if and else · Java · CodeMIRAI

if

Java

public class Main {
    public static void main(String[] args) {
        int temperature = 31;
        if (temperature > 30) {
            System.out.println("It's hot. Drink some water.");
        }
        System.out.println("Have a good day.");
    }
}

The condition goes in brackets and must be a boolean. The block in braces runs only when it's true; the program then carries on after it either way.

else and else if

Java

public class Main {
    public static void main(String[] args) {
        int score = 74;
        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("Score " + score + " is grade " + grade);
    }
}

Conditions are checked from the top, and only the first true branch runs. The order matters: test score >= 60 first, and an A-grade score would stop there.

Combining conditions

Java

public class Main {
    public static void main(String[] args) {
        int age = 16;
        boolean withParent = true;
        if (age >= 18 || withParent) {
            System.out.println("You can watch the film.");
        } else {
            System.out.println("Sorry, not without a parent.");
        }

        int year = 2024;
        boolean leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
        System.out.println(year + " is a leap year: " + leap);
    }
}

Nested decisions

Java

public class Main {
    public static void main(String[] args) {
        boolean loggedIn = true;
        boolean isAdmin = false;
        if (loggedIn) {
            if (isAdmin) {
                System.out.println("Welcome to the admin panel.");
            } else {
                System.out.println("Welcome back.");
            }
        } else {
            System.out.println("Please sign in.");
        }
    }
}

Nesting is fine a level or two deep. Deeper than that, combine conditions with &&, or move the logic into a method.

The ternary operator

Java

public class Main {
    public static void main(String[] args) {
        int items = 1;
        String label = items == 1 ? "item" : "items";
        System.out.println(items + " " + label);

        int a = 8, b = 13;
        int bigger = a > b ? a : b;
        System.out.println("Bigger: " + bigger);
    }
}

condition ? valueIfTrue : valueIfFalse chooses between two values. It's neat for short choices; for anything longer, an if is clearer.

Classic mistakes

Java

public class Main {
    public static void main(String[] args) {
        String answer = new String("yes");
        if (answer == "yes") {
            System.out.println("== says they match");
        }
        if (answer.equals("yes")) {
            System.out.println("equals says they match");
        }
    }
}

Only the second message appears. Compare Strings with equals.

Two more to watch for:

A stray semicolon. if (score > 50); ends the if right there, and the block after it always runs.
Missing braces. Without braces, only the next single statement belongs to the if. Add a second line later, and it runs every time. Always use braces.

The good news: if (x = 5) doesn't compile in Java, because an assignment isn't a boolean, so the classic C typo is caught for you.