Deadline

This work should be completed before the exercise on Friday 11th September.

Instructions

For instructions on how to do and submit the assignment, please see the assignments section of the course instructions.

Preparation

You must read and answer the questions in the OLI material for Module 2.

• Read Module 2: Looking Inside Classes
• If you have not done so, goto https://kth.oli.cmu.edu/, signup and register for the course key dd1337-ht20

You may wish to also read from, Objects First with Java.

Study sections 2.1 -- 2.12 and 2.15 -- 2.18 in Objects First with Java (5th or 6th ed) (don't mind the mentions of if/else, that is covered next week).

Exercises 2.1 -- 2.42 are good practice, but do not need to submit this homework, just you must be prepared to discuss it.

You can find the code for TicketMachine in the bluej-projects repo.

Github Task:

Submit code for tasks:

• 2.44 -- 2.45
• 2.58 -- 2.61

Assistant's note: Remember that the name of the class and the name of the file must match exactly, including capitalization. That is to say, if the public class in the file is called TicketMachine, then the file must be called TicketMachine.java, or the code will not compile.

Please commit any written answers to the docs folder, and commit any Java code developed to the src folder of your KTH Github repo. Remember to push to KTH Github.

Exercise 2.44

Give the class two constructors. One should take a single parameter that specifies the price, and the other should take no parameter and set the price to be a default value of your choosing. Test your implementation by creating machines via the two different constructors.

Assistant's note: Please note that there are two versions of TicketMachine in Chapter 2. You may use choose either one with no problem for the exercises.

Exercise 2.45

Implement a method empty, that simulates the effect of removing all money from the machine. This method should have a void return type, and its body should simply set the total field to zero. Does this method need to take any parameters? Test your method by creating a machine, inserting some money, printing some tickets, checking the total, and then emptying the machine. Is the empty method a mutator or an accessor?

Exercise 2.58

Why does the following version of refundBalance not give the same results as the original?

public int refundBalance() {
    balance = 0;
    return balance;
}

What tests can you run to demonstrate that it does not?

Exercise 2.59

What happens if you try to compile the TicketMachine class with the following version of refundBalance?

public int refundBalance() {
    return balance;
    balance = 0;
}

What do you know about return statements that helps to explain why this version does not compile?

Exercise 2.60

What is wrong with the following version of the constructor of TicketMachine?

public TicketMachine(int cost) {
    int price = cost;
    balance = 0;
    total = 0;
}

Try out this version in the better-ticket-machine project. Does this version compile? Create an object and then inspect its fields. Do you notice something wrong about the value of the price field in the inspector with this version? Can you explain why this is?

Exercise 2.61

Add a new method, emptyMachine, that is designed to simulate emptying the machine of money. It should reset total to be zero but also return the value that was stored in total before it was reset.

Grading Criteria

Each week we will communicate grading criteria through the issue tracker. Grading criteria set the basic standards for a pass, komp or fail, so it is essential you review them each week. These will change over time as your skills develop, so make sure you read the grading criteria issue carefully and tick off all the requirements.