Java: Armstrong, Palindrome & Prime Numbers

Often, such topics are given to students as assignment. This is necessary to clear the concepts or basics of Java programming. This article might help students and newcomers to Java.

Before beginning to write code, you should know what these numbers are. So, let's start with a little introduction into these numbers.
Armstrong Number
A number is called Armstrong Number, if the sum of its digits to the power of number of digits is number itself. For example,
we have a number, 1634. To determine whether 1634 is an Armstrong, we need to check:
Does 1⁴ + 6⁴ + 3⁴ + 4⁴ equal 1634 ?
Yes! So 1634 is Armstrong Number.
Similarly, 153 is Armstrong because 1³ + 5³ + 3³ equals to 153.
Palindrome Number
A number is said to be a Palindrome, if the reverse of its digit is number itself. For eg. 121, 959, 1441, etc.
Prime Number
A natural number greater than 1 is called a Prime Number, if it has no divisor other than 1 and itself. For eg. 2, 3, 5, 7, ...
The Java program written below has a class named VariousNumbers which contain three public static functions excluding main, listed below:

1. public static boolean Armstrong(int) // Return true, if an integer is found to be an Armstrong, else return false.
2. public static boolean Palindrome(int) // Return true, if an integer is found to be a Palindrome, else return false.
3. public static boolean Prime(int) // Return true, if an integer is found to be a Prime, else return false.
Code

public class VariousNumbers {

    public static void main(String[] args) {
        System.out.println("Armstrong Numbers 1 to 10000 >>");
        for (int i = 1; i <= 10000; i++) {
            if (Armstrong(i) == true) {
                System.out.print(i + " ");
            }
        }

        System.out.println("\nPalindrome Numbers 100 to 300 >>");
        for (int i = 100; i <= 300; i++) {
            if (Palindrome(i) == true) {
                System.out.print(i + " ");
            }
        }

        System.out.println("\nPrime Numbers up to 100 >>");
        for (int i = 1; i <= 100; i++) {
            if (Prime(i) == true) {
                System.out.print(i + " ");
            }
        }
    }

    public static boolean Armstrong(int num) {
        int num1 = num;
        /* Converting Integer to String. It'll help to find number of
        digits in the Integer by using length() */
        String str = Integer.toString(num);
        int rem;
        int result = 0;
        while (num > 0) {
            rem = num % 10;
            num = num / 10;
            result = result + ((int) Math.pow(rem, str.length()));
        }
        if (result == num1) {
            return true;
        } else {
            return false;
        }
    }

    public static boolean Palindrome(int num) {
        int num1 = num;
        int rem;
        int result = 0;
        while (num > 0) {
            rem = num % 10;
            num = num / 10;
            result = (result + rem) * 10;
        }
        result /= 10;
        if (result == num1) {
            return true;
        } else {
            return false;
        }
    }

    public static boolean Prime(int num) {
        if (num < 2) {
            return false;
        }
        int div = num / 2;
        for (int i = 2; i <= div; i++) {
            if (num % i == 0) {
                return false;
            }
        }
        return true;
    }
}

Output

Armstrong Numbers 1 to 10000 >>
1 2 3 4 5 6 7 8 9 153 370 371 407 1634 8208 9474
Palindrome Numbers 100 to 300 >>
101 111 121 131 141 151 161 171 181 191 202 212 222 232 242 252 262 272 282 292
Prime Numbers up to 100 >>
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97