradStudent
jane = new GradStudent("Jane", "jan@e.mail");

The disparity between the types is allowed (as long as GradStudent is a subtype of Student). However, how shall we answer the question: what is jane's data type?

To answer this question, we need to make a distinction between the apparent vs. actual type of an object:

• When you declare a variable (SomeClass obj), you are setting its apparent type to be the declared type.

• When you instantiate an object (obj = new SomeClass()) you are setting its actual type to be the instantiated type.

Therefore, the apparent type of jane is Student, while the actual type of jane is GradStudent.

Exercise Suppose we declared Student jane and GradStudent john. Which of the following are not valid Java statements? (there may be more than one correct answer)

(a) jane = new Student("Jane", "jan@e.mail");
(b) jane = new GradStudent("Jane", "jan@e.mail");
(c) john = new GradStudent("John", "john@e.mail");
(d) john = new Student("John", "john@e.mail");

To understand the above statement, we must explore another pillar of object-oriented programming: polymorphism.