CSCI 162 - Lecture 2 - Classes and Objects

Instructor's Class Notes

1. Java Classes and Information Hiding - Chapter 2
• Classes and Objects Overview from CSCI 161 lecture 13 and lecture 14
• Classes and their Members - Instance data, constructors, and methods are called the class members.
1. Instance Variables - The variables that define the state of the class. Often these will be private so that invariants may be maintained through accessor methods. Public instance variables are frowned upon as they don't allow information hiding.
2. Constructors - A method that is responsible for initializing instance variables.
3. Methods - Used to access or modify the instance variables (state) of a class:
1. Accessor method - a method that provides access to the state of an object but does not change it.
   Four reasons to implement accessor methods:
• A programmer that uses your class need not worry about how it is implemented.
• Using an accessor method allows for later changes in the implementation of the method and class thus reducing the need for the calling program to change.
• When a class implements an accessor methods the use of the class is restricted to those methods making test cases limited and easier to test, thus increasing reliability.
• The pattern of "private data, public methods" forbids other programmers from using your instance vqriables in unintended ways.
2. Modification method - aka mutator, is a method that can change the state of an object. Typically used along with accessor methods by a calling program.
• Throttle Class - From the book, Figure 2.1
1. Documentation here. Generated by Javadoc within Eclipse. Select "Project" then "Generate Javadoc..."
2. Source here
3. Using the class:
   

   final int SIZE = 8;     // The size of the Throttle
   final int SPOT = 3;     // Where to move the Throttle's lever
                           
   Throttle small = new Throttle(SIZE);        // Declare instance of a Throttle
                           
   small.shift(SPOT);
   System.out.print("My small throttle is now at position ");
   System.out.println(SPOT + " out of " + SIZE ".");
   System.out.println("The flow is now: " + small.getFlow());
                       

   Output is:
   

   My small throttle is now at position 3 out of 8.
   The flow is now: 0.375
                       

4. Terminology:
1. Reference - When creating an instance of an class that object is called a "reference" variable.
2. Null Reference - Sometimes a reference variable has been declared but does not refer to anything. This is a null reference and the value of the vriable is called null.
3. Null Pointer Exception - Trying to access a method through a reference variable that is null. This will generate a run-time exception.
4. Assignment - Reference variables can be assigned one to another with the = assignment operator
5. Test for Equality - You can test if two reference variables are equal with the == equality operator, where equality means that both are not null and both refer to the exact same object (not two different objects that happen to have the same values for their instance variables.)