Lesson 26 min read

Operators & Expressions

The math and logic behind every decision

Operators — Java's Toolbox

Operators are the verbs of your code. Variables hold the data, but operators do stuff with it — add, compare, combine, flip. Think of them like buttons on a calculator, except way more powerful. Mastering operators is your first step toward understanding algorithm complexity.

Java has several families of operators:

Arithmetic — math stuff: + - * / %
Relational — comparing things: == != < > <= >=
Logical — combining true/false: && || !
Assignment — storing results: = += -= *=
Increment/Decrement — quick +1 or -1: ++ --

Arithmetic & the Sneaky Integer Division

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

        System.out.println("Add: " + (a + b));      // 22
        System.out.println("Subtract: " + (a - b));  // 12
        System.out.println("Multiply: " + (a * b));  // 85
        System.out.println("Divide: " + (a / b));    // 3, NOT 3.4!
        System.out.println("Remainder: " + (a % b)); // 2

        // To get the real decimal answer, make one side a double
        System.out.println("Real divide: " + ((double) a / b)); // 3.4
    }
}

Output

Add: 22
Subtract: 12
Multiply: 85
Divide: 3
Remainder: 2
Real divide: 3.4

Relational & Logical Operators

Relational operators ask yes-or-no questions and give you back true or false. "Is 5 greater than 3?" → true.

Logical operators combine those answers:

&& (AND) — both sides must be true. Like needing both a ticket AND an ID to get in.
|| (OR) — at least one side must be true. Like a restaurant that accepts cash OR card.
! (NOT) — flips true to false and vice versa. Like an "opposite day" button.

Comparisons & Logic in Action

public class Main {
    public static void main(String[] args) {
        int age = 15;
        boolean hasPermission = true;

        // Relational operators
        System.out.println("Is teenager? " + (age >= 13 && age <= 19));
        System.out.println("Is adult? " + (age >= 18));

        // Logical operators
        boolean canWatch = age >= 13 || hasPermission;
        System.out.println("Can watch PG-13? " + canWatch);

        // NOT operator
        boolean isSleeping = false;
        System.out.println("Is awake? " + !isSleeping);

        // Combining multiple conditions
        boolean canDrive = age >= 16 && hasPermission && !isSleeping;
        System.out.println("Can drive? " + canDrive);
    }
}

Output

Is teenager? true
Is adult? false
Can watch PG-13? true
Is awake? true
Can drive? false

Ternary Operator & Increment/Decrement

public class Main {
    public static void main(String[] args) {
        // Ternary: condition ? valueIfTrue : valueIfFalse
        int score = 85;
        String result = score >= 60 ? "Pass" : "Fail";
        System.out.println("Result: " + result);

        // It's a one-line if/else!
        int temp = 30;
        String weather = temp > 25 ? "Hot" : "Cool";
        System.out.println("Weather: " + weather);

        // Increment & Decrement
        int lives = 3;
        System.out.println("Lives: " + lives);
        lives--;  // lost a life
        System.out.println("After hit: " + lives);
        lives++;  // found a 1-up!
        System.out.println("Found 1-up: " + lives);

        // Pre vs Post increment
        int x = 5;
        System.out.println("Post: " + x++); // prints 5, THEN adds 1
        System.out.println("Now x is: " + x); // 6
        System.out.println("Pre: " + ++x);  // adds 1 FIRST, then prints 7
    }
}

Output

Result: Pass
Weather: Hot
Lives: 3
After hit: 2
Found 1-up: 3
Post: 5
Now x is: 6
Pre: 7

Note: Watch out for == vs =! Single = means "put this value in the box." Double == means "are these two things equal?" Mixing them up is one of the most common beginner bugs. And remember: for Strings, always use .equals() instead of ==.

Challenge

Quick check

What does 17 / 5 give you in Java (both are ints)?

← Variables & Data Types Strings →