Let's write a Ticket class

• Some 'client' code:

  Ticket tkt1 = new Ticket("Lughnasa - Banshees welcome", "1st August");
  Ticket tkt2 = new Ticket("Imbolg (Free Chicken Sacrifices!)", "1st Feb");
  Ticket tkt3 = new Ticket("Beltane - Bring a Virgin", "1st May");
  System.out.println(tkt1);
  

• The interface:
  • public Ticket(String occasion, String date)
  • public String toString()
• The implementation will need at least two instance variables: for the occasion and for the date

Unique numbers for Ticket objects

• Suppose each ticket object must have a unique number, starting from 1
• We'll definitely need a third instance variable:

  private String occasion;
  private String date;
  private int ticketNumber;

• And we're going to need another variable that keeps track of what number to use for the next ticket
• Then the constructor will be as follows:

  public Ticket(String occasion, String date) {
      this.occasion = occasion;
      this.date = date;
      ticketNumber = nextTicketNumber;
      nextTicketNumber = nextTicketNumber + 1;
  }

• What kind of variable is nextTicketNumber?

nextTicketNumber as an instance variable

• Suppose nextTicketNumber is an instance variable:

  private int nextTicketNumber = 1;

• Create tkt1, tkt2 and tkt3:

nextTicketNumber as a class variable

• Suppose nextTicketNumber is a class variable:

  private static int nextTicketNumber = 1;

• Create tkt1, tkt2 and tkt3:

Summary of Ticket so far

class Ticket {

    private String occasion;
    private String date;
    private int ticketNumber;
    
    private static int nextTicketNumber = 1;
    
    public Ticket(String occasion, String date) {
        this.occasion = occasion;
        this.date = date;
        ticketNumber = nextTicketNumber;
        nextTicketNumber = nextTicketNumber + 1;
    }
    
    public String toString() {
        // not yet written
    }
       
}

Class variables

• Instance variables:
  • created per-instance (i.e. one in each object we create)
• Class variables (sometimes called 'static variables'):
  • created per-class (i.e. the variable is 'shared' among the objects of the class)
  • very rare (except when final)
  • declaration: at 'top-level' within a class definition, including the word static
  • initialization: Java automatically initializes with a default value. But you can also initialize them yourself in the declaration. (Advanced: You can also use 'static initializers')
  • scope:
    • everywhere in this class definition, you can use nextTicketNumber
    • in other class definitions, you must prefix with the class name, e.g. Ticket.nextTicketNumber (but you may still not be able to access the variable depending on its visibility)
  • visibility: as per instance variables (private is preferred, unless the variable is also final, in which case it might be public)

The usefulness of named constants

• Compare this:

  class Lecturer {
  
      private String name;
      private Student[] supervisees;
      
      public Lecturer(String name) {
          this.name = name;
          supervisees = new Student[3];
      }
      
      public void addSupervisee(Student stdnt) {
          for (int i = 0; i < 3; i = i + 1) {
              if (supervisees[i] == null) {
                  supervisees[i] = stdnt;
                  break;
              }
          }
      }
      
  }

• with this:

  class Lecturer {
  
      private String name;
      private Student[] supervisees;
      
      private static final int MAX_SUPERVISEES = 3;
      
      public Lecturer(String name) {
          this.name = name;
          supervisees = new Student[MAX_SUPERVISEES];
      }
      
      public void addSupervisee(Student stdnt) {
          for (int i = 0; i < MAX_SUPERVISEES; i = i + 1) {
              if (supervisees[i] == null) {
                  supervisees[i] = stdnt;
                  break;
              }
          }
      }
      
  }

• The second version is more readable and more easily changed

Public named constants

• If you think other classes will find your named constant to be useful, you can make it public, e.g.

  public static final int MAX_SUPERVISEES = 3;

• There are many public named constants in the standard Java library
  • java.lang.Math.PI and java.lang.Math.E
  • java.lang.Integer.MAX_VALUE and java.lang.Integer.MIN_VALUE,...
  • java.lang.System.out and java.lang.System.in,...
  • ...and many more!