3.7 Class Variables and Methods

1. Class variables: one shared copy

Class variables belong to the class, and all objects of the class share a single copy. They are marked with the keyword static before the variable type 3.7.B.1. Instance variables are the opposite: each object has its own copy.

This class is used on the whole page:

public class Ticket
{
    public static final int MAX_TICKETS = 100;   // class variable, final
    private static int nextNumber = 1;           // class variable

    private String event;                        // instance variable
    private int number;                          // instance variable

    public Ticket(String eventName)
    {
        event = eventName;
        number = nextNumber;
        nextNumber++;
    }

    public int getNumber()
    {
        return number;
    }

    public static int ticketsIssued()
    {
        return nextNumber - 1;
    }

    public static int ticketsLeft()
    {
        return MAX_TICKETS - ticketsIssued();
    }

    public static boolean isEarlier(Ticket a, Ticket b)
    {
        return a.number < b.number;
    }

    public String toString()
    {
        return event + " #" + number;
    }
}

Ticket t1 = new Ticket("Concert");
Ticket t2 = new Ticket("Concert");
Ticket t3 = new Ticket("Match");
System.out.println(t1 + ", " + t2 + ", " + t3);
System.out.println(Ticket.ticketsIssued());

Output:

Concert #1, Concert #2, Match #3
3

Memory after the code runs:

class Ticket:   MAX_TICKETS 100   nextNumber 4     (one copy, shared)

t1 [ ref ] ---> Ticket object: event "Concert"  number 1
t2 [ ref ] ---> Ticket object: event "Concert"  number 2
t3 [ ref ] ---> Ticket object: event "Match"    number 3

Printing a Ticket uses its toString method (background only: writing toString is outside the AP® exam). Every constructor call read and changed the same nextNumber. That is how each ticket gets a different number.

2. Using a public class variable from outside

A class variable that is public is accessed outside the class with the class name and the dot operator, because it is associated with the class, not with objects of the class 3.7.B.2. This is a new program, so no tickets exist before it runs:

System.out.println("Limit: " + Ticket.MAX_TICKETS);
Ticket t = new Ticket("Play");
System.out.println("Left: " + Ticket.ticketsLeft());

Output:

Limit: 100
Left: 99

You do not need any Ticket object to read Ticket.MAX_TICKETS. A private class variable such as nextNumber cannot be used from outside at all: other classes use the public class method ticketsIssued instead.

3. final: a value that cannot change

When a variable is declared final, its value cannot be modified 3.7.B.3. Constants are usually public static final, with a name in capital letters.

Ticket.MAX_TICKETS = 500;

Compiler says:

error: cannot assign a value to static final variable MAX_TICKETS

final works for local variables too:

final double TAX_RATE = 0.08;
double price = 50.0;
System.out.println(price + price * TAX_RATE);

Output:

54.0

4. What class methods can do

A class method (a static method) can access or change the values of class variables, and it can call other class methods 3.7.A.2.

In Ticket, the class method ticketsIssued reads the class variable nextNumber, and the class method ticketsLeft calls the class method ticketsIssued. A class method can also change a class variable:

public class Scoreboard
{
    private static int games = 0;
    private static int totalGoals = 0;

    public static void recordGame(int goals)
    {
        games++;
        totalGoals += goals;
    }

    public static double averageGoals()
    {
        return (double) totalGoals / games;
    }

    public static String report()
    {
        return games + " games, " + averageGoals() + " goals per game";
    }
}

Scoreboard.recordGame(3);
Scoreboard.recordGame(0);
Scoreboard.recordGame(2);
System.out.println(Scoreboard.report());

Output:

3 games, 1.6666666666666667 goals per game

5. What class methods cannot do (without an object)

Class methods cannot access or change instance variables, or call instance methods, unless an instance of the class is passed to them through a parameter 3.7.A.1. A class method is called on the class, so there is no “current object” whose instance variables it could use.

public class Seat
{
    private int row;

    public Seat(int r)
    {
        row = r;
    }

    public static boolean isFrontRow()
    {
        return row == 1;
    }
}

Compiler says:

error: non-static variable row cannot be referenced from a static context

The fix is to pass an object in. Ticket.isEarlier does exactly that: it receives two Ticket objects and uses their instance variables.

Ticket a = new Ticket("Concert");
Ticket b = new Ticket("Concert");
System.out.println(Ticket.isEarlier(a, b));
System.out.println(Ticket.isEarlier(b, a));

Output:

true
false

Calling an instance method from a class method without an object fails the same way:

public class Clock
{
    private int minutes;

    public int getMinutes()
    {
        return minutes;
    }

    public static int getHours()
    {
        return getMinutes() / 60;
    }
}

Compiler says:

error: non-static method getMinutes() cannot be referenced from a static context

	Can use instance variables and instance methods?	Can use class variables and class methods?
Instance method	Yes	Yes
Class method	Only through an object parameter 3.7.A.1	Yes 3.7.A.2