Class Variables and Methods: Revision Notes (Computer Science A 3.7)

These notes recap Topic 3.7. Full explanations and worked examples are in the study guide.

Recap

Class variables belong to the class. All objects share a single copy. They are declared with static before the type.
A public class variable is accessed outside the class with the class name and dot operator, for example Member.MAX_BORROW.
Class methods (static) can access and change class variables and call other class methods.
Class methods cannot access or change instance variables or call instance methods unless they are passed an instance as a parameter.
A final variable cannot be modified. Constants are usually public static final.
Typical uses: counting objects created, and giving each object a unique identifier.

Key relationships

Feature	Declared	Copies	Accessed with
Instance variable	`private int id;`	One per object	An object (inside its methods)
Class variable	`private static int nextId;`	One for the class	Class methods, instance methods, constructors
Constant	`public static final int MAX = 4;`	One, unchangeable	`ClassName.MAX`
Instance method	`public int getId()`	Operates on one object	An object: `a.getId()`
Class method	`public static int getCount()`	Belongs to the class	`ClassName.getCount()`

Worked reminder

Member gives each new object id = nextId then increments nextId. After three constructions the ids are 1, 2 and 3, and Member.getCount() is 3. In Tally, hit() increases both the per-object mine and the shared all. After p.hit() twice, q.hit(), Tally.reset() and q.hit() the values are p.mine 2, q.mine 2 and all 1, so the output is 2 2 1. Shop.withTax(80.0) is 100.0 when TAX_RATE is 0.25.

Compile errors to recognise

Message	Cause
`non-static variable id cannot be referenced from a static context`	Class method uses an instance variable with no instance
`non-static method getId() cannot be referenced from a static context`	Class method calls an instance method with no instance
`cannot assign a value to static final variable LIMIT`	Assigning to a constant

Mistakes to avoid

Declaring a counter as an instance variable when it needs to be shared.
Resetting a shared counter inside the constructor.
Using this or an instance variable in a class method (Topic 3.9 covers this).
Calling a class method through an object instead of the class name (it works, but is poor style).
Confusing static with final.

Quick self-check

Three Order objects are created. How many copies of a static int ordersPlaced exist? Answer: one.
Can public static int total() { return count; } compile if count is an instance variable? Answer: No.
How do you read public static final int LIMIT from another class? Answer: ClassName.LIMIT.
Which is the right place to increment a shared counter of objects created? Answer: in the constructor.