Chapter 3.5, Problem 20SC

Program Plan Intro

Card for a car object:

Consider a “Car” class, which simulates fuel consumption in a car.

On the front side of the card, the methods that the object can execute are written. The methods for efficiency, adding gas, driving a given distance, and checking the amount of gas left in the tank that is as follows:

Car myCar Car(mpg) addGas(amount) drive(distance) getGasLeft

On the back side of the card, the values of the instance variables are noted down. After creating the objects, the values of the instance variables will be as follows:

  

gasLeft	milesPerGallon
0	25

Here, the initial value of “gasLeft” will be zero and the efficiency calculated in miles per gallon will be 25mpg.

Tracing the methods of Self Check 18:

The following method calls are given:

//Call the methods

Car myCar(25);

myCar.addGas(20);

myCar.drive(100);

myCar.drive(200);

myCar.addGas(5);

Cross out the old values whenever a mutator method is called, and write down the new one below.

gasLeft	milesPerGallon
$\cancel{0}$  $2\cancel{0}$  $1\cancel{6}$ $\cancel{8}$ 13	25

• Initially, the value of “gasLeft” and “milesPerGallon” was “0” and “25” respectively.
• When the method myCar.addGas(20)is called, the value “0” is cut-off and the value of “gasLeft” will be updated to “20”.
  • It is because the addGas method adds the value of gas and thus, the gas left will be 0+20=20.
• When the method myCar.drive(100)is called, the value “20” is cut-off and the value of “gasLeft” will be updated to “16”.
  • It is because a gas of value “4” is required to travel a distance of “100miles” and thus, the gas left will be 20-4=16.
• When the method myCar.drive(200)is called, the value “16” is cut-off and the value of “gasLeft” will be updated to “8”.
  • It is because a gas of value “8” is required to travel a distance of “200miles” and thus, the gas left will be 16-8=8.
• When the method myCar.addGas(5)is called, the value “8” is cut-off and the value of “gasLeft” will be updated to “13”.
  • It is because the addGas method adds the value of gas and thus, the gas left will be 8+5=13.

Instance variable added in Self Check 19:

It is given that the odometer of the car is simulated by adding a method “getMilesDriven”.

After adding the instance variables to the card, the card will be as follows:

gasLeft	milesPerGallon	totalMiles
0	25	0

The initial value of the variable “totalMiles” is assigned to zero.