Data Types in Java — Revision Notes

Definition

Syntax

int wholeNumber;
double decimalNumber;
char singleCharacter;
boolean trueOrFalse;

Example

public class Main {
    public static void main(String[] args) {
        int age = 20;
        double gpa = 8.9;
        char grade = 'A';
        boolean passed = true;
        System.out.println(age + " " + gpa + " " + grade + " " + passed);
    }
}

Output

Important Points

• Java has exactly 8 primitive types: byte, short, int, long, float, double, char, boolean.
• boolean can only be true or false — never an integer.
• int is 4 bytes, long is 8 bytes, always — regardless of the machine running it.
• Non-primitive types (String, arrays, objects) are called reference types.

Common Mistakes

• Trying to assign an integer (like 1 or 0) directly to a boolean — Java doesn't allow this.
• Losing precision by assigning a double value into a float or int without an explicit cast.
• Confusing primitive char (single character) with String (text).

Interview Tip