Variables

Learn how to declare, initialise, and name variables in Java - the containers that store data in your programs.

Overview

A variable is a named storage location in memory that holds a value. In Java, every variable must be declared with a type before it can be used. The type tells Java what kind of data the variable stores (a number, text, a true/false value, etc.) and how much memory to allocate. Java variables follow strict naming rules and a strong naming convention (camelCase). A variable can be declared and initialised on the same line, or declared first and assigned a value later. Once declared, a variable's type cannot change - Java is statically typed.

Key Points

•Declaration: `int age;` - tells Java the type and name but does not set a value yet
•Initialisation: `age = 25;` - assigns a value to a previously declared variable
•Declaration + Initialisation: `int age = 25;` - the most common pattern
•Variable names use camelCase: myAge, firstName, totalScore (start with lowercase)
•Names cannot start with a digit, cannot contain spaces, and cannot be Java keywords (int, class, etc.)
•Java is statically typed - a variable declared as int can only ever hold integers

Examples

Declaring and using variables

public class Main {
    public static void main(String[] args) {
        // Declare then initialise
        int score;
        score = 95;

        // Declare and initialise on one line
        String name = "Alice";
        double height = 1.75;
        boolean isStudent = true;

        System.out.println(name + " scored " + score);
        System.out.println("Height: " + height + "m");
        System.out.println("Student: " + isStudent);
    }
}

String concatenation with + converts numbers and booleans to text automatically.

Updating a variable's value

public class Main {
    public static void main(String[] args) {
        int lives = 3;
        System.out.println("Lives: " + lives); // 3

        lives = lives - 1;
        System.out.println("Lives: " + lives); // 2

        lives = 0;
        System.out.println("Lives: " + lives); // 0
    }
}

You can overwrite a variable's value at any time. The old value is gone.

Valid and invalid variable names

// Valid names
int age = 25;
String firstName = "Bob";
double averageScore = 88.5;
boolean isLoggedIn = false;
int _count = 0;

// Invalid names - these will NOT compile
// int 1stPlace = 1;    ← cannot start with a digit
// String my name = ""; ← no spaces allowed
// int class = 5;       ← 'class' is a reserved keyword
// double int = 3.14;   ← 'int' is a reserved keyword

Java reserved keywords (int, class, for, if, etc.) cannot be used as variable names.

Common Mistakes

Using a variable before assigning it a value - Java will not compile: "variable might not have been initialized"
Using PascalCase (MyAge) for variables - that convention is for class names; variables use camelCase (myAge)
Trying to change a variable's type: declaring `int age = 25` then later writing `age = "twenty-five"` will not compile

Note

Java 10+ supports `var` for local variables: `var name = "Alice";`. The compiler infers the type from the right-hand side. Use it when the type is obvious to reduce verbosity.

Variables

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

Which of the following is a valid Java variable declaration?