Variables & Data Types

IB Syllabus: B2.1.1 — Identify and describe variables, constants, and the common data types: int, double, boolean, char, String.

Key Concepts
1. Declaration and Initialisation
2. Constants
Data Types
1. Primitive Types (IB-assessed)
2. String
Naming Conventions
Operators
1. Arithmetic Operators
2. String Concatenation
Type Casting
1. Implicit (Widening) — Automatic
2. Explicit (Narrowing) — Manual
User Input with Scanner
1. Common Scanner Methods
Worked Examples
Quick Code Check
Trace Exercise
Code Completion
Spot the Bug
Output Prediction
Practice Exercises
1. Core
2. Extension
3. Challenge
GitHub Classroom
Connections

Key Concepts

A variable is a named container for storing data. In Java, you must declare the type before using a variable.

Declaration and Initialisation

int age;           // declared — memory reserved, no value yet
age = 16;          // initialised — value assigned

int lives = 3;     // declared and initialised in one line

Constants

Use final for values that should never change after assignment:

final double PI = 3.14159;
final int MAX_ATTEMPTS = 3;

By convention, constants use UPPER_SNAKE_CASE. The compiler will give an error if you try to reassign a final variable.

Data Types

A data type tells Java what kind of value a variable holds and how much memory to reserve.

Primitive Types (IB-assessed)

Type	Size	What it stores	Example
`int`	32 bits	Whole numbers	`int score = 42;`
`double`	64 bits	Decimal numbers	`double price = 9.99;`
`boolean`	—	`true` or `false` only	`boolean passed = true;`
`char`	16 bits	A single Unicode character	`char grade = 'A';`

char uses single quotes ('A'), String uses double quotes ("Alice"). Mixing them is a compile error.

String

String is not a primitive — it is an object. This matters later in OOP, but for now:

String name = "Alice";                        // double quotes, capital S
String greeting = "Hello, " + name;           // "Hello, Alice"
String empty = "";                            // empty string, not null

Compare Strings with .equals(), never ==:

if (name.equals("Alice")) { ... }   // correct
if (name == "Alice") { ... }        // WRONG — compares memory addresses

Naming Conventions

What	Rule	Example
Variables & methods	Start lowercase, camelCase	`totalScore`, `playerName`
Classes	Start uppercase, PascalCase	`GradeCalculator`
Constants	All caps, underscores	`MAX_ATTEMPTS`, `TAX_RATE`

Operators

Arithmetic Operators

Operator	Meaning	Example	Result
`+`	Addition	`5 + 3`	`8`
`-`	Subtraction	`5 - 3`	`2`
`*`	Multiplication	`5 * 3`	`15`
`/`	Division	`7 / 2`	`3` (integer!)
`%`	Modulo (remainder)	`7 % 2`	`1`

Integer division trap: When both operands are int, Java discards the decimal part. 7 / 2 gives 3, not 3.5. Cast one operand to get a decimal result: (double) 7 / 2 gives 3.5.

String Concatenation

The + operator joins Strings together, and automatically converts numbers to text:

String name = "Alice";
int age = 16;
System.out.println("Name: " + name + ", Age: " + age);
// Output: Name: Alice, Age: 16

Watch evaluation order: "A" + 1 + 2 gives "A12" (concatenation left-to-right), but 1 + 2 + "A" gives "3A" (addition first, then concatenation).

Type Casting

Sometimes you need to convert a value from one type to another.

Implicit (Widening) — Automatic

Java automatically promotes smaller types to larger types:

int total = 365;
double average = total;          // int automatically widened to double

Explicit (Narrowing) — Manual

Converting to a smaller type requires a cast — information may be lost:

double price = 9.99;
int truncated = (int) price;     // 9 — decimal part dropped, NOT rounded

char letter = 'A';
int code = (int) letter;         // 65 (Unicode value)

int code2 = 66;
char letter2 = (char) code2;     // 'B'

(int) 3.9 gives 3, not 4. Casting to int truncates — it does not round.

User Input with Scanner

The Scanner class reads input typed by the user. It requires an import statement.

import java.util.Scanner;

public class Greeting {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter your name: ");
        String name = input.nextLine();

        System.out.print("Enter your age: ");
        int age = input.nextInt();

        System.out.println("Hello, " + name + "! You are " + age + " years old.");
    }
}

Common Scanner Methods

Method	What it reads
`nextInt()`	Next integer
`nextDouble()`	Next decimal number
`next()`	Next word (stops at whitespace)
`nextLine()`	Entire line including spaces

nextLine() trap: After calling nextInt() or nextDouble(), the newline character \n remains in the buffer. A following nextLine() will read that leftover newline as an empty string. Fix: add a throwaway input.nextLine() call between them.

Worked Examples

Example 1: Area of a Circle

import java.util.Scanner;

public class AreaOfCircle {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        final double PI = 3.14159;

        System.out.print("Enter the radius: ");
        double radius = input.nextDouble();

        double area = PI * radius * radius;

        System.out.println("Area = " + area);
    }
}

Input: 5.0 → Output: Area = 78.53975

Example 2: Temperature Converter

import java.util.Scanner;

public class TempConverter {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter temperature in Celsius: ");
        double celsius = input.nextDouble();

        double fahrenheit = celsius * 9.0 / 5.0 + 32;

        System.out.println(celsius + "°C = " + fahrenheit + "°F");
    }
}

Input: 100 → Output: 100.0°C = 212.0°F

Notice 9.0 / 5.0 instead of 9 / 5. Using 9 / 5 would give 1 (integer division), producing an incorrect result.

Example 3: Swap Two Variables

int a = 10;
int b = 20;

int temp = a;    // save a's value
a = b;           // overwrite a with b
b = temp;        // put original a into b

System.out.println("a = " + a);   // a = 20
System.out.println("b = " + b);   // b = 10

Key insight: You need a temporary variable to swap — you cannot swap two values directly without losing one.

Example 4: Integer Division and Modulo

int total = 17;
int groupSize = 5;

int fullGroups = total / groupSize;   // 3
int remaining = total % groupSize;     // 2

System.out.println("Full groups: " + fullGroups);
System.out.println("Remaining: " + remaining);

Use cases for %:

Check if even: n % 2 == 0
Extract last digit: n % 10
Wrap around (clock, array index): index % length

Quick Code Check

Q1. What is the result of 7 / 2 in Java?

Q2. Which data type stores a single character such as 'A'?

Q3. What does (int) 3.9 evaluate to?

Q4. What does "Score: " + 42 produce?

Q5. What is the result of 17 % 5?

Q6. What does the final keyword do to a variable?

Trace Exercise

Trace this code — fill in the value of each variable after each line executes.

int a = 10;
int b = 3;
int c = a / b;
int d = a % b;
double e = (double) a / b;
a = a + d;

Line	a	b	c	d	e
`int a = 10`		—	—	—	—
`int b = 3`	10		—	—	—
`int c = a / b`	10	3		—	—
`int d = a % b`	10	3	3		—
`double e = (double) a / b`	10	3	3	1
`a = a + d`		3	3	1	3.333...

Code Completion

Complete the missing parts of this program that calculates the area of a rectangle. The program reads two dimensions from the user using Scanner and computes the area using the correct data type and method call.

import java.util.Scanner;

Scanner input = new Scanner(System.in);
System.out.print("Enter width: ");
 width = input.nextDouble();
System.out.print("Enter height: ");
double height = input.;

double area = ;
System.out.println("Area = " + area);

Complete the code to swap two variables using a temporary variable. Swapping requires a third variable to hold one value so it is not lost during reassignment.

int x = 5;
int y = 10;

int temp = ;
x = ;
y = ;

System.out.println("x = " + x);   // should print 10
System.out.println("y = " + y);   // should print 5

Spot the Bug

This code should calculate the average of three test scores, but operator precedence and integer division cause it to produce the wrong answer. Find the bug.

int score1 = 80;
int score2 = 90;
int score3 = 85;
double average = score1 + score2 + score3 / 3;
System.out.println("Average: " + average);

What is wrong and how do you fix it?

This code applies the formula F = C * 9/5 + 32 to convert Celsius to Fahrenheit, but integer division causes it to produce incorrect results. Find the bug.

int celsius = 100;
double fahrenheit = celsius * 9 / 5 + 32;
System.out.println(celsius + "C = " + fahrenheit + "F");

What is wrong and how do you fix it?

Output Prediction

What does this code print? This code demonstrates the difference between integer division (/), the modulo operator (%), and explicit type casting to double for decimal results. Type the exact output.

int x = 5;
int y = 2;
System.out.println(x / y);
System.out.println(x % y);
System.out.println((double) x / y);

Expected:

2
1
2.5

What does this code print? This code demonstrates how Java evaluates + left-to-right: when a String appears first, subsequent numbers are concatenated as text, but when numbers appear first, they are added arithmetically until a String is encountered.

String s = "Hello";
System.out.println(s + " " + 3 + 4);
System.out.println(3 + 4 + " " + s);

Expected:

Hello 34
7 Hello

Practice Exercises

Core

Name and age — Write a program that reads the user’s name and age, then prints a greeting: "Hello, Alice! You are 16 years old."
Average calculator — Read three test scores from the user and print the average as a decimal number.
Coin converter — Read a number of cents from the user and output how many dollars and cents that is. Use integer division and modulo.
Rectangle calculator — Read the width and height of a rectangle. Print its area and perimeter.

Extension

Time converter — Read a total number of seconds and convert it to hours, minutes, and seconds. Example: 3661 seconds = 1 hour, 1 minute, 1 second.
String explorer — Read a word from the user. Print its length, first character, last character, and the word in uppercase.
Number decomposer — Read a 3-digit number (e.g., 472) and extract the hundreds, tens, and units digits using / 10 and % 10.

Challenge

BMI calculator — Read weight (kg) and height (m) from the user. Calculate and display the BMI (weight / height²) with appropriate formatting.
Letter grade with rounding — Read a decimal score and round it to the nearest integer using type casting (hint: add 0.5 before casting). Then assign a letter grade.

GitHub Classroom

Practice declaring variables, using operators, reading input, and type casting.

Connections

Prerequisites:

None — this is the starting point for Java programming

Related Topics:

String Methods — built-in methods for manipulating text
Exception Handling — handling runtime errors
Debugging — finding and fixing errors

What’s Next:

Selection (if/else) — making decisions in code
Iteration (Loops) — repeating code

Once you understand variables and types, selection (if/else) is the natural next step — you already know how to write conditions like score >= 50.

Variables & Data Types

Table of Contents

Key Concepts

Declaration and Initialisation

Constants

Data Types

Primitive Types (IB-assessed)

String

Naming Conventions

Operators

Arithmetic Operators

String Concatenation

Type Casting

Implicit (Widening) — Automatic

Explicit (Narrowing) — Manual

User Input with Scanner

Common Scanner Methods

Worked Examples

Example 1: Area of a Circle

Example 2: Temperature Converter

Example 3: Swap Two Variables

Example 4: Integer Division and Modulo

Quick Code Check

Trace Exercise

Code Completion

Spot the Bug

Output Prediction

Practice Exercises

Core

Extension

Challenge

GitHub Classroom

Connections