Question

Background for the next few questions: You’ve already seen, on this exam, a class called Point. I’ve also asked you to define a subclass of Point called Point2D. Assume that there also exists a class called SpecializedPoint2D, which is a subclass of Point2D. (You aren’t responsible for coding the SpecializedPoint2D class.) The SpecializedPoint2D class has a no-arg constructor and many other methods; you don’t need to know any of the details of this class.

What will get printed by the following code snippet?

Point p = new SpecializedPoint2D();

System.out.println(p instanceof Point2D);

System.out.println(p instanceof Point);

Answer 1

Both print statements will print true. Because:

p is initialized as SpecializedPoint2D, which is a sub class of Point2D. Every child class object is automatically an instance of its super class. For example, if A is the super class of B, then every objects of B are instances of A as well as B. So System.out.println(p instanceof Point2D) will print true. Also this is applicable to any level of inheritance. p is a SpecializedPoint2D object, which is a child of Point2D which is a child of Point. So each and every objects of SpecializedPoint2D is automatically an instance of Point. Which makes the statement System.out.println(p instanceof Point); prints true.

Note: This is applicable in one direction only. For example, if p was initialized as a Point object, it returns false for ‘p instanceof Point2D’, because p may not have enough data to be a Point2D object.

So key points to be noted:

SpecializedPoint2D instance of Point2D: true

SpecializedPoint2D instance of Point: true

Point2D instance of Point: true

Point2D instance of SpecializedPoint2D: false

Point instance of Point2D: false

Point instance of SpecializedPoint2D: false

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