Java Variables

A variable is a named box that holds a value: a number, a piece of text, a true-or-false answer. Your program can read the value, change it, and use it in calculations. In Java every variable has a type that says what kind of value it can hold, and the type never changes.

Declaring a variable

You write the type, the name, and usually a first value:

public class Main {
    public static void main(String[] args) {
        String name = "Asha";
        int age = 21;
        double height = 1.62;
        System.out.println(name);
        System.out.println(age);
        System.out.println(height);
    }
}

Asha
21
1.62

• String holds text. Notice the capital S.
• int holds whole numbers like 21 or -5.
• double holds numbers with a decimal point like 1.62.

The Data Types topic lists all the types. These three cover most beginner programs.

Changing a value

The = sign means "store this value", not "is equal to". You can store a new value at any time, and the old one is gone:

public class Main {
    public static void main(String[] args) {
        int score = 10;
        System.out.println("Score: " + score);
        score = 15;
        System.out.println("Score: " + score);
        score = score + 5;
        System.out.println("Score: " + score);
    }
}

Score: 10
Score: 15
Score: 20

score = score + 5 reads the current value (15), adds 5, and stores the result back in score. The + between text and a number joins them into one piece of text.

You can also declare a variable first and give it a value later, but Java will not let you read it before it has a value.

Several variables and constants

You can declare several variables of the same type in one line. A variable marked final can be given a value only once; it is a constant, and by convention its name is written in capital letters:

public class Main {
    public static void main(String[] args) {
        int x = 5, y = 7, z = 9;
        final double PI = 3.14159;
        System.out.println(x + y + z);
        System.out.println(PI * 2);
    }
}

21
6.28318

If you try to write PI = 3; later, the program does not compile. Use final for values that should never change, like a tax rate or the number of days in a week.

var: let Java work out the type

Since Java 10 you can write var instead of the type when the value makes the type obvious. The variable still has a fixed type; Java only saves you from writing it:

public class Main {
    public static void main(String[] args) {
        var city = "Pune";      // a String
        var population = 7_400_000;  // an int
        System.out.println(city + " has " + population + " people.");
    }
}

Pune has 7400000 people.

The underscores in 7_400_000 only make the number easier to read; Java ignores them.

Rules for names

• A name may contain letters, digits, _ and $, but it cannot start with a digit.
• It cannot be a Java keyword such as int, class or public.
• Names are case-sensitive: age and Age are two different variables.
• Use clear names in camelCase: totalPrice, studentCount, isReady.

Practice exercises

1. Store your name, your age and your height in three variables and print them in one sentence.
2. Make a variable count with the value 1, then add 1 to it three times, printing it after each change.
3. Declare a final int DAYS_IN_WEEK = 7; and print how many days there are in 5 weeks.