Static vs Instance Members

One Copy vs One Per Object

Instance fields belong to each object: every Student has its own name. Static fields belong to the class: there is exactly one copy shared by all objects.

	Instance member	Static member
Belongs to	each object	the class
Copies	one per object	one in total
Accessed with	`obj.field`	`ClassName.field`
Can use `this`	yes	no (there is no current object)
Typical use	object data (name, balance)	counters, constants, utility methods

Counting Objects with a Static Field

Example

public class Main {
    public static void main(String[] args) {
        new User("Asha");
        new User("Ravi");
        User u = new User("Meena");
        System.out.println("Last user id: " + u.id);
        System.out.println("Total users: " + User.count);
    }
}

class User {
    static int count = 0;    // shared by all users
    int id;                  // each user has its own
    String name;

    User(String name) {
        count++;
        this.id = count;
        this.name = name;
        System.out.println("Created " + name + " with id " + id);
    }
}

Output

Created Asha with id 1
Created Ravi with id 2
Created Meena with id 3
Last user id: 3
Total users: 3

Static Methods and Static Blocks

A static method can be called without an object, like Math.max(). It can only use static fields directly. A static block runs once, when the class is first loaded, and is used to set up static data.

Example

public class Main {
    public static void main(String[] args) {
        System.out.println("Main started");
        System.out.println("GST on 1000: " + Tax.gst(1000));
        System.out.println("GST on 500: " + Tax.gst(500));
    }
}

class Tax {
    static final double RATE;

    static {
        System.out.println("Tax class loaded");
        RATE = 0.18;
    }

    static double gst(double amount) {
        return amount * RATE;
    }
}

Output

Main started
Tax class loaded
GST on 1000: 180.0
GST on 500: 90.0

Note: Notice "Tax class loaded" prints only once, and only when Tax is used for the first time.

Common Mistake

A static method cannot touch instance fields, because there is no object to read them from.

class Counter {
    int value;

    static void reset() {
        value = 0;   // compile error: non-static variable value cannot be referenced from a static context
    }
}

Tip: Rule of thumb: if a method does not use any instance field, it can be static (for example, helper methods in a Utils class).