Instance vs. Static Members

An instance field or method belongs to a specific object; a static one belongs to the class itself, shared by every instance.

class Counter {
    static int totalCreated = 0; // shared by every Counter
    int id;                        // unique per Counter

    Counter() {
        totalCreated++;
        id = totalCreated;
    }
}

Counter a = new Counter();
Counter b = new Counter();
System.out.println(Counter.totalCreated); // 2 — accessed via the class
System.out.println(a.id);                  // 1
System.out.println(b.id);                  // 2

main itself is static — the JVM calls it without creating an instance of the class first, which is also why a static method can't directly access an instance field.

Common mistakes

• Trying to access an instance field from a static method without an object to reference it through — there's no implicit object for static code to use.