Variables: boxes in memory

Riya's canteen wallet

What is a variable?

Words you'll meet in this lesson

Variables at a glance

Three kinds of variables, and where each lives

Java sorts variables by where you declare them. That decides how long they live, who can see them and which part of memory holds them.

Ways to write a variable

The life of one variable

1. Declare it

   int balance;

   javac reserves a slot for balance in main's stack frame and remembers it holds an int (4 bytes). No value is stored yet, and javac won't let you read it.

2. Initialise it

   balance = 500;

   The number 500 is written into the slot. In bytecode this is sipush 500 then istore_1.

3. Read and update it

   balance = balance - 20;

   The right side runs first: read 500, subtract 20, get 480. Then the result is written back into the same slot. The old value 500 is overwritten; it isn't kept anywhere.

4. Lock a value with final

   final int CHAI_PRICE = 20;
   CHAI_PRICE = 25;   // error: cannot assign a value to final variable CHAI_PRICE

   The check happens at compile time: javac refuses any second assignment. At run time a final local is just a normal slot.

5. Watch it go out of scope

   {
       int tip = 5;
       balance = balance - tip;
   }
   // tip can't be used here

   A variable exists only inside the { } block where it was declared. After the closing brace the name tip is unknown to javac, and its slot can be reused.

Boxes appearing, changing and disappearing: both views

Step through Riya's wallet code. Left: the code and output. Right: main's stack frame, slot by slot.

Variables in real software

Every app you use is full of variables. Here's how the three kinds show up in real systems.

Watch it with an example: a metro card through one day

One variable, balance, holds the card's money. Watch the value in the box change while the name stays the same.

Examples are simplified to show where variables fit. Real systems differ in detail.

Riya's wallet program

public class Main {
    static int ordersToday = 0;          // static variable

    public static void main(String[] args) {
        int balance = 500;               // local variable
        final int CHAI = 20;             // constant
        balance = balance - CHAI;
        ordersToday = ordersToday + 1;
        {
            int tip = 5;                 // exists only in this block
            balance = balance - tip;
        }
        System.out.println("Balance: " + balance);
        System.out.println("Orders today: " + ordersToday);
    }
}

• static int ordersToday = 0;A static variable: one shared copy for the whole class.
• int balance = 500;A local variable, declared and initialised in one line.
• final int CHAI = 20;A constant. Any later CHAI = … is a compile error.
• balance = balance - CHAI;The right side is calculated first (500 − 20), then stored back.
• { int tip = 5; … }tip lives only inside this block.

Every variable, step by step

Watch the static variable stay with the class while local variables appear in main's frame, change, and disappear.

Worked examples: real problems, solved and traced in memory

Variables are named boxes on the stack. In these three programs, watch each box appear when it's declared, change when it's assigned, and disappear when its block ends.

start 500; −120, +200, −45

After lunch: Rs 380
After cashback: Rs 580
After bus ticket: Rs 535

bills 1200 and 350

GST on bill 1: Rs 216.0
GST on bill 2: Rs 63.0

first = 10, second = 16

First slot: 16:00
Second slot: 10:00

Check yourself

Quick revision

Interview and viva questions

Practice problems

Declare, assign and update variables, and use final for values that must not change. Every output is fixed. The last three are business tasks.

19, Nagpur

19 years, from Nagpur

public class Main {
    public static void main(String[] args) {
        int age = 19;
        String city = "Nagpur";
        System.out.println(age + " years, from " + city);
    }
}

(none)

Score: 30

public class Main {
    public static void main(String[] args) {
        int score = 0;
        score = score + 10;
        score = score + 10;
        score = score + 10;
        System.out.println("Score: " + score);
    }
}

6 weeks

42

public class Main {
    public static void main(String[] args) {
        final int DAYS_IN_WEEK = 7;
        System.out.println(6 * DAYS_IN_WEEK);
    }
}

5, 9

a = 9, b = 5

public class Main {
    public static void main(String[] args) {
        int a = 5, b = 9;
        a = a + b;
        b = a - b;
        a = a - b;
        System.out.println("a = " + a + ", b = " + b);
    }
}

total 400, tip 40

Total with tip: 440

public class Main {
    public static void main(String[] args) {
        int total = 400;
        {
            int tip = 40;
            total = total + tip;
        }
        System.out.println("Total with tip: " + total);
    }
}

50; −12, +30, −25

After sale: 38
After delivery: 68
After sale: 43

public class Main {
    public static void main(String[] args) {
        int stock = 50;
        stock = stock - 12;
        System.out.println("After sale: " + stock);
        stock = stock + 30;
        System.out.println("After delivery: " + stock);
        stock = stock - 25;
        System.out.println("After sale: " + stock);
    }
}

2000, 640

Rs 1700.0
Rs 544.0

public class Main {
    public static void main(String[] args) {
        final double MEMBER_DISCOUNT = 0.15;
        System.out.println("Rs " + 2000 * (1 - MEMBER_DISCOUNT));
        System.out.println("Rs " + 640 * (1 - MEMBER_DISCOUNT));
    }
}

as given

Savings: Rs 14000

public class Main {
    public static void main(String[] args) {
        int salary = 32000;
        int rent = 9500, food = 6000, travel = 2500;
        int savings = salary - rent - food - travel;
        System.out.println("Savings: Rs " + savings);
    }
}

Real-world scenario questions (asked in company interviews)

Variable mistakes cause some of the most expensive bugs in production. Decide your answer first, then open the model answer.