Booleans & if…else

Programs make decisions: show a discount if the basket is big enough, reject a password that's too short, greet differently in the morning. if, else if and else run different code depending on whether a boolean condition is true.

if

JAVA

void main() {
    int temperature = 31;
    if (temperature > 30) {
        System.out.println("It's hot today.");
    }
    System.out.println("Done.");
}

Output

It's hot today.
Done.

The condition in parentheses must be a boolean expression. If it's true, the block runs; otherwise it's skipped.

if…else

JAVA

void main() {
    double basket = 42.0;
    if (basket >= 50) {
        System.out.println("Free shipping!");
    } else {
        System.out.println("Spend " + (50 - basket) + " more for free shipping.");
    }
}

Output

Spend 8.0 more for free shipping.

else if chains

JAVA

void main() {
    int score = 73;
    String grade;
    if (score >= 90) {
        grade = "A";
    } else if (score >= 75) {
        grade = "B";
    } else if (score >= 60) {
        grade = "C";
    } else {
        grade = "F";
    }
    System.out.println("Grade: " + grade);
}

Output

Grade: C

Conditions are checked from the top, and only the first true branch runs. Order matters: if score >= 60 came first, every passing score would get a C.

Combining conditions

JAVA

void main() {
    int age = 16;
    boolean withAdult = true;
    if (age >= 18 || withAdult) {
        System.out.println("Entry allowed");
    }
    if (age >= 13 && age <= 19) {
        System.out.println("Teenager");
    }
    if (!(age >= 18)) {
        System.out.println("Minor");
    }
}

Output

Entry allowed
Teenager
Minor

Nested if

JAVA

void main() {
    boolean loggedIn = true;
    boolean isAdmin = false;
    if (loggedIn) {
        if (isAdmin) {
            System.out.println("Admin dashboard");
        } else {
            System.out.println("User dashboard");
        }
    } else {
        System.out.println("Please log in");
    }
}

Output

User dashboard

Deep nesting gets hard to read. Often a combined condition or an early return (lesson 20) flattens the logic.

Braces are optional — but use them

JAVA

void main() {
    int stock = 0;
    if (stock > 0)
        System.out.println("Available");
        System.out.println("Ships today");
}

Output

Ships today

Without braces, only the first statement belongs to the if. The indentation suggests both lines are conditional, but "Ships today" always prints. Always using braces prevents this classic bug.

Comparing correctly

JAVA

void main() {
    String answer = new String("yes");
    double total = 0.1 + 0.2;
    System.out.println(answer == "yes");
    System.out.println(answer.equals("yes"));
    System.out.println(total == 0.3);
    System.out.println(Math.abs(total - 0.3) < 1e-9);
}

Output

false
true
false
true

Compare strings with equals, and compare doubles with a small tolerance rather than ==.

The conditional operator

JAVA

void main() {
    int hour = 20;
    String greeting = hour < 12 ? "Good morning" : hour < 18 ? "Good afternoon" : "Good evening";
    System.out.println(greeting);
}

Output

Good evening

The ternary operator chooses between two values. Chaining them works but quickly becomes hard to read; a switch (next lesson) or if chain is often clearer.

Try it yourself

Classify a year as a leap year: divisible by 4, except centuries, unless divisible by 400.

Output

2100 is not a leap year

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