Static vs Instance in AP CSA

Instance members belong to a specific object. Static members belong to the class itself and are shared across all objects. The Math class is all static — you never write new Math().

Instance Members

Each object gets its own copy of instance variables. Instance methods can access instance variables with this.

public class Circle {
    private double radius;          // instance variable -- each Circle has its own

    public Circle(double r) {
        radius = r;
    }

    public double getArea() {       // instance method -- needs a specific Circle
        return Math.PI * radius * radius;
    }
}

Static Members

Static variables are shared by all objects. Static methods belong to the class and cannot access instance variables directly.

public class Circle {
    private double radius;
    private static int count = 0;   // shared across ALL circles

    public Circle(double r) {
        radius = r;
        count++;                    // every new Circle increments the shared count
    }

    public static int getCount() { // static method -- called on the class
        return count;
    }
}

// Usage
Circle c1 = new Circle(5.0);
Circle c2 = new Circle(3.0);
System.out.println(Circle.getCount());  // 2 -- called on CLASS, not object