I wrote a primality test program in Java

Primality test in Java

I've been taking Java lectures in earnest since yesterday, and I learned about do-while statements. I decided to write a primality test program, so this is a memorandum.


package sample;

import java.util.Scanner;

public class Sosu {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int num;
		boolean b = true;

		while (true) {
			System.out.println("Primality test");
			System.out.print("Please enter an integer greater than or equal to 2.");
			num = sc.nextInt();

			if (num < 2) {
				System.out.println("An integer less than 2.");
			} else {
		if (num == 2) {
			System.out.println(num + "Is a prime number.");
		} else {
			for (int i = 2; i < num; i++) {
				if (num % i == 0) {
					System.out.println(num + "Is not a prime number.");
					b = false;
			if (b) {
				System.out.println(num + "Is a prime number.");



  1. Play the while statement whose input content is less than 2 and have it re-entered.
  2. Escape the while statement with break, and judge if the input is 2 or more with the if statement.
  3. When the input is 2, it is a prime number.
  4. If the input is greater than 2, divide it by a number greater than or equal to 2 and less than num, and if it is divisible somewhere, it is not a prime number.
  5. Those that are not divisible are prime numbers. The final judgment is made by boolean.


To be honest, there seems to be a better way, so if I come up with something I've learned in the future, I'll correct it. Also, I would be grateful if you could give me any advice, such as how to do this here in its current form.

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