Unit 2 · Java Fundamentals · 9 min

Variables and data types

A variable is a named box in memory that holds a value. In Java you must say what kind of value the box will hold — that is its data type.

Why this matters

Choosing the right type prevents wrong results (like losing the decimal part of a marks average) and is the base of everything that follows.

Real-life analogy

A variable is a labelled container in your kitchen. The label says both the name (sugar) and the kind of thing it holds (powder, not liquid).

Syntax

dataType variableName = value;

Example

Java

public class Types {
    public static void main(String[] args) {
        int age = 19;
        double percentage = 78.5;
        char grade = 'A';
        boolean isPassed = true;
        String name = "Ananya Rao";

        System.out.println(name + " is " + age + " years old");
        System.out.println("Percentage: " + percentage + ", Grade: " + grade);
        System.out.println("Passed? " + isPassed);
    }
}

Output

Ananya Rao is 19 years old
Percentage: 78.5, Grade: A
Passed? true

Step by step

1`int age = 19;` stores a whole number. int holds values roughly between -2 billion and 2 billion.
2`double percentage = 78.5;` stores a decimal number.
3`char grade = 'A';` stores one character in single quotes.
4`boolean isPassed = true;` stores only true or false.
5`String name = "Ananya Rao";` stores text in double quotes. String is a class, not a primitive type.
6The `+` operator joins text with values when one side is a String.

Key points

8 primitive types: byte, short, int, long, float, double, char, boolean
String is a class (non-primitive) but is used constantly
Use single quotes for char, double quotes for String
A variable must be declared before it is used

Common mistakes

Writing `int marks = 78.5;` — a decimal cannot fit in an int
Using double quotes for a char value
Using a variable before giving it a value

Try it yourself

Declare variables for a student's name, roll number, marks and pass status, then print them.
Try storing 10/3 in an int and in a double, and compare the results.

Open the practice notebook