Java Inheritance, Dynamic Dispatch Exam Question

Question

This is a question from UW CSE 143. I am just studying past exams to get better at Java.

    public class Box extends Pill {

        public void method2() {
            System.out.println("Box 2");
        }

        public void method3() {
            method2();
            System.out.println("Box 3");
        }

    }

    public class Cup extends Box {

        public void method1() {
            System.out.println("Cup 1");
        }

        public void method2() {
            System.out.println("Cup 2");
            super.method2();
        }

    }

    Box var3 = new Cup();

Question: What is the outprint if var3.method3() is called?

I have no idea why the answer is Cup 2/Box 2/Box 3

Where is cup 2 from? I get the dynamic type is Cup. But if Cup class does not have method3, so it goes to the parent class for method3.

Please format the source code.

VHS
– VHS

2020-06-04 06:29:25 +00:00
Commented Jun 4, 2020 at 6:29 — VHS
– VHS, Commented Jun 4, 2020 at 6:29

Eran · Accepted Answer · 2020-06-04 06:33:28Z

1

var3.method3()

executes the Box method (since Cup doesn't override that method):

public void method3() {
    method2();
    System.out.println("Box 3");
}

method2() executes Cup's method2(), since the dynamic type of var3 is Cup, and Cup overrides method2() of Box:

public void method2() {
    System.out.println("Cup 2");
    super.method2();
}

This prints "Cup 2" and then super.method2() executes the super class method:

public void method2() {
    System.out.println("Box 2");
}

This prints "Box 2".

Finally, when we return to method3(), "Box 3" is printed.

Hence the output is

Cup 2
Box 2
Box 3.

answered Jun 4, 2020 at 6:33

Eran

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1 Comment

BrownBear2018 Over a year ago

Yeah I am just surprised that method2() executes Cup's method2() because it is inside the box class.

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