static and final

Why this matters

Counters, constants and utility methods all depend on these two keywords, and they appear in nearly every exam paper.

Example

class Student {
    static String college = "ABC Degree College";   // shared by all students
    static int count = 0;                           // shared counter
    final int rollNumber;                           // set once, never changed
    String name;

    Student(String name, int rollNumber) {
        this.name = name;
        this.rollNumber = rollNumber;
        count++;
    }

    static void showCount() {                       // no object needed
        System.out.println("Total students: " + count);
    }
}

public class Main {
    public static void main(String[] args) {
        new Student("Ananya Rao", 12);
        new Student("Arjun", 15);
        System.out.println(Student.college);
        Student.showCount();
    }
}

Step by step

1. 1`static String college` exists once in memory, shared by every Student object.
2. 2`count++` inside the constructor increases the shared counter each time an object is created.
3. 3`final int rollNumber` must be assigned exactly once — here in the constructor.
4. 4`Student.showCount()` is called on the class name, without any object.
5. 5A static method cannot use non-static fields directly, because there is no 'current object'.

Key points

• static members belong to the class, shared by all objects
• static methods cannot use this or instance fields
• final variable = constant, final method = cannot be overridden, final class = cannot be extended
• static final is the usual way to declare constants

Common mistakes

• Using an instance field inside a static method
• Trying to reassign a final variable
• Expecting each object to have its own copy of a static field

Try it yourself

• Add a static counter to your BankAccount class to count accounts created.
• Declare a static final double PI and use it in an area calculation.