Java Static Keyword

In a hostel, each room has its own fan and its own light, but the whole building shares one water tank and one notice board. Java classes work the same way. Ordinary fields belong to each object, like a room's fan. A static member belongs to the class itself: there's one copy, shared by every object, and it exists even when no object has been created yet.

One Shared Copy

public class Bank {
    static class BankAccount {
        static String bankName = "PoolBank";
        static int totalAccounts = 0;
 
        String holder;
        long balance;
 
        BankAccount(String holder, long balance) {
            this.holder = holder;
            this.balance = balance;
            totalAccounts++;
        }
 
        static String branchCode(String city) {
            return city.substring(0, 3).toUpperCase() + "001";
        }
    }
 
    public static void main(String[] args) {
        System.out.println("Accounts before: " + BankAccount.totalAccounts);
        new BankAccount("Navika", 5000);
        new BankAccount("Srihitha", 12000);
        new BankAccount("Advitha", 800);
        System.out.println(BankAccount.bankName + " accounts: "
                + BankAccount.totalAccounts);
        System.out.println("Branch: " + BankAccount.branchCode("Pune"));
    }
}

Accounts before: 0
PoolBank accounts: 3
Branch: PUN001

How it works

1. Lines 3 and 4 are static variables: one bankName and one totalAccounts for the whole class.
2. Line 21 reads totalAccounts before any object exists, through the class name.
3. Each constructor call increases the same counter (line 12), so it ends at 3.
4. Lines 15 to 17: a static method works only with its parameter and is called as BankAccount.branchCode(...).

Try it yourself: remove static from totalAccounts (line 4). What do the counts become, and why does line 21 stop compiling?

Where static Can Go

What Static Code Can't Do

Static methods and blocks run without an object, so they can't use instance fields, instance methods or this directly:

StaticCtx.java:1: error: non-static variable this cannot be referenced from a static context
public class StaticCtx { int count; static void inc() { this.count++; } }
                                                        ^

Instance methods, on the other hand, can use static members freely.

Common Mistakes

Making a per-object field static. A static balance would give every account the same balance.

Calling static members through an object. navika.totalAccounts compiles but misleads; write BankAccount.totalAccounts.

Making everything static to avoid "static context" errors. That throws away objects and their state. Create an object instead.

Interview Corner

Practice

1. Easy: Add a static nextId to a Ticket class so each new ticket gets a unique number: 1, 2, 3...
2. Harder: Write a class with a static method isValidPan(String pan) that checks a 10-character PAN format (5 letters, 4 digits, 1 letter) without creating any object.

Key Takeaways