Java Variables

A locker at a railway station has a number on the door and something inside it. You find your bag by the number, and you can swap what's inside without changing the number. A variable is the same idea in a program: a named place in memory that holds a value. The name stays fixed; the value can change.

Declare, Assign, Use

Every variable in Java has three things: a type, a name and a value.

public class Wallet {
    public static void main(String[] args) {
        int balance;
        balance = 500;
        System.out.println("Balance: Rs. " + balance);
        balance = balance - 120;
        System.out.println("After chai and samosa: Rs. " + balance);
    }
}

Balance: Rs. 500
After chai and samosa: Rs. 380

How it works

1. Line 3 declares the variable: type int, name balance. Java sets aside room for one whole number.
2. Line 4 assigns a value with =. Read = as "becomes", not "equals".
3. Line 6 reads the current value, subtracts 120 and stores the result back in the same variable.

You'll usually declare and assign in one line: int balance = 500;. This is called initializing the variable.

Try it yourself: add a line that adds Rs. 1000 to balance (a top-up), then print it.

Three Kinds of Variables

Where you declare a variable decides its kind:

All Three in One Program

public class Coach {
    static int totalCoaches = 0;
    String code;
 
    public static void main(String[] args) {
        Coach s4 = new Coach();
        s4.code = "S4";
        totalCoaches++;
 
        Coach b2 = new Coach();
        b2.code = "B2";
        totalCoaches++;
 
        int seatsPerCoach = 72;
        System.out.println(s4.code + " and " + b2.code);
        System.out.println("Coaches: " + totalCoaches);
        System.out.println("Seats: " + totalCoaches * seatsPerCoach);
    }
}

S4 and B2
Coaches: 2
Seats: 144

How it works

1. Line 2: totalCoaches is static, so there's exactly one, shared by the whole class.
2. Line 3: code is an instance variable. Lines 6 and 10 create two Coach objects with new, and each gets its own code: one holds "S4", the other "B2".
3. Lines 8 and 12 add 1 to the single shared counter, so it ends at 2.
4. Line 14: seatsPerCoach is local to main. It exists only while main runs, and no other method can see it.

Objects and new are covered properly in Level 2. For now, notice that instance variables come one per object, and static variables one per class.

Try it yourself: create a third coach, A1, and update the counter. What do the last two lines print now?

Common Mistakes

Declaring the same name twice in one method. int total = 0; followed later by int total = 5; fails with variable total is already defined. Leave out the type the second time: total = 5;.

Expecting each object to have its own static variable. Changing a static variable through one object changes it for all of them, because there's only one.

Using a variable outside its braces. A variable declared inside { } disappears at the closing brace. The next lesson, "Scope and Lifetime of Variables", explains this.

Interview Corner

Practice

1. Easy: Declare variables for a train's name, its number of coaches and whether it has a pantry car. Print a sentence using all three.
2. Harder: In Coach, add an instance variable int bookedSeats. Give S4 40 booked seats and B2 55, then print the total booked seats across both coaches.

Key Takeaways