static and final - Java

Two keywords appear all over Java code: static and final. They answer two different questions: "does this belong to the class or to each object?" and "can this change?"

static fields: shared by all objects

An ordinary (instance) field gets a separate copy in every object. A static field has one copy shared by the whole class.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        new Visitor("Asha");
        new Visitor("Ravi");
        new Visitor("Meera");
        System.out.println("Total visitors: " + Visitor.count);
    }
}

class Visitor {
    static int count = 0;      // one counter for the class
    String name;               // one name per object

    Visitor(String name) {
        this.name = name;
        count++;
        System.out.println(name + " is visitor #" + count);
    }
}

Output

Asha is visitor #1
Ravi is visitor #2
Meera is visitor #3
Total visitors: 3

static methods

A static method can be called on the class without creating an object, like Math.sqrt(16) or Integer.parseInt("5"). Because there is no object, a static method cannot use this or instance fields directly.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        System.out.println(MathUtil.square(9));
        System.out.println(MathUtil.isEven(10));
        System.out.println(Math.max(3, 7) + Math.abs(-2));
    }
}

class MathUtil {
    static int square(int x) {
        return x * x;
    }

    static boolean isEven(int x) {
        return x % 2 == 0;
    }
}

Output

81
true
9

That is also why main is static: the JVM calls it before any object exists.

final variables: constants

A final variable can be assigned only once. Combined with static, it makes a class-wide constant, written in UPPER_SNAKE_CASE by convention.

Java

Open in playground

public class Main {
    static final double GST_RATE = 0.18;
    static final int MAX_LOGIN_ATTEMPTS = 3;

    public static void main(String[] args) {
        final int base = 1000;
        // base = 2000;   // compile error: cannot assign a value to final variable
        System.out.println(base * (1 + GST_RATE));
        System.out.println("Attempts allowed: " + MAX_LOGIN_ATTEMPTS);

        final int[] marks = {1, 2, 3};
        marks[0] = 99;     // allowed: the array's contents can change
        System.out.println(marks[0]);
    }
}

Output

1180.0
Attempts allowed: 3
99

final on a reference means the variable cannot point to a different object; the object itself can still change unless it is immutable.

final methods and classes

A final method cannot be overridden by a child class.
A final class cannot be extended at all. String is a final class.

static blocks

A static { ... } block runs once, when the class is first loaded. It is useful for setting up complex static data.

Java

Open in playground

public class Main {
    static final int[] SQUARES = new int[5];

    static {
        for (int i = 0; i < SQUARES.length; i++) {
            SQUARES[i] = i * i;
        }
        System.out.println("static block ran");
    }

    public static void main(String[] args) {
        System.out.println(java.util.Arrays.toString(SQUARES));
    }
}

Output

static block ran
[0, 1, 4, 9, 16]