UNIT 3: CLASS CREATION · TOPIC 3.7

3.7 Class Variables and Methods

static members belong to the class, not to any single object. One copy, shared by everyone, accessible without creating an object.

What you need to know

A class variable (static variable) is declared with static: private static int count; There is one copy shared by all objects of the class.
Common uses: counting how many objects have been created, or constants: public static final double RATE = 0.05;
A class method (static method) is declared with static and called on the class: ClassName.method(). It can be called without any object existing.
Static methods cannot access instance variables or call instance methods directly — there's no object to refer to. They can access static variables.
Instance methods can access static variables and methods.
Static variables are initialized when the class is first loaded, before any object is created. Their default values are the same as instance variables.
Access a static variable from an instance method or from outside (if public) via the class name: Student.count.

Worked example

public class Ticket
{
    private static int nextId = 1;        // shared by all tickets
    public static final double FEE = 2.50; // constant
    private int id;                        // each ticket has its own

    public Ticket()
    {
        id = nextId;
        nextId++;
    }
    public static int getNextId() { return nextId; }   // OK: static uses static
    public int getId() { return id; }
    // public static int getId() { return id; }  // ERROR: no object
}
Ticket a = new Ticket();   // id 1
Ticket b = new Ticket();   // id 2
System.out.println(Ticket.getNextId());   // 3

Exam tip: "Which variable should be static?" — anything that's the same for every object or tracks the class as a whole. "Why won't this compile?" — a static method touching an instance variable. Trace static variables as one shared cell, not one per object.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

static = belongs to the class. One copy for all objects, existing even if zero objects have been created. Non-static = belongs to each object.
Uses for static variables: counters across all instances (how many Tickets sold), shared configuration, and constants (public static final double TAX = 0.07;). Constants are the most common on the exam.
Static methods can't use this, can't access instance variables, and can't call instance methods without an object — because there may be no object. They can access static variables and call other static methods. main is static, which is why it needs to create objects to call instance methods.
Instance methods can use static members freely. A getPrice() instance method can read a static TAX constant. The restriction is one-directional.
Accessing static members: ClassName.member from anywhere it's visible. object.staticMember compiles but is misleading and the exam avoids it. Inside the class, bare name.
Static initialization: static variables are set when the class loads, once. A static count = 0 then incremented in the constructor counts all objects ever created.
Tracing static state: keep one row for the static variable, separate from any object's rows. Every object shares that row.

Mistakes that cost points

Static method touching an instance variable. Compile error. The single most common static question.
Thinking each object has its own copy of a static variable. One copy, shared.
Using this in a static method. Compile error.
Calling an instance method from main without an object. Compile error. Create an object first.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

Q1 A class Player has private static int numPlayers = 0; incremented in its constructor. After creating three Player objects, what is numPlayers?

A 0
B 1
C 3
D Each object has its own value

Show answer

Answer: C. One shared variable, incremented three times.

Q2 Which of the following will not compile inside a class with private int size; and private static int total;?

A public int getSize() { return size; }
B public static int getTotal() { return total; }
C public static int getSize() { return size; }
D public int getTotal() { return total; }

Show answer

Answer: C. A static method can't access an instance variable — there's no object.

Key vocabulary

Class variable / static variable: a single variable shared by all objects of a class
Class method / static method: a method called on the class that can't use instance variables
Constant: a static final variable whose value never changes