Java Operators, Scanner Input, if-else, switch and Loops

This lesson covers everything you need to write small programs: operators, reading input, decisions and loops. The ideas are the same as in other languages, so the focus is on Java syntax and traps.

(Keep this picture in mind: every int you create in main lives on the stack; objects like Scanner live on the heap.)

1. Operators you'll use

• Arithmetic: + - * / %
• Increment: i++ (use then add), ++i (add then use)
• Comparison: == != > < >= <=
• Logical: && (and), || (or), ! (not) — they short-circuit
• Ternary: condition ? a : b

public class Main {
    public static void main(String[] args) {
        int i = 5;
        int a = i++;   // a = 5, then i = 6
        int b = ++i;   // i = 7, then b = 7
        System.out.println(a + " " + b + " " + i);
        int marks = 72;
        String result = marks >= 40 ? "Pass" : "Fail";
        System.out.println(result);
    }
}

5 7 7
Pass

2. Reading input with Scanner

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        // Input: Asha
        // Input: 20 30
        Scanner sc = new Scanner(System.in);
        String name = sc.nextLine();
        int x = sc.nextInt();
        int y = sc.nextInt();
        System.out.println("Hi " + name + ", sum = " + (x + y));
    }
}

Hi Asha, sum = 50

nextInt(), nextDouble(), next() (one word), nextLine() (whole line).

Trap: after nextInt(), the Enter key is still waiting. A following nextLine() reads that empty line. Call an extra sc.nextLine() to skip it.

3. if, else if, else

public class Main {
    public static void main(String[] args) {
        int marks = 86;
        if (marks >= 90) {
            System.out.println("A");
        } else if (marks >= 75) {
            System.out.println("B");
        } else {
            System.out.println("C");
        }
    }
}

B

4. switch

public class Main {
    public static void main(String[] args) {
        int day = 3;
        switch (day) {
            case 1:
                System.out.println("Monday");
                break;
            case 3:
                System.out.println("Wednesday");
                // no break → falls through!
            case 4:
                System.out.println("Thursday");
                break;
            default:
                System.out.println("Other");
        }
        String type = switch (day) {          // Java 14+ arrow form: no fall-through
            case 6, 7 -> "Weekend";
            default -> "Weekday";
        };
        System.out.println(type);
    }
}

Wednesday
Thursday
Weekday

Forgetting break makes execution fall through into the next case. The arrow form (->) avoids this.

5. Loops

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.print(i + " ");
        }
        System.out.println();

        int n = 1234, sum = 0;
        while (n > 0) {
            sum += n % 10;
            n /= 10;
        }
        System.out.println("digit sum " + sum);

        int k = 10;
        do {
            System.out.println("runs once even though k > 5");
        } while (k < 5);

        int[] marks = {70, 85, 92};
        for (int m : marks) {          // for-each
            System.out.print(m + " ");
        }
        System.out.println();
    }
}

1 2 3 4 5
digit sum 10
runs once even though k > 5
70 85 92

• do-while runs the body at least once (the check is at the end).
• break exits a loop; continue skips to the next round.

6. A pattern with nested loops

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j);
            }
            System.out.println();
        }
    }
}

1
12
123
1234

Common mistakes

• Using = instead of == in conditions.
• Forgetting break in a classic switch.
• Comparing Strings with == (next lessons explain why to use .equals()).
• The nextInt() + nextLine() trap with Scanner.

Quick recap

• i++ uses then increments; ++i increments then uses.
• Scanner: nextInt, nextLine; mind the leftover newline.
• if / else if / else; switch with break or the arrow form.
• for, while, do-while (at least once), for-each for arrays and collections.