COSC121 Assignment 1 Solution

Focus: OOP basics, Inheritance, and Polymorphism
Q1. [15 marks] Suppose that Fruit, Apple, Orange, GoldenDelicious, and McIntosh are defined in the following inheritance hierarchy:


And assume that the following code is given:
Fruit fruit = new GoldenDelicious(); Orange orange = new Orange();
Answer the following questions, and explain your answer.
a. Is fruit instanceof Fruit?
b. Is fruit instanceof Orange?
c. Is fruit instanceof Apple?
d. Is fruit instanceof GoldenDelicious?
e. Is fruit instanceof McIntosh?
f. Is orange instanceof Orange?
g. Is orange instanceof Fruit?
h. Is orange instanceof Apple?
i. Can fruit invoke makeAppleCider method?
j. Can orange invoke makeAppleCider method?
k. Can fruit invoke makeOrangeJuice method?
l. Can orange invoke makeOrangeJuice method?
m. Is the statement Orange p = new Apple() legal?
n. Is the statement McIntosh p = new Apple() legal?
o. Is the statement Apple p = new McIntosh() legal?


Q2. [10 marks] Consider the following class hierarchy.

A method saySomething() displays the type of object, e.g., for the Apple class, the method displays “I am an Apple”. The overloaded method saySomething(String s) displays the type then the value of s, e.g., for class Apple and s = “hello”, the method displays
“Apple says: hello!”
a) What is displayed on the screen after running the code below.
public class A2 { public static void main(String[] args) { Fruit[] fruits = new Fruit[5]; fruits[0] = new Fruit(); fruits[1] = new Apple(); fruits[2] = new Orange(); fruits[3] = new GoldenDelicious(); fruits[4] = new McIntosh(); for(Fruit f1: fruits){
f1.saySomething();
}
Fruit[] apples = new Apple[3]; apples[0] = new Apple(); apples[1] = new GoldenDelicious(); apples[2] = new McIntosh();
for(Fruit f2: apples){
//add code here
}
}
}

b) Write the missing code where indicated above in order to invoke the method saySomething(String s) using the reference variable f2. Use any value for s. Explain the output (i.e., why we see each text line in the output). Hint: object casting.
c) Is the following statement valid? Explain.
apples[0] = new Orange();


Note about the array f2 in Q2 above:
- The array is of the type Fruit. However, each element is of the type Apple. This means:
o the array elements can only be of the type Apple or a subtype of Apple.
o The reference f2 can only invoke methods known to Fruit (i.e. it cannot for example invoke methods in the Apple class if they are not defined in Fruit)
- We do this (array type is different from elements type) when we what to limit the programmer to use only the methods in the array type (e.g. Fruit in our example) and put a restriction on the type of objects that can be stored in the array (e.g. only Apples can be stored in f2. We cannot store oranges in the array).


Submission Instructions
For this assignment, you need to do the following:
1- Create a new Microsoft Word file which name consists of your student number followed by the assignment number, e.g., “1234567_assignment_A1.docx”. 2- Write you answers in the above file and submit it to Canvas.
Note that you can resubmit an assignment, but the new submission overwrites the old submission and receives a new timestamp.