Decisions, Switch & Loops

if, else if and else choose what runs; combine conditions with &&, || and !. The ternary condition ? a : b picks between two values.

Modern Java has switch expressions: case "A" -> "Excellent"; with arrows, no fall-through and a value you can assign. Use default to cover everything else.

Loops: the classic for (int i = 0; i < n; i++), the enhanced for (int s : scores) for every item, while for repeating until a condition changes, and do...while when the body must run at least once. break exits a loop; continue skips to the next round.

Grades.javaJava

public class Grades {
    static String gradeFor(int score) {
        if (score >= 75) return "A";
        else if (score >= 65) return "B";
        else if (score >= 45) return "C";
        else if (score >= 30) return "D";
        else return "F";
    }

    static String remarkFor(String grade) {
        return switch (grade) {
            case "A" -> "Excellent";
            case "B" -> "Very good";
            case "C" -> "Good";
            case "D" -> "Satisfactory";
            default -> "Fail";
        };
    }

    public static void main(String[] args) {
        int[] scores = {82, 67, 49, 31, 18};

        for (int score : scores) {
            String grade = gradeFor(score);
            System.out.println(score + " -> " + grade + " (" + remarkFor(grade) + ")");
        }

        for (int i = 0; i < scores.length; i++) {
            String status = scores[i] >= 30 ? "pass" : "fail";
            System.out.println("Student " + (i + 1) + ": " + status);
        }

        for (int n = 1; n <= 15; n++) {
            if (n % 15 == 0) System.out.print("FizzBuzz ");
            else if (n % 3 == 0) System.out.print("Fizz ");
            else if (n % 5 == 0) System.out.print("Buzz ");
            else System.out.print(n + " ");
        }
        System.out.println();

        int countdown = 3;
        while (countdown > 0) {
            System.out.println("Starting in " + countdown + "...");
            countdown--;
        }
    }
}

Key points

Prefer switch expressions with ->: no fall-through, and they return a value.
Enhanced for for every item; indexed for when you need positions.
Keep conditions simple — return early from methods instead of nesting.

Exercise

Print the multiplication table (1–12) for a number given as a command-line argument. Then write a program that keeps doubling an investment of 100,000 TSh at 12% per year with a while loop until it exceeds 1,000,000, printing each year.

Show solution

Try the exercise yourself first — then compare your approach with this one.

The table loops from 1 to 12, reading the number from the first argument. The investment loop repeats while the balance is still at or below one million, growing it by 12% and counting years.

Loops.javaJava

public class Loops {
    public static void main(String[] args) {
        int number = args.length > 0 ? Integer.parseInt(args[0]) : 7;
        for (int i = 1; i <= 12; i++) {
            System.out.printf("%2d x %d = %d%n", i, number, i * number);
        }

        double balance = 100_000;
        int year = 0;
        while (balance <= 1_000_000) {
            balance *= 1.12;
            year++;
            System.out.printf("Year %2d: %,.0f TSh%n", year, balance);
        }
        System.out.println("It takes " + year + " years to pass 1,000,000 TSh.");
    }
}

Key points

Exercise

Check your understanding