Lesson 8 of 20

Operators and Expressions

Perform arithmetic, comparisons, assignments, and logical operations, and understand operator precedence.

Learning objectives

• Use arithmetic operators in calculations.
• Compare values with relational operators.
• Combine conditions using logical operators.
• Use compound assignment and increment/decrement operators.
• Control evaluation order with parentheses.

1. Arithmetic operators

Java supports addition +, subtraction -, multiplication *, division /, and remainder %.

Example: arithmetic

public class ArithmeticDemo {
    public static void main(String[] args) {
        int a = 17;
        int b = 5;

        System.out.println("a + b = " + (a + b));
        System.out.println("a - b = " + (a - b));
        System.out.println("a * b = " + (a * b));
        System.out.println("a / b = " + (a / b));
        System.out.println("a % b = " + (a % b));
    }
}

Because both operands in a / b are integers, the result is integer division: 3, not 3.4.

2. Relational operators

Comparisons return a Boolean result.

int score = 72;

System.out.println(score >= 50); // true
System.out.println(score == 100); // false
System.out.println(score != 0);   // true

3. Logical operators

Use && for logical AND, || for logical OR, and ! for NOT.

Example: eligibility rule

int age = 20;
boolean hasLicense = true;

boolean canRent = age >= 18 && hasLicense;
System.out.println("Can rent: " + canRent);

4. Assignment and increment

int points = 10;
points += 5; // same as points = points + 5
points++;    // add 1
System.out.println(points); // 16

5. Operator precedence

Multiplication and division are evaluated before addition and subtraction. Parentheses make the intended order explicit.

double result1 = 10 + 5 * 2;     // 20
double result2 = (10 + 5) * 2;   // 30

Mini project: BMI calculator

public class BmiCalculator {
    public static void main(String[] args) {
        double weightKg = 70.0;
        double heightM = 1.75;

        double bmi = weightKg / (heightM * heightM);
        boolean healthyRange = bmi >= 18.5 && bmi < 25.0;

        System.out.printf("BMI: %.2f%n", bmi);
        System.out.println("In standard healthy range: " + healthyRange);
    }
}

Common mistakes

Summary

Operators turn stored values into useful expressions. In Lesson 9, you will use Boolean expressions to make decisions with if, else, and switch.