Java Variables

Java variables are named containers that store values your program works with. Every variable in Java has a type, which decides what kind of value it can hold, and that type cannot change later.

Declaring Variables

The pattern is type name = value;. You can also declare first and assign later.

public class Main {
    public static void main(String[] args) {
        String name = "Ayesha";
        int age = 27;
        double height = 1.65;
        boolean student = false;

        int score;      // declared
        score = 88;     // assigned later

        System.out.println(name + " is " + age + " years old.");
        System.out.println("Height: " + height + ", student: " + student);
        System.out.println("Score: " + score);
    }
}

Ayesha is 27 years old.
Height: 1.65, student: false
Score: 88

Changing a Variable

A normal variable can be given a new value of the same type at any time.

public class Main {
    public static void main(String[] args) {
        int level = 1;
        level = 2;
        level = level + 1;
        System.out.println("Level " + level);
    }
}

Level 3

Type Inference with var

Since Java 10 you can write var for local variables and the compiler figures out the type from the value. The variable is still strongly typed: once var count = 5 is an int, it stays an int.

public class Main {
    public static void main(String[] args) {
        var city = "Lahore";      // String
        var population = 13_000_000; // int (underscores improve readability)
        var ratio = 0.75;          // double
        System.out.println(city + ": " + population + " people, ratio " + ratio);
    }
}

Lahore: 13000000 people, ratio 0.75

Constants with final

Mark a variable final when its value should never change. Trying to reassign it is a compile error.

public class Main {
    public static void main(String[] args) {
        final double TAX_RATE = 0.17;
        double price = 200;
        System.out.println("Tax: " + price * TAX_RATE);
        // TAX_RATE = 0.2; // error: cannot assign a value to final variable
    }
}

Tax: 34.0

Java Variable Naming Rules

• Names may contain letters, digits, underscores and dollar signs.
• A name cannot start with a digit.
• Names are case sensitive: total and Total are different variables.
• Reserved words such as int, class or new cannot be used as names.
• By convention use camelCase and descriptive names like orderTotal, not x.

Frequently asked questions

Should I use var in Java?

Use var when the type is obvious from the right-hand side, such as var list = new ArrayList<String>(). Write the type explicitly when it makes the code clearer.

Can I declare multiple variables in one line in Java?

Yes, if they share a type: int x = 1, y = 2, z = 3;. For readability, most style guides prefer one declaration per line.

What happens if I use a local variable before assigning it?

The compiler reports "variable might not have been initialized". Local variables have no default value, unlike fields in a class.