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  • #CT2106 - Object-Oriented Programming
  • Previous Topic: Introduction to Java
  • Next Topic: More Java Code
  • Relevant Slides: Lecture02.pdf
  • What is the structure of a class? #card card-last-interval:: 21.53 card-repeats:: 4 card-ease-factor:: 2.32 card-next-schedule:: 2022-12-01T00:46:31.046Z card-last-reviewed:: 2022-11-09T12:46:31.046Z card-last-score:: 3
    • Every class has the following structure: 
      • public class ClassName 
{
  Fields
  Constructors
  Methods
}

  • Fields

    • What are Fields? #card card-last-interval:: 86.42 card-repeats:: 5 card-ease-factor:: 2.66 card-next-schedule:: 2023-02-09T06:22:08.706Z card-last-reviewed:: 2022-11-14T20:22:08.706Z card-last-score:: 5
      • Fields, also known as instance variables, store values for an object.
      • Fields define the state of an object.
      • In BlueJ, use Inspect to view the state.
      • Some values change frequently, others rarely, or not at all.

  • Encapsulation

    • What is Encapsulation? #card card-last-interval:: 9.68 card-repeats:: 3 card-ease-factor:: 2.42 card-next-schedule:: 2022-11-24T08:35:40.664Z card-last-reviewed:: 2022-11-14T16:35:40.665Z card-last-score:: 5
      • In encapsulation, the ^^variables of a class will be hidden from other classes^^ and can only be accessed through the methods of their current class.
      • This is also known as data hiding.
    • Why use encapsulation? #card card-last-interval:: 22.66 card-repeats:: 4 card-ease-factor:: 2.38 card-next-schedule:: 2022-12-07T11:20:26.959Z card-last-reviewed:: 2022-11-14T20:20:26.960Z card-last-score:: 5
      • In OOP, ^^each object is responsible for its own data.^^
        • This allows an object to have greater control over which data is available to be viewed externally, and how external objects can mutate the object's state.

    • Encapsulation Type: Private

      • What is the effect of making a field private? #card card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-11-17T15:33:20.443Z card-last-reviewed:: 2022-10-20T08:33:20.443Z card-last-score:: 5
        • Making a field private encapsulates their values inside their object.
        • No external class or object can access a private field.

  • Constructors

    • What are constructors? #card card-last-interval:: 25.4 card-repeats:: 4 card-ease-factor:: 2.32 card-next-schedule:: 2022-12-10T01:49:12.088Z card-last-reviewed:: 2022-11-14T16:49:12.088Z card-last-score:: 3
      • Constructors:
        • Initialise an object.
        • Have the same name as their class.
        • Have a close association with the fields:
          • They contain the initial values stored in the fields.
          • They contain the parameter values often used for these.
    • What is the point of the keyword this? #card card-last-score:: 5 card-repeats:: 4 card-next-schedule:: 2022-12-18T07:50:05.102Z card-last-interval:: 33.64 card-ease-factor:: 2.9 card-last-reviewed:: 2022-11-14T16:50:05.103Z
      • The this keyword refers to the current object in a method or constructor.
      • The most common use of this is to distinguish between class attributes & parameters of the same name.
      • If the input parameter variables in your constructor have the same name as your fields, you must use the this keyword to distinguish between the two.
      • this = "belonging to this object".
      • E.g., 
        • public Bicycle(int speed, int gear, int cadence)
{
  this.speed = speed;
  this.gear = gear;
  this.cadence = cadence;
}

  • Methods

    • What are methods? #card card-last-score:: 5 card-repeats:: 5 card-next-schedule:: 2022-12-18T19:12:15.540Z card-last-interval:: 33.96 card-ease-factor:: 2.04 card-last-reviewed:: 2022-11-14T20:12:15.540Z
      • Methods implement the behaviour of an object.
      • They have a consistent structure comprised of a header and a body.

    • Accessor Methods

      • What are accessor methods? #card card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-09T18:39:30.677Z card-last-reviewed:: 2022-11-11T11:39:30.678Z card-last-score:: 5
        • Accessor methods provide information about the state of an object.
        • An accessor method always returns a type that is not void.
        • An accessor method returns a value (result) of the type given in the header.
        • The method will contain a return statement to return the value.

    • Mutator Methods

      • What are mutator methods? #card card-last-interval:: 29.26 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-13T22:48:10.566Z card-last-reviewed:: 2022-11-14T16:48:10.566Z card-last-score:: 5
        • Mutator methods alter (mutate) the state of an object.
        • Achieved through changing the value of one or more fields.
          • They typically contain one or more assignment statements.
          • Often receive parameters.
        • image.png

      • Mutator Methods: Set

        • Each field may have a dedicated set mutator method.
        • These have a simple, distinctive form:
          • void return type
          • method name related to the field name
          • a single formal parameter, with the same type as the type of the field
          • a single assignment statement
        • A typical "set" method: 
          • public void setGear (int number)
{
  gear = number;
}

      • Protector Mutators

        • A set method does not always have to assign unconditionally to the field.
          • The parameter may be checked for validity and rejected if innappropriate.
        • Mutators thereby protect fields.
        • Mutators also support encapsulation.

        • Protecting a Field

          • public void setGear (int gearing)
{
  // this conditional statement prevents innapropriate action.
  // if protects the "gear" field from values that are too large or too small.
  if (gearing >= 1 && gearing <= 18)
  {
    gear = gearing;
  }
  else
  {
    System.out.println("Exceeds maximum gear ratio. Gear not set");
  }
}

    • Method Structure

      • The header:
        • The head tells us:
          • the visibility of the method to objects of other class.
          • whether or not the method returns a result.
          • the name of the method.
          • whether or not the method takes parameters.
        • E.g., 
          • public int getSpeed()
      • The body encloses the method's statements.

  • C vs Java

    • Unlike C, an OOP program will not have a pool of global variables that each method can access.
    • Instead, ^^each object has its own data^^, and other objects rely on the accessor methods of the object to access the data.

  • Conditional Statements

    • Conditional statements in Java have the same format as in C.
    • if (condition) {
  do something;
}
else {
  do somethingElse;
}
    • image.png