Operators & Expressions

Goal: calculate and compare values with Java operators and use the Math class. Duration: 90 min

1. Arithmetic operators

% is very useful:

boolean isEven = number % 2 == 0;
int lastDigit = number % 10;
int minutes = totalSeconds / 60, seconds = totalSeconds % 60;

2. Assignment and increment

int x = 10;
x += 5;   // x = x + 5   → 15
x -= 3;   // 12
x *= 2;   // 24
x /= 4;   // 6
x %= 4;   // 2

int i = 5;
int a = i++;  // a = 5, then i = 6  (post-increment)
int b = ++i;  // i = 7, then b = 7  (pre-increment)

Rule: in real code use i++ alone on a line; avoid mixing it inside expressions.

3. Comparison operators → boolean

==, !=, >, <, >=, <=

int balance = 500;
boolean canPay = balance >= 300;     // true

⚠️ Never compare Strings with ==. Use .equals() (lesson 4).

4. Logical operators

boolean adult = age >= 18;
boolean hasIncome = salary > 0;
boolean canGetLoan = adult && hasIncome && !blacklisted;

Short-circuit: in a && b, if a is false, b is not evaluated. This protects us:

if (count != 0 && total / count > 10) { ... }  // no division by zero

5. Precedence

() → ++ -- ! → * / % → + - → comparisons → && → || → =

int r = 2 + 3 * 4;      // 14
int s = (2 + 3) * 4;    // 20

When in doubt, use parentheses: they make code readable.

6. The Math class

Math.max(4, 9)        // 9
Math.min(4, 9)        // 4
Math.abs(-7)          // 7
Math.pow(2, 10)       // 1024.0
Math.sqrt(144)        // 12.0
Math.round(4.6)       // 5
Math.floor(4.9)       // 4.0
Math.ceil(4.1)        // 5.0
Math.random()         // 0.0 ≤ x < 1.0
int dice = (int) (Math.random() * 6) + 1;   // 1..6

7. Step by step: leap year and BMI

public class Calculations {
    public static void main(String[] args) {
        int year = 2028;
        boolean leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
        System.out.println(year + " is leap: " + leap);

        double weightKg = 78, heightM = 1.80;
        double bmi = weightKg / Math.pow(heightM, 2);
        System.out.println("BMI: " + Math.round(bmi * 10) / 10.0);   // 24.1
    }
}

8. Common mistakes

• ❌ = (assignment) instead of == (comparison).
• ❌ 5 / 2 * 2.0 → 4.0, not 5.0 (integer division happens first).
• ❌ Using & / | instead of && / || (no short-circuit).

9. Exercises

1. Given a 3-digit number, print the sum of its digits (472 → 13).
2. Check if a number is between 10 and 50 (inclusive).
3. Convert 7384 seconds to hh:mm:ss.
4. Calculate the monthly loan payment: P * r / (1 - Math.pow(1 + r, -n)) for P = 10000, annual 12%, 24 months.

10. Homework

ATM logic without if: given amount = 1780, print how many 200, 100, 50, 20, 10 AZN notes are needed (use / and %).

11. Self-check

1. What is 17 % 5?
2. What is the value of x after int x = 3; x += x * 2;?
3. Why is && safer than & in count != 0 && total / count > 1?