Calculations & Arithmetic

What is it?

Java supports standard math operations. You can use these in expressions, assign results to variables, or pass them directly to methods.

Operators

How does it work?

int sum  = 5 + 3;   // 8
int diff = 10 - 4;  // 6
int prod = 3 * 4;   // 12
int quot = 7 / 2;   // 3  (NOT 3.5 - integer division truncates)
int rem  = 7 % 3;   // 1  (remainder after 7 ÷ 3 = 2, leftover 1)

Operator Precedence

Java follows standard math order of operations - BODMAS / PEMDAS:

1. Parentheses ( ) first
2. Then *, /, % (left to right)
3. Then +, - (left to right)

int result = 2 + 3 * 4;   // 14, not 20 - multiplication first
int result = (2 + 3) * 4; // 20 - parentheses override

The Integer Division Trap

This is the #1 gotcha in arithmetic:

int a = 7 / 2;      // Result: 3  (the .5 is silently dropped!)
double b = 7 / 2;   // Still 3.0! - the division happens as int first

double c = 7.0 / 2; // 3.5  ← correct: one double forces double division
double d = (double) 7 / 2; // 3.5  ← casting to double

Warning If both operands are int, the result is always int - the decimal is truncated (not rounded). To get a decimal result, at least one operand must be a double.

Modulo % - What’s it actually for?

Modulo gives you the remainder after division. It’s more useful than it looks:

// Is a number even or odd?
int n = 8;
if (n % 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

// Wrap around (e.g., clock: after 23 comes 0)
int hour = (currentHour + 1) % 24;

Compound Assignment Operators

Shorthand for updating a variable:

int score = 0;
score += 10; // score is now 10
score++;     // score is now 11

Converting String Input to Numbers

User input always comes in as String. To do math with it:

Scanner scanner = new Scanner(System.in);
String raw = scanner.nextLine();

int number    = Integer.parseInt(raw);    // "42" → 42
double price  = Double.parseDouble(raw);  // "9.99" → 9.99

Gotchas

• 7 / 2 = 3, not 3.5 - integer division truncates silently
• % works on negative numbers too, but the result takes the sign of the dividend: -7 % 3 = -1
• Dividing by zero (n / 0) throws ArithmeticException at runtime - guard against it

Related: String, Comparison, Condition