GoldbachConjecture.java

package com.thealgorithms.maths;

import static java.lang.String.format;

import java.util.Scanner;

/**
 * This is a representation of the unsolved problem of Goldbach's Projection, according to which every
 * even natural number greater than 2 can be written as the sum of 2 prime numbers
 * More info: https://en.wikipedia.org/wiki/Goldbach%27s_conjecture
 * @author Vasilis Sarantidis (https://github.com/BILLSARAN)
 */

public final class GoldbachConjecture {
    private GoldbachConjecture() {
    }

    /**
     * Checks whether a number is prime or not
     * @param n the input number
     * @return true if n is prime, else return false
     */
    private static boolean isPrime(int n) {
        int i;
        if (n <= 1 || (n % 2 == 0 && n != 2)) {
            return false;
        } else {
            for (i = 3; i < Math.sqrt(n); i += 2) {
                if (n % i == 0) {
                    return false;
                }
            }
        }
        return true;
    }

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter a number");
        int n = scanner.nextInt();
        int flag = 0;

        if (n % 2 == 0 && n > 2) {
            for (int i = 0; i <= n / 2 && flag == 0; i++) {
                if (isPrime(i) && isPrime(n - i)) {
                    System.out.println(format("%d + %d = %d", i, n - i, n));
                    flag = 1;
                }
            }
        } else {
            System.out.println("Wrong Input");
        }
    }
}