Variables & Data Types

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

Table of Contents

1. Key Concepts
   1. Declaration and Initialisation
   2. Constants
2. Data Types
   1. Primitive Types (IB-assessed)
   2. String
3. Naming Conventions
4. Operators
   1. Arithmetic Operators
   2. String Concatenation
5. Type Casting
   1. Implicit (Widening) - Automatic
   2. Explicit (Narrowing) - Manual
6. User Input with Scanner
   1. Common Scanner Methods
7. Worked Examples
   1. Example 1: Area of a Circle
   2. Example 2: Temperature Converter
   3. Example 3: Swap Two Variables
   4. Example 4: Integer Division and Modulo
8. Quick Code Check
9. Trace Exercise
10. Code Completion
11. Spot the Bug
12. Output Prediction
13. Practice Exercises
    1. Core
    2. Extension
    3. Challenge
14. GitHub Classroom
15. Connections

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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.

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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

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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

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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).

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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.

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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.

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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

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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.

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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.

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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

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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?

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Trace Exercise

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

int a = 10;
int b = 4;
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 = 4	10		-	-	-
int c = a / b	10	4		-	-
int d = a % b	10	4	2		-
double e = (double) a / b	10	4	2	2
a = a + d		4	2	2	2.5

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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);

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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

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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?

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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?

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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

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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

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Practice Exercises

Core

1. 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."

2. Average calculator - Read three test scores from the user and print the average as a decimal number.

3. Coin converter - Read a number of cents from the user and output how many dollars and cents that is. Use integer division and modulo.

4. Rectangle calculator - Read the width and height of a rectangle. Print its area and perimeter.

Extension

1. 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.

2. String explorer - Read a word from the user. Print its length, first character, last character, and the word in uppercase.

3. Number decomposer - Read a 3-digit number (e.g., 472) and extract the hundreds, tens, and units digits using / 10 and % 10.

Challenge

1. BMI calculator - Read weight (kg) and height (m) from the user. Calculate and display the BMI (weight / height²) with appropriate formatting.

2. 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.

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GitHub Classroom

Variables & Data Types

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

Core + Extension Open in GitHub

Codespace Setup Instructions (click to expand)

First Time Setup

1. Click “Open in GitHub” above, then click the green “Accept this assignment” button
2. Check the email registered with your GitHub account – click the link there to open your repository. This avoids access issues and means your teacher does not need to share links manually
3. Wait for your repository to be created, then click “Open in Codespace”
4. Wait for the Codespace to finish loading (you’ll see VS Code in your browser)

Important: Switch to Standard Mode

1. Look at the bottom status bar. If it says Java: Lightweight Mode, click on it and select Standard
2. Wait for Java to finish loading (status bar will say Java: Ready)
3. Without Standard Mode, the Run button won’t work and you’ll get “Main method not found” errors

Dismiss Popups

You’ll see two notification popups. Dismiss them both:

• “Would you like VS Code to periodically run git fetch?” → Click No
• “There’s a pull request associated with main” → Click Don’t Show Again

These are housekeeping popups, not part of the assignment.

How to Code

1. Open src/VariablesAndTypes.java. This is the only file you need to edit
2. Complete each // TODO section
3. Click the Run button (▶ above main) to test your output
4. Compare your output with the expected output written in the comments above each task

How to Check Your Code

You can check your work in two ways:

Option A: Run your code directly

• Click the Run button (▶) above main in VariablesAndTypes.java
• Compare your printed output with the expected output shown in the comments above each task

Option B: Run the autograder tests

• Open the Terminal (Menu → Terminal → New Terminal)
• Type bash run-tests.sh and press Enter
• Green ✓ = test passed, Red ✗ = test failed
• Each test tells you what it expected. Read the error message to fix your code

How to Submit

1. Click the Source Control icon (branch icon) in the left sidebar
2. Type a message like completed tasks in the message box
3. Click the green Commit button
4. If asked “stage all changes?” → click Always
5. Click Sync Changes → if asked about push/pull → click OK, Don’t Show Again
6. Done! Autograding runs automatically. Your teacher can see your results in the GitHub Classroom dashboard

How to Review Feedback

1. Go to your repository on GitHub
2. Click the “Pull requests” tab at the top
3. Open the pull request called “Feedback”
4. Click the “Files changed” tab. You’ll see your teacher’s comments on specific lines of your code
5. Read through each comment carefully, then fix your code in Codespaces
6. Run bash run-tests.sh in your Codespaces Terminal to check your fixes
7. Commit and push again (repeat steps 12-17). The autograder will re-run automatically

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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.