Java Variables and Data Types

A variable is a named box that stores a value your program can use and change.

Creating Variables

To create a variable, write its type, its name, an equals sign and a value:

public class Main {
  public static void main(String[] args) {
    String name = "Ali";
    int age = 19;
    double fee = 3500.50;

    System.out.println(name);
    System.out.println(age);
    System.out.println(fee);
  }
}

Ali
19
3500.5

You can change a variable later by giving it a new value:

int marks = 70;
marks = 85;   // marks is now 85

Primitive Data Types

Java has eight basic (primitive) types. These are the ones you will use most:

byte and short also exist for small whole numbers, but they are used less often.

The String Type

String holds text in double quotes. It is not a primitive type; it is a class, which is why it starts with a capital S. You will learn more in the Strings chapter.

Joining Text and Variables

Use + to join text and variables in one line:

public class Main {
  public static void main(String[] args) {
    String student = "Ayesha";
    int marks = 92;
    char grade = 'A';
    boolean passed = true;

    System.out.println(student + " got " + marks + " marks");
    System.out.println("Grade: " + grade + ", Passed: " + passed);
  }
}

Ayesha got 92 marks
Grade: A, Passed: true

Naming Rules

• Names can contain letters, digits, _ and $, but cannot start with a digit
• Names are case sensitive: total and Total are different
• You cannot use Java keywords like int or class as names
• Use camelCase for variables: totalPrice, studentName

Constants with final

Add final when a value must never change:

final int DAYS_IN_WEEK = 7;
// DAYS_IN_WEEK = 8;  would cause an error

The var Keyword

Since Java 10, var lets Java work out the type from the value:

var city = "Karachi";   // Java knows this is a String
var count = 12;         // Java knows this is an int

Type Casting

Casting changes a value from one type to another.

• Widening (small to big) happens automatically: int to double
• Narrowing (big to small) must be written by hand with (type)

public class Main {
  public static void main(String[] args) {
    int items = 9;
    double asDouble = items;        // automatic
    double average = 7.8;
    int rounded = (int) average;    // manual, the decimal part is cut off

    System.out.println(asDouble);
    System.out.println(rounded);
  }
}

9.0
7

Exercise