Classic number programs · Java

These programs come up again and again in classes, tests and interviews. Each is a small method built from loops and conditions; the output is shown under each example.

Even or odd, and leap years

A leap year divides by 4, except century years, which must also divide by 400:

Java

public class Main {
    static boolean isLeap(int year) {
        return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
    }

    public static void main(String[] args) {
        int[] numbers = {7, 12, 0};
        for (int n : numbers) {
            System.out.println(n + (n % 2 == 0 ? " is even" : " is odd"));
        }
        int[] years = {2024, 2026, 1900, 2000};
        for (int y : years) {
            System.out.println(y + (isLeap(y) ? " is a leap year" : " is not a leap year"));
        }
    }
}

Output

7 is odd
12 is even
0 is even
2024 is a leap year
2026 is not a leap year
1900 is not a leap year
2000 is a leap year

Prime numbers

A prime has exactly two divisors, 1 and itself. Test divisors only up to the square root (d * d <= n): a bigger factor would pair with a smaller one already tested.

Java

public class Main {
    static boolean isPrime(int n) {
        if (n < 2) {
            return false;
        }
        for (int d = 2; d * d <= n; d++) {
            if (n % d == 0) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        System.out.println(isPrime(29) + " " + isPrime(91));
        int count = 0;
        for (int n = 2; n <= 50; n++) {
            if (isPrime(n)) {
                System.out.print(n + " ");
                count++;
            }
        }
        System.out.println();
        System.out.println(count + " primes up to 50");
    }
}

Output

true false
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 
15 primes up to 50

The Fibonacci series

Java

public class Main {
    public static void main(String[] args) {
        int a = 0, b = 1;
        for (int i = 1; i <= 10; i++) {
            System.out.print(a + " ");
            int next = a + b;
            a = b;
            b = next;
        }
        System.out.println();
    }
}

Output

0 1 1 2 3 5 8 13 21 34

Palindromes

Reverse a number's digits and compare; for text, StringBuilder can reverse a string:

Java

public class Main {
    static boolean isPalindrome(int n) {
        int original = n, reversed = 0;
        while (n > 0) {
            reversed = reversed * 10 + n % 10;
            n /= 10;
        }
        return reversed == original;
    }

    public static void main(String[] args) {
        System.out.println(121 + " " + isPalindrome(121));
        System.out.println(123 + " " + isPalindrome(123));
        String word = "level";
        String back = new StringBuilder(word).reverse().toString();
        System.out.println(word + " " + word.equals(back));
    }
}

Output

121 true
123 false
level true

Compare strings with .equals(), never ==, which checks whether they are the same object.

Armstrong numbers

The sum of the digits, each raised to the number of digits, equals the number itself (153 = 1 + 125 + 27):

Java

public class Main {
    static boolean isArmstrong(int n) {
        int digits = String.valueOf(n).length();
        int sum = 0, rest = n;
        while (rest > 0) {
            int d = rest % 10, power = 1;
            for (int k = 0; k < digits; k++) {
                power *= d;
            }
            sum += power;
            rest /= 10;
        }
        return sum == n;
    }

    public static void main(String[] args) {
        for (int n = 1; n <= 1000; n++) {
            if (isArmstrong(n)) {
                System.out.print(n + " ");
            }
        }
        System.out.println();
    }
}

Output

1 2 3 4 5 6 7 8 9 153 370 371 407

GCD and LCM

Euclid's method: replace (a, b) with (b, a % b) until the remainder is 0. Then LCM = a x b / GCD:

Java

public class Main {
    static int gcd(int a, int b) {
        while (b != 0) {
            int r = a % b;
            a = b;
            b = r;
        }
        return a;
    }

    public static void main(String[] args) {
        int x = 48, y = 36;
        System.out.println("GCD of " + x + " and " + y + " is " + gcd(x, y));
        System.out.println("LCM of " + x + " and " + y + " is " + x * y / gcd(x, y));
    }
}

Output

GCD of 48 and 36 is 12
LCM of 48 and 36 is 144

Swapping two values

Java

public class Main {
    public static void main(String[] args) {
        int a = 5, b = 9;
        int temp = a;
        a = b;
        b = temp;
        System.out.println("a = " + a + ", b = " + b);
    }
}

Output

a = 9, b = 5