Answer to Question #290918 in Java | JSP | JSF for Goldbach

Question

Write a program to check and display whether the number is Goldbach number or not using only for loop or while loop & java.util.* package

The output must be like below-

Enter a number:

6

Goldbach number

Answer 1

import java.util.*;


public class App {


	/**
	 * The start point of the program
	 * 
	 * @param args
	 * 
	 */
	public static void main(String[] args) {
		Scanner keyBoard = new Scanner(System.in);
		System.out.println("Enter a number: ");
		int number = keyBoard.nextInt();


		boolean[] isPrime = new boolean[number];


        for (int i = 2; i < number; i++)
            isPrime[i] = true;


        // determine primes < n using Sieve of Eratosthenes
        for (int factor = 2; factor*factor < number; factor++) {
            if (isPrime[factor]) {
                for (int j = factor; factor*j < number; j++)
                    isPrime[factor*j] = false;
            }
        }


        // count primes
        int primes = 0;
        for (int i = 2; i < number; i++)
            if (isPrime[i]) primes++;
        
        int[] numbers = new int[primes];
        int count = 0;
        for (int i = 0; i < number; i++)
            if (isPrime[i]) numbers[count++] = i;


        
        int left = 0, right = count-1;
        while (left <= right) {
            if      (numbers[left] + numbers[right] == number) break;
            else if (numbers[left] + numbers[right]  < number) left++;
            else right--;
        }
        if (numbers[left] + numbers[right] == number)
            System.out.println("Goldbach number");
        else
            System.out.println("NOT Goldbach number");
		
		
		keyBoard.close();
	}


}