Instance Variable vs Static Variable

In a cricket team, each player has their own runs scored, but the team has one total score on the board. If you stored the team total inside each player, you'd have eleven different "totals" and none of them right. Fields work the same way: instance variables hold each object's own data, and static variables hold the one value the whole class shares. Choosing the wrong one is a classic bug.

Two Counters

public class Gates {
    static class Gate {
        int passed;
        static int totalPassed;
 
        void tap() {
            passed++;
            totalPassed++;
        }
    }
 
    public static void main(String[] args) {
        Gate a = new Gate();
        Gate b = new Gate();
        a.tap(); a.tap(); a.tap();
        b.tap();
        System.out.println("Gate A: " + a.passed);
        System.out.println("Gate B: " + b.passed);
        System.out.println("Station total: " + Gate.totalPassed);
    }
}

Gate A: 3
Gate B: 1
Station total: 4

How it works

1. Line 3: passed is an instance variable: each gate has its own.
2. Line 4: totalPassed is a static variable: one value for the class.
3. Each tap() adds 1 to that gate's passed and 1 to the shared total.

Try it yourself: make passed static too. What do the three lines print now?

Side by Side

Constants Are Static Too

Values that never change and are the same for every object should be static final, not instance fields:

Common Mistakes

Storing per-object data in a static variable. Every object would overwrite the same value.

Storing a shared counter in an instance variable. Each object would count only itself.

Accessing static variables through objects. a.totalPassed works but hides the fact that it's shared; write Gate.totalPassed.

Interview Corner

Practice

1. Easy: In a Student class, decide which should be static: name, rollNo, schoolName, studentCount.
2. Harder: Write a Ticket class where each ticket gets a unique id (instance) taken from a static nextId counter that starts at 1001.

Key Takeaways