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Section 9.1 Worked Example: Writing Classes - Attributes

Subgoals for Writing a Class.

  1. Name it
  2. Differentiate class-level static vs. instance/object-level variables
  3. Differentiate class-level static vs. instance/object behaviors/methods
  4. Define instance variables (that you want to be interrelated)
    1. Name
    2. Data Type
    3. private
  5. Define class variables static as needed
    1. Name
    2. Data Type
    3. public / private / final
  6. Create constructor (behavior) that creates initial state of object
    1. Overloaded constructor (with as many parameters)
    2. public
    3. Same name as class
    4. No return type
    5. Default - no parameters
    6. Logic - initialize all variables
    7. Repeat as needed, adding parameters
  7. Create 1 accessor and 1 mutator behaviors per attribute
    1. Accessors
      1. Name is get_<attr_name>
      2. Public
      3. Return type same data type as attribute
      4. No parameters
      5. Logic - return value
    2. Mutators
      1. Name is set_<attr_name>
      2. Public
      3. Return type is void
      4. Parameter is same data type as attribute
      5. Logic validates input parameter and sets attribute value
  8. Write toString method
    1. public
    2. Returns String
    3. No parameters
    4. Logic - convert needed attributes to a format that can be printed
  9. Write equals method
    1. public
    2. Returns boolean
    3. Parameter - instance of the class
    4. Logic - compare attributes for equity
  10. Create additional methods as needed

Subsection 9.1.1

You can watch this video or read through the content below it.
Problem: We will be writing a class to represent an instance of time, like a specific time in the day.
For the first part, determine the appropriate attributes (data) to be stored and declare them

Subsection 9.1.2 SG1: Name it

We will call the class TimeType

Subsection 9.1.3 SG2: Differentiate class-level (static) vs. instance/object-level variables

class-level (static) data: one value shared between ALL instances instance-level data: each instance has its own copy
For time, we want all time instances to be in the same format so static data for format, everything else instance

Subsection 9.1.4 SG3: Differentiate class-level (static) vs. instance/object behaviors/methods

All methods will be instance level as the format will be fixed an unchangeable

Subsection 9.1.5 SG4: Define class variables (static) as needed

public class TimeType {

   public static final boolean FORMAT24 = true;

Note the use of the final keyword to define FORMAT24 as a constant, which makes it safe to expose as public. An alternate implementation might choose to make this value mutable, but private, using static methods to ssssssssaccess and alter it.

Subsection 9.1.6 SG5: Define instance variables (that you want to be interrelated)

public class TimeType {

   //static var
   public static final boolean FORMAT24 = true;

   //instance vars
   private int hour;
   private int minute;
   private int second;

Figure 9.1.1.

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