Variables and Data Types - Java

Every Java variable has a type, written before its name. The compiler checks that you only store values of that type in it, which catches many mistakes before the program ever runs.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        int age = 20;
        double cgpa = 8.7;
        char section = 'B';
        boolean isEnrolled = true;
        String name = "Kabir";

        System.out.println(name + " is " + age + " years old.");
        System.out.println("CGPA: " + cgpa + ", section " + section);
        System.out.println("Enrolled? " + isEnrolled);
    }
}

Output

Kabir is 20 years old.
CGPA: 8.7, section B
Enrolled? true

Primitive types and String

int and long store whole numbers (long for very big ones).
double stores decimals.
char stores one character in single quotes.
boolean is true or false.
String (capital S) stores text in double quotes. It is a class, not a primitive, so it comes with useful methods.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        String word = "Programming";
        System.out.println(word.length());
        System.out.println(word.toUpperCase());
        System.out.println(word.substring(0, 7));
        System.out.println(word.contains("gram"));
    }
}

Output

11
PROGRAMMING
Program
true

Converting between types

Java converts a smaller type to a larger one automatically, but going the other way needs an explicit cast. To turn text into a number, use Integer.parseInt or Double.parseDouble.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        int items = 7;
        double perPerson = items / 2.0;      // int widened to double
        int rounded = (int) perPerson;        // cast drops the decimals
        int fromText = Integer.parseInt("250");

        System.out.println(perPerson);
        System.out.println(rounded);
        System.out.println(fromText * 2);
    }
}

Output

3.5
3
500

var and final

Inside a method you can write var and let the compiler work out the type from the value. Mark a variable final when it must never be reassigned.

Java

Open in playground

public class Main {
    public static void main(String[] args) {
        var city = "Jaipur";          // compiler infers String
        final double GST_RATE = 0.18;
        double bill = 500 * (1 + GST_RATE);
        System.out.println(city + " bill: " + bill);
    }
}

Output

Jaipur bill: 590.0