CSCI 241 Labs: PreLab 11

CSCI 241 Labs: PreLab 11
Concepts in Object-Oriented Design

Introduction

Chapter 9 of our text introduces you to making your own classes and objects in Java. These include:

static methods and data
overloading constructors and methods
passing objects to methods
returning objects from methods

The syntax behind these features is the easy part. Learning how and when to use them is more difficult, and to a certain extent, will only come with much practice. This prelab will walk you through the design of a Thermometer class that uses many of these features.

Temperature

There are multiple different temperature scales used for different purposes. Most of us in the United States are still more comfortable using the Fahrenheit scale than any other. In Fahrenheit water freezes at 32 degrees and boils at 212 degrees. Most of the rest of the world uses the Celsius scale. In Celsius water freezes at 0 degrees and boils at 100 degrees. There is also the strange concept of absolute zero. Absolute zero is the coldest possible temperature in the entire universe. It is -273.15 degrees Celsius and -459.67 degree Fahrenheit. Remember that the next time the temperature outside reaches 30 below!

Some baseline temperatures in the two scales:

temperature degrees Celsius degrees Fahrenheit
symbol °C °F
boiling point of water 100.0 212.0
freezing / melting point of water / ice 0.0 32.0
absolute zero -273.15 -459.67

temperature	degrees Celsius	degrees Fahrenheit
symbol	°C	°F
boiling point of water	100.0	212.0
freezing / melting point of water / ice	0.0	32.0
absolute zero	-273.15	-459.67

Temperature conversions

There are simple mathematical formulas we can use to convert a temperature in one scale to the other:

°F = °C x 1.8 + 32.0
°C = (°F - 32.0) / 1.8

Designing a Thermometer Class

When we design an instantiable class, the two basic questions we ask should be:
What kind of data do we want in our objects? and
Which methods do we want in our objects?
We also need to determine if we wish to include static data and methods in our classes.

Let's consider designing a Thermometer class.

Class Constants
The table above gives absolute zero and the freezing and boiling points of water using the different scales. These data are useful constants to let users look up at any time. Our thermometer class should declare them to be public class constants.
Next we note that the conversion equations given above use some constants (32, 1.8 and 273.15) multiple times. This is a sure sign that they are useful constants. Two of them were already discussed above. The 1.8 is the ratio between the size of Fahrenheit and Celsius degrees. It takes 1.8 Fahrenheit degrees to make 1 Celsius degree. It is a useful for our conversions, but is not of general interest to the public. It should be declared as a private class constant.
Converting Between Temperature Scales
Look at the thermometer in the image below. It really serves two purposes:
1. It tells us what the current temperature is in either Fahrenheit or Celsius. (It is about 63.5 °F = 17.5 °C)
2. It can also be used to convert between Fahrenheit and Celsius and back. We simply read across from one scale to the other. For example: 40 °C = 104 °F.
We want to design a Thermometer class that can also serve both functions. The thermometer in the image can only convert between Celsius and Fahrenheit.
First, we note that the conversions are the same regardless of the current temperature. This is a sure sign that our conversion methods can be class methods. When we define the methods we will use the static modifier.
To do the conversions, our Thermometer class should have 2 class methods. The method prototypes are:
- public static double celsiusToFahrenheit(double celsius)
- public static double fahrenheitToCelsius(double fahrenheit)
Instance Variables and Methods
We also want to instantiate Thermometer objects that will keep track of the current temperature. Thermometers are pretty simple objects. They keep track of the current temperature. That is about it. Our Thermometer class will have only one data member, currentTemperature, which we will store as the current Celsius temperature.
It will have a number of different methods. These methods are all instance methods since they refer to the currentTemperature instance variable.
1. It will have two constructors: a default constructor that sets the currentTemperature to 0.0 and an alternate constructor that takes the initial Celsius temperature as a parameter.
  (Note that it is not possible in Java to have an additional constructor that takes a Fahrenheit temperature as the initial value. Why?)
2. It will have two "setter" methods:
  - void setTemperatureUsingCelsius (double degreesC) which simply sets the current temperature.
  - void setTemperatureUsingFahrenheit (double degreesF) which converts degreesF to Celsius and sets the current temperature.
3. It will have two "getter" methods:
  - double getCelsius () which simply returns the current temperature.
  - double getFahrenheit () which converts the current temperature to Fahrenheit and returns it.

Something to Think About

Many modern thermometers also keep track of maximum and minimum temperatures. We already have all the coding skills we need to make our thermometers do this. What additional data members and methods would you add to the class so that thermometer objects will know their max and min temperatures?