Variables

Learn how to store and work with data in your Java programs using variables.

What is a Variable?

A variable is a container that stores data in your program. Think of it like a labelled box where you can put information and retrieve it later.

int age = 25;
String name = "Alice";

In the example above:

• age is a variable that stores the number 25
• name is a variable that stores the text "Alice"

Declaring Variables

To create a variable in Java, you need to:

1. Specify the type (what kind of data it holds)
2. Give it a name
3. Optionally, assign it a value

Syntax

// Declaration only
int age;

// Declaration with initialisation
int age = 25;

// Multiple declarations
int x = 10, y = 20, z = 30;

Variable Names

Rules for Naming Variables

1. Must start with: a letter, underscore _, or dollar sign $
2. Cannot start with: a number
3. Can contain: letters, numbers, underscores, dollar signs
4. Cannot use: Java keywords (like int, class, public)
5. Case-sensitive: age and Age are different variables

Good Naming Conventions

// Good - descriptive and clear
int userAge = 25;
String firstName = "John";
double accountBalance = 1250.50;

// Bad - unclear meaning
int x = 25;
String s = "John";
double d = 1250.50;

// Use camelCase for variable names
int numberOfStudents = 30;  // Good
int number_of_students = 30; // Not Java convention

Assigning Values

You can change a variable's value after declaring it:

int score = 0;
System.out.println(score); // Prints: 0

score = 10;
System.out.println(score); // Prints: 10

score = score + 5;
System.out.println(score); // Prints: 15

Variable Types

Variables can store different types of data:

// Whole numbers
int age = 25;

// Decimal numbers
double price = 19.99;

// Text
String message = "Hello";

// True/false values
boolean isStudent = true;

We'll learn more about these types in the Data Types lesson.

Constants

If you want a variable that never changes, use the final keyword:

final double PI = 3.14159;
final int MAX_USERS = 100;

// This would cause an error:
// PI = 3.14; // Cannot change a final variable

Constants are typically written in UPPER_CASE.

Variable Scope

Variables only exist within the block {} where they're declared:

public class Example {
    public static void main(String[] args) {
        int x = 10; // x exists here

        if (true) {
            int y = 20; // y exists only inside this block
            System.out.println(x); // Can access x
            System.out.println(y); // Can access y
        }

        System.out.println(x); // Still can access x
        // System.out.println(y); // ERROR: y doesn't exist here
    }
}

Common Mistakes

1. Using Before Declaring

// Wrong
System.out.println(age); // Error: age doesn't exist yet
int age = 25;

// Correct
int age = 25;
System.out.println(age);

2. Not Initialising

// Wrong
int score;
System.out.println(score); // Error: score has no value

// Correct
int score = 0;
System.out.println(score);

3. Wrong Type

// Wrong
int age = "25"; // Error: can't store text in an int

// Correct
int age = 25;
String ageText = "25";

Practice Exercise

Create a Java program that:

1. Declares variables for:

   • Your name (String)
   • Your age (int)
   • Your height in metres (double)
   • Whether you're a student (boolean)

2. Print each variable with a label:

   Name: Alice
   Age: 25
   Height: 1.75
   Student: true

3. Update your age by adding 1, then print it again

Solution Template

public class VariablesPractice {
    public static void main(String[] args) {
        // Declare your variables here

        // Print them out

        // Update age and print again
    }
}

Key Takeaways

• Variables store data that can be used and changed in your program
• Every variable has a type, name, and value
• Use descriptive names in camelCase
• Variables only exist within their scope (the {} block they're declared in)
• Use final for values that shouldn't change

Next Steps

Now that you understand variables, let's learn about the different data types you can store in them.